Arousal, Extraversion, and Individual Differences in Resource Availability

Transcription

1 Journal of Personality and Social Psychology Copyright 1990 by the American Psychological Association, Inc. 1990, Vol. 59, No. 1, /90/$00.75 Arousal, Extraversion, and Individual Differences in Resource Availability Gerald Matthews, D. Roy Davies, and Jackie L. Lees Aston University Birmingham, England Three studies of the effects ofextraversion and self-report arousal on attentional task performance were conducted, using sustained attention, visual/memory search, and letter transformation tasks. Differing predictions were derived from the Yerkes-Dodson Law and from Humphreys and Revelle's theory of individual differences in multiple attentional resources. Higher arousal was consistently associated with more efficient performance of demanding versions of the attentional tasks. This finding supports Humphreys and Revelle's hypothesis that arousal is positively associated with the resources required for sustained information transfer. No support for the Yerkes-Dodson Law was found in any study: Relationships between arousal and performance were largely linear. Interactive effects of extraversion and arousal on response criterion were found with self-paced tasks. Extraversion may affect more than 1 type of cognitive process, with the strategic demands of the task influencing the kind of extraversion effect found in any given experiment. Extraversion and Performance: The Role of Arousal Numerous psychometric studies of personality questionnaires show that one of the most reliable personality traits is extraversion-introversion (H. Eysenck & M. Eysenck, 1985). Questionnaire measures ofextraversion predict individual differences in many types of behavior, from simple conditioned reflexes to complex social behaviors (H. Eysenck & M. Eysenck, 1985). It is also well established that extraversion is associated with individual differences in attentional performance. For example, introverts are more susceptible to adverse effects of increased processing demands and of increased environmental stress (M. Eysenck, 1982). Extraversion measures also predict psychophysiological functioning: extraverts tend to show lower cortical and autonomic activity and reactivity, at least under conditions of moderate external stimulation (Stelmack, 1981). Extraversion research thus presents a considerable challenge to personality theorists because of the need to integrate psychometric, neurological, and behavioral levels of description. The most successful theory of extraversion to date, that of H. Eysenck (1967, 1981), supposes that the neurological level is of primary importance for explanatory purposes. H. Eysenck (1967) described a simplified, conceptual nervous system in which a major element is a neural feedback loop linking the cerebral cortex to the ascending reticular activating system, with the function of regulating the level of cortical arousal elicited by stimulation. H. Eysenck proposed that the excitability of this reticulo-cortical loop is higher in introverts than in extraverts, so that under moderate ambient stimulation introverts will tend to have higher resting levels of cortical arousal. Under We are grateful to Trevor Harley for programming assistance, to Adrian Wells for assistance with testing subjects, and to two anonymous referees for comments on a previous draft of this article. Correspondence concerning this article should be addressed to Gerald Matthews, who is now at the Department of Psychology, University of Dundee, Dundee, Scotland DDI 4HN. high levels of stimulation, transmarginal inhibition (TMI), a protective decrease in cortical arousal, may be induced, which will reduce or eliminate the normal negative correlation between extraversion and cortical arousal (H. Eysenck, 198 I). H. Eysenck (1967) proposed two further hypotheses that allow individual differences in arousal, and hence in extraversion, to be linked to behavior. First, the relationship between arousal and performance is described by the Yerkes-Dodson Law (Yerkes & Dodson, 1908), which states that the relationship between arousal and performance is an inverted-u function. In addition, the optimal level of arousal decreases as task difficulty increases. For easier tasks performance is better when arousal is high, whereas for more difficult tasks, or more difficult versions of the same task, performance is better when arousal level is low. The Eysenck theory then successfully predicts the characteristic interactive effect of extraversion and environmental stress on performance. Extraverts (being underaroused) will exhibit superior performance under high levels of stimulation but introverts will perform best under low levels of stimulation. For example, loud noise improves the signal detection performance of extraverts, whereas it exerts little or no effect on that of introverts (Davies & Hockey, 1966). Second, moderate levels of arousal are most pleasurable, so that people will actively regulate arousal level. In particular, extraverts will tend to seek stimulation, whereas introverts will avoid it. For example, compared with introverts, extraverts tend to adopt a lower or less stringent criterion for responding (M. Eysenck, 1982). It may be that the higher response rate normally associated with a low response criterion provides extraverts with the additional stimulation they seek. The Eysenck (1967) theory has two important implications for extraversion research. First, individual differences in arousal are of central importance: Effects of extraversion on behavior are often arousal-mediated. The causal chain relating extraversion to performance is that extraversion affects arousal, which in turn affects performance. The relationships between (a) extraversion and arousal, and (b) arousal and performance, 150

2 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 151 must be separately specified and tested. Second, at least two different kinds of extraversion effect on performance must be distinguished. On the one hand, the individual differences in arousal associated with extraversion may have direct, largely automatic effects on cognitive functioning, as described by the Yerkes-Dodson Law. On the other hand, performance may be affected by the person's strategy for controlling arousal level. Evidence reviewed by Revelle (1987) shows that personality traits do indeed appear to affect performance at several qualitatively different levels. Revelle, Humphreys, Simon, and Gilliland (1980) have suggested two important modifications to the personality-arousal relationship postulated by H. Eysenck (1967). First, their data showed that the impulsivity subfactor of extraversion was a stronger predictor of performance than the second subfactor, sociability. Second, on the basis of a consistent triple interaction between impulsivity, time of day, and the stimulant drug caffeine, they suggested that the relationship between impulsivity or extraversion and arousal varies with time of day. Impulsivity is negatively correlated with arousal in the morning but positively correlated with arousal in the evening. In a later article, Humphreys and Revelle (1984) also suggested modifications to the Yerkes-Dodson Law, proposing that arousal affects performance through its effects on attentional resources. Although definitions of resources differ, resources are viewed here as constituting one or more reservoirs of"fuel" (Hirst & Kalmar, 1987) for processing. Resources have been variously described as unitary or multiple (see Wickens, 1984). Multiple resources comprise several independent capacity reservoirs, each of which energizes a specific type of processing. Humphreys and Revelle's (1984) multiple resource theory distinguishes between (a) sustained information-transfer (SIT) resources, required for a variety of attentional tasks (such as letter cancellation and vigilance); and (b) short-term memory (STM) resources, required for short-term retention of information (as in digit span and running memory tasks). Complex tasks, such as intelligence tests, require both types of resources. Humphreys and Revelle claimed that as arousal increases, the availability of SIT resources rises, but the availability of STM resources falls. They review evidence suggesting that high arousal facilitates performance of simple attentional tasks but impairs STM. They derived the inverted-ij curve relating arousal to performance for complex tasks only. Arousal will only affect performance of tasks that are resource- rather than data-limited (Norman & Bobrow, 1975). Hence, the Humphreys and Revelle (1984) model not only explains the inverted-li curve for complex tasks but also differs from the Yerkes-Dodson Law in predicting monotonic arousal-performance regressions for tasks limited by one type of resource only. Experimental manipulation of SIT and STM resources therefore enables a straightforward test of the Humphreys and Revelle model. The Assessment of Individual Differences in Arousal Both arousal-mediation theories (H. Eysenck, 1967; Humphreys & Revelle, 1984) can be decomposed into two independently testable subtheories, one relating extraversion to arousal and one relating arousal to performance. The two rela- tionships concerned can be tested independently by using a valid and reliable measure of individual differences in arousal. Until recently, however, the use of external stressors has generally been preferred to direct measures of arousal in performance research because of difficulties with the validity of psychophysiological arousal measures (Broadbent, 1971). Unfortunately, stressors sometimes fail to have the expected effect on arousal, and there may be individual differences in the magnitude and direction of the arousal response to stressors (Matthews, 1985). Nonlinearity of relationships between external stress and internal arousal resulting from TMI (H. Eysenck, 1981) may explain some of these findings. Stressors may also have effects on cognition other than those mediated by arousal, such as distraction, so that different stressors have differing effects across a range of indicators of information-processing style and etficiency (Hockey & Hamilton, 1983). These problems of the validity of external stress manipulations indicate a need for direct measures of arousal to be reinstated in tests of theories of arousal and performance. At the least, it is necessary to complement studies of stress manipulations with studies using direct measures. A promising alternative to the recording of psychophysiological measures is to use a self-report measure of arousal, such as that developed by Thayer (1967). Thayer's (1978, 1986) two separate dimensions of self-report arousal, labelled energetic arousal and wnse arousal, both correlate positively with psychophysiological measures of autonomic arousal under performance testing conditions (Thayer, 1967, 1970). He also claimed that self-report arousal scales are more valid as measures of the activity of broad, integrated neural arousal systems than are single psychophysiological measures (Thayer, 1978). Because only energetic arousal reliably predicts performance, Thayer (1978) identifies energetic arousal with H. Eysenck's (1967) reticulo-cortical arousal. Thus, in the present experiments, energetic arousal effects were of primary interest, but tense arousal effects were tested in supplementary analyses. The assumption underlying use of self-report arousal measures is that they measure relatively transient states of arousal: Test-retest stabilities for such measures are considerably lower than their internal consistencies. For example, Matthews, Jones, and Chamberlain (1990) tested 158 subjects on 6 separate days during a 3-week period. Although the internal consistency of the energetic arousal measure used was 0.88, the mean interoccasion correlation was only Likewise, experimental evidence shows that self-report arousal measures are too sensitive to external stress manipulations to show the stability characteristic of trait measures. Matthews et al. (1990) reported shifts in mean energetic arousal of up to 2.59 standard deviations as a result of applying external stressors. Although the data are compatible with a state but not a trait interpretation of arousal scales, there is a further possibility. Revelle (personal communication, July 11, 1988) pointed out that individuals' self-ratings of arousal may be affected by individual differences in characteristic baseline levels of arousal, so that arousal ratings are not directly comparable across subjects. In other words, a self-report arousal score may indicate arousal relative to an individual's baseline rather than an absolute arousal value. Thus, only a part of the interindividual variance in arousal scores will reflect absolute arousal values; a second

3 152 G. MATTHEWS, R. DAVIES, AND J. LEES part will reflect interindividual variation in baseline. There is little direct evidence that would allow this interpretation to be distinguished from a straightforward state interpretation of self-report arousal scales. Johansson (1976) reported a psychophysical study of the direct magnitude estimation scaling of mood ratings that suggests that subjects are able to rate independently their current and usual moods, which favors the state interpretation of mood scales. A stronger test would require arousal measures to be taken across an extensive and systematically sampled range of situations to establish individual baselines. However, in view oftbe lack of strong positive evidence for the baseline-relative interpretation, it is reasonable to adopt the working hypothesis that a substantial part of the variance in self-report arousal measures is associated with individual differences in absolute level of arousal state. Aims of Research The general aim of this series of experiments was to test the Eysenck (1967) and Humphreys and Revelle (1984) theories by using tasks that are known from prior research to be strongly resource-limited. A self-report energetic arousal measure was used to conduct independent tests of the extraversion-arousal and arousal-performance relationships. Only one study in this research program has previously tested the Humphreys and Revelle (1984) theory. Matthews et al. (1989) used a battery of six tasks differing in their information-processing characteristics but failed to find the effects of arousal on performance predicted by either the Yerkes-Dodson Law or by the Humphreys and Revelle (1984) model. It is possible, however, that the resource usage of the tasks in this study was insufficient for predicted arousal effects to be found. Matthews et al. (1989) suggested that those tasks showing the characteristic interactive effect of extraversion and arousal on performance in this study were characterized by rapid but routine stimulus encoding rather than by heavy resource utilization. Here, resource demands were manipulated within-tasks to provide a more sensitive test of the dependence of personality and arousal effects on resource usage than in previous studies. Differing predictions were derived from the Eysenck (1967) arousal theory and from the multiple resource model of Humphreys and Revelle (1984). The first category of tasks selected were held to require SIT resources only: externally paced and self-paced versions of Nuechterlein, Parasuraman, and Jiang's (1983) sustained attention task, requiring discrimination of degraded digits. The second category of tasks were those on which both SIT and STM resources could be varied: Shiffrin and Schneider's (1977) visual and memory search task, and the letter transformation task used by Hamilton, Hockey, and Rejman (1977). General predictions from the multiple resource model were that in the first category of tasks, high arousal should be associated with superior performance when task demands are high. In the second task category, high arousal should facilitate performance when SIT resource demands are high and STM demands low, but should impair performance for tasks requiring STM but not SIT resources. In contrast, the Yerkes-Dodson Law predicts that arousal effects should vary with task difficulty rather than with the precise informationprocessing demands of the task. Task difficulty must be de- fined empirically: Ideally, the more difficult of two tasks should have both a higher error rate and a longer mean response time. Although the main focus of the research was on the direct effects of arousal on performance efficiency predicted by the two theories, we monitored a range of dependent measures to distinguish effects of arousal on efficiency from effects on task strategy, such as changes in speed-accuracy tradeoff or response criterion. The primary personality measure was extraversion rather than impulsivity, because in earlier studies (Matthews, 1989; Matthews & Chappelow, 1986; Matthews et al 1989), extraversion was found to be more predictive of performance than impulsivity. Both arousal-mediation theories predict that an analysis of performance data, including direct measures of both arousal and extraversion, should show no effects of extraversion. In other words, the relationship between a direct measure of arousal and performance should be the same in both extraverted and introverted groups, given that empirically there is a high degree of overlap between the distributions of arousal values within the two personality groups (Matthews, 1985). Instead, relationships between extraversion and arousal should be found. Specifically, the Eysenck (1967) theory predicts a negative correlation between extraversion and arousal level throughout the day. The Humphreys and Revelle (1984) theory, on the basis of the correlation between extraversion and impulsivity, predicts a negative extraversion-arousal correlation in the morning only, and a positive correlation in the evening. Extraversion should thus affect the personality characteristics of high and low arousal groups rather than performance per se. Experiments 1A and IB: Externally Paced and Self-Paced Sustained Attention It is important in sustained attention research to apply signal detection theory (SDT) analyses of performance (Green & Swets, 1966), distinguishing perceptual sensitivity or efficiency (indexed by d' or P(A)) from response criterion (indexed by B or log ~). Previous research (reviewed by Davies & Parasuraman, 1982) suggests that perceptual sensitivity may be higher in introverts, whereas criterion placement tends to be more stringent (Carr, 1969; Harkins & Geen, 1975). In addition, extraverts sometimes show a more pronounced decrement in detection rate with time on task (e.g, Davies & Hockey, 1966). The arousal theory explanation for these personality findings is that extraverts are more adversely affected by the low arousal induced by the task than are introverts. However, these personality studies suffer from three serious shortcomings. First, few of the studies used SDT measures, so extraversion effects on sensitivity cannot be distinguished from effects on criterion. Second, few of the studies manipulated or controlled for time of day. Third, the theoretical rationale for the particular tasks used is often unclear. The sustained attention task selected for the present investigation was a computerized version of the digit discrimination task used by Nuechterlein et al. (1983). Nuechterlein et al. demonstrated that in this task, perceptual sensitivity declines extremely rapidly over a period of 5-10 min, when the digits are visually degraded. Parasuraman, Warm, and Dember (1987) claimed that the rapid sensitivity decrement is attributable to a reduction in resource availability with time at work. Parasuraman (1985) used a secondary auditory probe reaction time to

4 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 153 Low arousal High arousal P E R F 0 R M A N C E DIFFICULT/ DEORADED EASY/CONTROL CONDIT ION CORTICAL AROUSAL Figure 1. Predicted effects of arousal on performance in easy and difficult tasks, limited by a single resource, according to Humphreys and ReveUe (1984). measure resource utilization of versions of the degraded digits tasks differing in event rate, and hence demands. Only the faster, more demanding version showed a sensitivity decrement and also showed an increase over time in probe reaction time (RT). Because primary and secondary task stimuli were presented in different modalities, reducing overlap of specific processing structures, this study provides strong evidence for the resource hypothesis. Previous evidence indicates that performance in self-paced vigilance tasks is broadly similar to that observed in externally paced tasks (Craig, Davies, & Matthews, 1987). However, selfpaced tasks provide greater scope for trade-offs between speed and accuracy and active control of stimulus presentation. Thus, comparison of externally paced and self-paced tasks provides a test of whether effects of extraversion and arousal on performance are modified by the need to choose and maintain a speed-accuracy tradeoff strategy. Both degraded and nondegraded versions of the externally paced and self-paced vigilance tasks were administered to subjects. It was hypothesized that (a) the tasks required SIT resources only, (b) the degraded version is more strongly resourcelimited than is the nondegraded version, and (c) resource availability is depleted to a greater extent with time at work in the degraded version than in the nondegraded version. The Humphreys and Revelle (1984) theory predicts that facilitative effects of high arousal on sustained attention should be more pronounced in the degraded version, particularly toward the end of the task, as shown in Figure 1. In contrast, classical arousal theory predicts that because more difficult tasks have a lower optimal level of arousal, high arousal should exert a greater facilitative effect on performance in the nondegraded (control) version of the sustained attention task, as shown in Figure 2. Method Experiment 1A Subjects. Subjects were 50 male and 50 female Aston University undergraduates, aged years, with normal natural or corrected vision, who were paid a small fee for participation in the experiment. Individual difference measures. Extraversion was measured with the Sixteen Personality Factor Questionnaire (16PF; Cattell, Eber, & Tatsuoka, 1970). The 16PF extraversion measure (exvia) is psychometrically equivalent to Eysenek Personality Inventory (EPI) extraversion in British students (Saville & Blinkhorn, 1976a). Energetic and tense arousal measures were obtained from the University of Wales Institute of Science and Technology (UWlST) Mood Adjective Checklist (UMACL), an extension of Thayer's (1967, 1986) ^D-ACt, with 16 items for each primary arousal scale. Instructions for the UMACL emphasize that subjects should rate their current mood and not how they usually feel. Matthews et al. (1990) confirmed the factor structure and internal consistency of the checklist (N = 388). Matthews (1987,1989) reported significant positive correlations between OMACL arousal factors and measures of autonomic arousal. Task apparatus, stimuli, and responses. The task was presented on a monochrome visual display unit (VDU) by a British Broadcasting Corporation (BBC) Model B microcomputer fitted with a Paul Fray Ltd. SLIDER millisecond timer, and a separate response box. Stimuli consisted of 540 single digits, of which 135 were the target digit, zero, presented at a rate ofl digit per second, giving a signal probability of Each digit was displayed as part of a block of pixels in the P E R F O R i"1 A N C E EASY/CONTROL LOW arousal High arousal, CONDITION! RESOURCES Figure 2. Predicted effects of arousal on performance in easy and difficult tasks, according to the Yerkes-Dodson Law. Q

5 154 G. MATTHEWS, R. DAVIES, AND J. LEES Table 1 Descriptive Statistics and Alpha Values for Individual Difference Variables (Experiments 1A and 2) Variable M SD c~ 16PF extraversion Impuisivity Sociability Energetic Arousal I Energetic Arousal Tense Arousal I Tense Arousal Note, Energetic arousal data are given for two UMACL (University of Wales Institute of Science and Technology Mood Adjective Checklist) administrations. 16PF = 16 Personality Factor Questionnaire. centre of the VDU. In the control condition, these digits were easily perceived. In the degraded condition, 30% of the pixels of the digit display were reversed in polarity, so that the white digit appeared to contain black dots, and the black background contained white dots, making the digits difficult to perceive. Generation and presentation of stimuli were controlled by assembly language routines. Stimuli were viewed from a distance of 2.2 meters, and subtended an angle of approximately 0.47* horizontally and 0.57* vertically. Subjects were required to respond to targets only, by pressing a single key on a response box. Numbers of missed targets and false positives, and mean RT for correct target dcteetions were recorded for each of three successive blocks of 180 trials, each containing 45 targets. Accuracy data were transformed to give a nonparametric measure of sensitivity, P(A), and log B as a measure of criterion placement. Design and procedure. Personality questionnaires were completed by subjects in their own time prior to the experimental session. Subjects were then tested once only, either in the morning (9.~0-10.~0 a.m.) or in the early evening (6"30-7"30 p.m.). Fifty subjects were tested at each time of day. Subjects performed both control and degraded versions of the externally paced sustained attention task and a search task (reported later as Experiment 2). Order of task administration and order of administration of the two task conditions within the sustained attention task were counterbalanced. Subjects were given 180 practice trials prior to each version of the sustained attention task. The UMACL was administered twice, prior to both sustained attention and search tasks, giving two energetic arousal measures, the first being Energetic Arousal 1, the second Energetic Arousal 2. Subjects were allocated to (a) extraverted or introverted and (b) high energetic arousal or low energetic arousal groups on the basis of median splits, using the joint distribution of the two energetic arousal distributions. All task parameters were manipulated within-subjects. Thus, sustained attention data were analyzed as a (Time of Day Extraversion Energetic Arousal Stimulus Degradation Task Period) factorial design. Energetic arousal was set to the value taken immediately prior to sustained attention performance. Results The principal analyses were the factorial analyses of variance Control Period 1 (ANOVAS) indicated by the designs just mentioned. Supplemen- Period 2 tary analyses are reported for the most important dependent Period 3 measures. These analyses tested for effects of EPI impulsivity Degraded and sociability on performance and for curvilinear relation- Period 1 ships between both energetic and tense arousal and perfor- Period 2 Period 3 mance. Sex differences in performance were also monitored. Preliminary analyses. Table I gives descriptive statistics for the individual difference variables of the experiment, together with a values where these were measured. 16PF data were converted to standard ten scores by using Saville and Blinkhorn's (1976b) norms for British undergraduates. The sample was somewhat extraverted, compared with the normative mean of 5.5: Means for median-split extraverted and introverted groups were 8.16 and 5.27, respectively. Energetic arousal declined significantly, t(99) = 5.22, p <.001, across the two administrations of the UMACL, but the two arousal scores were quite strongly intercorrelated (r =.62, p <.001), suggesting that individual differences in energetic arousal were reasonably stable during the experiment. The Kolmogorov-Smirnov test showed no significant deviations from normality for the distributions of both Arousal I (Z =.53, ns) and Arousal 2 (Z =.95, ns). Correlations between extraversion and energetic arousal were nonsignificant:. 19 for Arousal 1, and.02 for Arousal 2 (N= 100). For the morning sample these correlations were 0.20 and 0.01, respectively (ns, N = 50); for the evening sample the corresponding correlations were. 18 and.06 (ns, N = 50). The correlation between change in energetic arousal and extraversion was also nonsignificant (r = -0.19). Extraversion was also independent of tense arousal, at both times of day. Time of day had no significant effect on either energetic or tense arousal level. Descriptive statistics for sustained attention task performance are given in Table 2. These show that difficulty increased with degradation and with time on task, as shown by both sensitivity and RT performance measures. Regression model ANOVAS were used to test effects of time of day, extraversion, energetic arousal, stimulus degradation, and task period on three dependent measures: P(A), log B, and mean RT of correct detections. Group means for these measures are given in Table 3. Effects on perceptual sensitivity. Both stimulus degradation, F(I, 92) = 105.9, p <.001, and later time on task, F(2, 184) = 19.3, p <.001, significantly depressed P(A). A significant Degradation Task Period interaction, F(2, 184) = 7.3, p =.001, indicated that the decline in P(A) with time on task was greater in the degraded condition. A number of effects involving the between-subjects factors reached significance. The main effect of energetic arousal reached significance, F(I, 92) = 4.8, p <.05: High arousal subjects were of higher sensitivity. This main effect was modified by two interactions with task parameters: Table 2 Descriptive Statistics for Sustained Attention Task (Experiment 1A) P(A) log B RT Condition M SD M SD M SD Note. RT = mean reaction time for correct detections (ms).

6 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 155 Table 3 Sustained Attention Performance as a Function of Time of Day, Extraversion, Arousal, Stimulus Degradation, and Task Period (Experiment IA) A.M. EM. Condition P(A) log 15 RT P(A) log/~ RT Extraverts: High arousal (N= 14) (N= 13) Control Period I Period Period Degraded Period I Period Period Extraverts: Low arousal (N= 12) (N= 11) Control Period I Period Period Degraded Period I Period Period Introverts: High arousal (N= 10) (N= 13) Control Period I Period i Period Degraded Period Period i0 424 Period Introverts: Low arousal (N= 14) (N= 13) Control Period Period Period Degraded Period I Period Period Note. RT = mean reaction time for correct detections (ms). Energetic Arousal Degradation, F(1, 92) = 6.7, p <.05, and Energetic Arousal Degradation Period, F(2,184) = 3.5, p <.05. P(A) scores of high and low arousal groups only differed in the degraded condition, with the difference increasing as time progressed. Three interactive effects involving extraversion were found: Extraversion Period, F(2, 184) = 3.6, 17 <.05, Extraversion Energetic Arousal Period, F(2,184) = 4.4, p <.05, and Extraversion Energetic Arousal Degradation Period, F(2, 184) = 8.5, p <.001. These interactions can be approximately characterized as follows (see Figure 3). Introver- sion, low energetic arousal, stimulus degradation, and later task period all seem to predispose the person to poor task performance, with combinations of these factors leading to performance decrements. Thus, particularly low levels of perceptual sensitivity were observed in low energetic arousal introverts working at the degraded version of the task in the final task period. One potential difficulty with the data is the possibility of a ceiling effect in the nondegraded, control condition. The correlations between sensitivity values in successive task periods give a lower bound for the reliability of the P(A) measure. The values of these correlations in the control condition~.66 (period l-period 2) and.88 (period 2-period 3)---suggest some loss of reliability in the first task period but not a major effect. These data imply that most subjects were performing below ceiling, though some subjects may have been at ceiling in period 1. Effects on criterion (log ~). Significant effects on log B were restricted to task parameters and time o fday. There were significant main effects of stimulus degradation, F(l, 92) = 92.2, p <.00 l, task period, F(2,184) = 9. l, p <.00 l, and time of day, F(l, 92) , p <.05: A more stringent criterion was adopted in the degraded condition, toward the end of task, and in the morning. Effects on correct detection latencies. Only the main effects of stimulus degradation, F(l, 92) = 426.2, p <.001, and period, F(2, 184) = 13.6, p <.001, were significant: Correct detections were slower in the degraded version of the task and in the final task period. Curvilinear effects of energetic and tense arousal on sensitivity. Because the Yerkes-Dodson Law predicts inverted-u regressions of performance on arousal, depending on task difficulty, a separate trend analysis for such effects on P(A) was conducted. This was done by assigning the energetic arousal scores to four levels of approximately equal frequency (quartiles), again using the joint distribution of the two sets of arousal scores. The sensitivity data were then analyzed within a (Energetic Arousal Degradation Task Period) design, in which the four levels of energetic arousal were analyzed as three orthogonal polynomial contrasts: linear, quadratic, and cubic. The only significant main or interactive effects of arousal in this analysis were those of the linear component, suggesting that there was no marked curvilinear regression of performance on arousal in any of the task conditions. A similar analysis was performed for tense arousal: No significant linear or curvilinear effects were found. These results should be qualified by noting that the analysis assumes that the four levels of arousal constitute an interval scale. Although arousal scores were normally distributed, this assumption can be questioned on the grounds that the correlation between self-report arousal and true absolute arousal values may be reduced by individual differences in arousal baseline, as discussed earlier. Effects of impulsivity and sociability on sensitivity. Humphreys and Revelle (1984) claimed that effects of impulsivity on performance are stronger than those of sociability. To test this hypothesis, the analysis of effects of independent variables on P(A) was repeated with extraversion replaced by median-split factors of, first, impulsivity, and, second, sociability. Only one main or interactive effect involving impulsivity reached significance: the Impulsivity Arousal Degrada-

7 156 G. MATTHEWS, R. DAVIES, AND J. LEES tion x Period interaction, F(2, 184) = 4.21, p <.05. This interaction was similar to the corresponding extraversion interaction shown in Figure 3. The Sociability x Arousal x Degradation x Period interaction also reached significance, F(2,184) = 3.25, p <.05. So also did two further interactions: Time of Day x Sociability Period, F(2,184) , p <.01, and Time of Day Sociability x Degradation x Period, F(2, 184) = 4.69, p =.01. These interactions were associated with a particularly marked difference in decrement in high and low sociable groups in the control condition in the morning, with the low sociables showing the greater decrement. This result is of no obvious theoretical significance. In general, these data show that the strongest results here were obtained with the extraversion factor rather than with the two EPI subfactors. Sex differences. There was a slight tendency for women to be more extraverted than men. Mean extraversion scores for the two groups were 7.05 and 6.38, respectively, t(98) = -1.91, p =.059. Twenty-eight out of 50 of the extraverted group were female. Sex differences in energetic arousal did not approach significance. Sex differences in performance were tested by re-analyzing the data as a 2 X 2 X 3 (Sex x Stimulus Degradation X Task Period) design, for three dependent variables: P(A), log ~, and correct detection latency. No significant main effects of sex, or interactions between sex and task parameters reached significance, implying that the effects of extraversion and arousal on performance were not an artifact of sex differences. Experiment 1B Method Subjects. Subjects were 20 male and 20 female Aston University undergraduates, aged years, who were unpaid volunteers. They were preselected as the 20 most extraverted and 20 most introverted subjects from an initial pool of 60. The Eysenck Personality Inventory (H. Eysenck & S. Eysenck, 1964) was used for personality assessment in this study. Design and procedure. Subjects completed control and degraded versions of the sustained attention task, in counterbalanced order. The task differed from the Experiment 1 sustained attention task primarily in being self-paced. Subjects responded to every stimulus by pressing one of two different keys to register a target or nontarget. The next stimulus in the sequence was presented immediately after the subject's response. One thousand eighty digits were presented, of which 25% were targets, in four successive blocks of 360 trials. Subjects were run in the morning only (9 a.m.- 12 noon). Thus, the experiment used a 2 x 2 x 2 x 4 (Extraversion x Arousal Stimulus Degradation x Task Period) design, with both task parameters varied within subjects. Because of time constraints, extraversion was measured with the EPI, and energetic and tense arousal with a short version of the UMACL, with eight items per scale. The adjective checklist was given twice, prior to each task version, giving Arousal I and Arousal 2 measures. The first administration provided the primary measure, for consistency with Experiment 1. As with the externally paced task, P(A) and log fl were taken as dependent variables. However, in view of the self-paced nature of the task, a perceptual efficiency measure d'/(rt) 1/2 (Craig & Condon, 1985) was taken to check that effects on sensitivity were not simply a consequence of effects on RT. Mean RT was averaged across RTs for correct detections and for correct rejections ofnontargets. Results Preliminary analyses. Descriptive statistics for individual difference variables and for task performance are given in Tables 4 and 5. The mean extraversion level for the whole sample AM PM CONTROL 1,00 - n.. " -''' DE6RADED B~--~._~.~ntrovert/high arousal ~ xtravert/high arousal ', Extravert/low arousal 0, "-* Introvert/low arousal "El I I I I I I I Period Figure 3. Sensitivity, P(A), of sustained attention as a function of time of day, extraversion, energetic arousal, stimulus degradation, and task period (Experiment 1A).

8 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 157 Table 4 Descriptive Statistics and Alpha Values for Individual Difference Variables (Experiment 1B) Variable M SD a EPI Extraversion EPI Extraversion (extraverted group) EPI Extraversion (introverted group) Impulsivity Sociability Energetic Arousal I Energetic Arousal Tense Arousal Tense Arousal Note. EPI = Eysenck Personality Inventory. Energetic arousal data are given for two UM^CL (University of Wales Institute of Science and Technology Mood Adjective Checklis0 administrations. was 13.6, which was again above the normative mean for British students ofl 1.4 (Saville& Blinkhorn, 1976a). The drop in energetic arousal across the two administrations was nonsignificant, t(39) = 1.71, and the two arousal measures were positively correlated (r = 0.68, p <.001). The extraverted and introverted groups did not differ significantly in either Energetic Arousal 1, t(38) = 0.03, Energetic Arousal 2, t(38) = 1.04, or change in energetic arousal, t(38) = Extraversion and tense arousal were also independent. Effects on sensitivity and efficiency. Effects of independent variables on performance were examined by using regression model ANOVAS, as before. Cell means are shown in Table 6. In both degraded and nondegraded conditions, subjects spontaneously tended to adopt a fairly risky speed-accuracy tradeoff. P(A) values were thus lower than in the externally paced task versions. However, as in Experiment IA, sensitivity was significantly reduced by stimulus degradation, F(I, 36) = 15.82, p <.001, and by later task period, F(3,108) = 20.31, p <.001. The Degradation x Period interaction was also significant, F(3, 108) = 5.43, p <.01: The sensitivity decrement with time on task was greater in the degraded condition. The main effect of arousal on P(A) just reached significance, F(1, 36) = 4.12, p =.05: Subjects higher in arousal were higher in sensitivity. However, energetic arousal did not interact significantly with any other variable, and no main or interactive effects ofextraversion reached significance. Effects on efficiency were mostly similar: the main effects of degradation, F(1,36) = , p <.01, task period, F(3,108) = 41.89, p <.001, and arousal, F(I, 36) = 6.27, p <.05, all reached significance (see Figure 4). However, the Stimulus Degradation X Period interaction proved not to be significant (F < 1), suggesting that the apparent differential sensitivity decrements in control and degraded conditions may have been an artifact of changes in RT. Effects on criterion (logfl). Log # was significantly lower (less stringent) in the degraded condition, F(I, 36) = 27.20, p <.001, although as in the externally paced task, it became higher in the later task periods, F(3, 108) = 4.17, p <.01. Thus, stimulus degradation affected criterion placement in a direction oppo- site to that observed in the externally paced task. The only significant effect of between-subjects factors was the interaction between extraversion, arousal, and degradation, F(I, 36) = 5.14, p <.05. In the control condition only, arousal was negatively related to log # in extraverts but positively related to log # in introverts. Effects on response latencies. Responses were slower in the degraded condition, F(1, 36) = 23.78, p <.001, and the Degradation X Period, F(3, 108) = 4.97, p <.01, interaction reached significance. Responses slowed in the control condition, but tended to speed up in the degraded condition. One individual difference effect reached significance: the Arousal Degradation x Period interaction, F(3, 108) , p <.05. In the control condition, there was a slight tendency for high arousal subjects to slow down less than low arousal subjects. In the degraded condition, only the low arousal subjects showed a marked increase in speed of response, a finding with no strong implications for theory. Curvilinear effects of arousal on ej~iciency. Because of the tradeoffs between rate of work and sensitivity noted previously, the efficiency measure was taken as the principal dependent variable for supplementary analyses. No significant curvilinear effects of energetic arousal on perceptual efficiency were found within a 4 x 2 x 4 (Energetic Arousal x Degradation X Task Period) analysis of trend. No significant linear or curvilinear effects of tense arousal were found in a comparable analysis. Effects ofimpulsivity and sociability on e~ciency. No significant main or interactive effects of either impulsivity or sociability were found, though the main effect of impnlsivity was almost significant, F(I, 36) = 4.00, p =.053. High impulsives tended to be higher in efficiency. Sex differences. In this sample there were no significant sex differences in extraversion or arousal: Both the extraverted and introverted groups comprised 10 men and 10 women. Sex differences in efficiency, log/~, and response latency were analyzed within a 2 x 2 x 4 (Sex x Degradation x Task Period) design. A significant main effect of sex on efficiency was found, F(I, 38) = 12.85, p <.01, but there were no interactions between sex and task parameters. Mean efficiency was lower in women in Table 5 Descriptive Statistics for Sustained Attention Task (Experiment 1B) P(A) Condition M SD d'/!/rt log # RT M SD M SD M SD Control Period I Period Period Period 4 Degraded Period I Period Period Period Note. RT = mean reaction time (ms) averaged across correct detections and correct rejections.

9 158 G. MATTHEWS, R. DAVIES, AND J. LEES Table 6 Sustained Attention Performance as a Function of Extroversion, Arousal, Stimulus Degradation, and Task Period (Experiment IB) Condition P(A) d'nrt log fl RT Extraverts: High arousal (N = 11) Control Period Period Period 3" Period Degraded Period I Period Period Period Extraverts: Low arousal (N = 9) Control Period I Period Period Period Degraded Period l Period Period Period Introverts: High arousal (N = 9) Control Period I Period Period Period Degraded Period I Period Period Period Introverts: Low arousal (/7 = 11) Control Period I Period Period Period Degraded Period Period Period Period Note. RT = mean reaction time (ms) averaged across correct detections and correct rejections. all conditions. There was also a significant sex difference in log /5, with women adopting a more stringent criterion in all conditions, F(1, 38) = 7.09, p <.01, but there were no significant sex differences in response latency. Thus, contrary to the usual finding that there are no appreciable sex differences in sustained attention (Davies & Parasuraman, 1982), women appeared to be disadvantaged in the self-paced version of the task. How- ever, there was no confounding between sex and individual difference variables. Discussion The data from both the externally and the self-paced sustained attention tasks support the finding of Neuchtedein et al. (1983) that stimulus degradation induces a rapid decrement in perceptual sensitivity. In the externally paced task, only a small decrement was found in the nondegraded condition. In the self-paced task, marked sensitivity and efficiency decrements were found in both nondegraded and degraded conditions. However, the differential sensitivity decrement found with this task appeared to be an artifact of subjects in the degraded condition tending to work faster during the later task periods. These findings suggest that resource demands were increased both by stimulus degradation and by the faster event rate associated with self-pacing. In the self-paced task, both control and degraded conditions may have been resource-limited. However, the failure to find a differential efficiency decrement in the self-paced task suggests that once the task becomes resource-limited, the resource-performance function may be approximately linear within the range of performance levels sampled here. Stimulus degradation affected criterion placement in opposite directions in the externally and self-paced versions of the task, suggesting that pacing did indeed affect subjects' strategies. The major finding of this first study was that energetic arousal significantly enhanced perceptual sensitivity in both tasks. Moreover, energetic arousal only improved sensitivity on those versions of the task that showed a strong decrement over time, that is, all versions except the undegraded, externally paced task. On the externally paced task, the beneficial effect of high arousal on performance also increased with time on task. In both tasks, arousal affected sensitivity in a linear rather than a curvilinear fashion. The data also confirm Thayer's 0978) finding that energetic arousal is a stronger predictor of performance than tense arousal, which had no significant effects here. These results provide clear support for the Humphreys and Revelle (1984) resource model. Arousal was related to performance only in task conditions showing the sensitivity decrements associated with high resource demands (Parasuraman, 1985). Conversely, the arousal data were inconsistent with the Yerkes--Dodson Law: Arousal was relatively more facilitative in more difficult task conditions, and arousalperformance regressions were linear. As in previous studies (e.g, Mattbews et al~ 1989), the extraversion data were inconsistent with arousal-mediation theories of extraversion effects on performance. Extraversion was not significantly correlated with energetic arousal at either time of day. In both tasks, effects ofextraversion on performance were found with arousal controlled for. Thus, the importance of the extraversion data is that they confirm that at least some effects of extraversion on performance need not be mediated by individual differences in arousal. In contrast with the simple effects of arousal, extraversion effects were inconsistent across the two tasks. On the externally paced task, perceptual sensitivity was sensitive to extraversion effects, but on the self-paced task only response criterion was affected. The greater sensitivity decre-

10 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES , % o e- o o hi I0- e, High arousal/ control LOW arousal/ control High arousal/ degraded Low arousal/ degraded 0.05 I I I I I Task period (360 responses) Figure 4. Efficiency (d'/vrt) of sustained attention as a function of energetic arousal, stimulus degradation, and task period (Experiment IB). ment displayed by introverts on the externally paced task is in the opposite direction to the general finding that extraverts normally show greater performance decrements with time at work (Davies & Parasuraman, 1982). The discrepancy may reflect the information-processing demands of the tasks concerned. Davies and Parasuraman's review of the relevant literature indicated that tasks showing a greater hit rate decrement in extraverts, such as the Bakan task (Bakan, 1959), tend to be more dependent on working memory than the digit discrimination task used here. Extraverts' decrement on the Bakan task has been shown to be eliminated rather than potentiated by a secondary task (Bakan, 1959), suggesting that the single4ask decrement found by Bakan was not a function of lack of resources. Low arousal introverts showed particularly marked sensitivity decrements on the degraded version of the externally paced task. If arousal increases availability of SIT resources, then these resources may be depleted more rapidly in introverts with time of work. Consistent with this hypothesis, M. Eysenck 0982) reviews evidence suggesting that introverts may be poorer than extraverts at dual-task performance, and proposes that introverts typically have fewer processing resources available. The failure of extraversion to affect perceptual sensitivity on the self-paced task suggests that any relationship between extraversion and resources is relatively weak. A difficulty with the extraversion data is that both samples were extraverted relative to normative undergraduate means for the 16PF and EPI. This finding has three possible interpretations. First, we may have selectively sampled extraverted students within Aston University. Second, Aston students may be unusually extraverted compared with students attending other British universities. Third, the distribution of extraversion scores in British students may have changed since the EPI and 16PF were last normed in this population, in (Saville & Blinkhorn, 1976a, 1976b). The first two possibilities would raise interpretational problems. Table 7 compares descriptive statistics for the extraversion distributions of the two volunteer samples, with those of a sample of 147 Aston students attending compulsory psychology practical classes. Because the extraversion level of Aston students is unaffected by the proportion of psychology courses taken (Jackson & Matthews, 1988), the latter data will serve as normative for Aston students. Because (a) mean levels of extraversion were similarly elevated relative to published norms (Saville & Blinkhorn, 1976a, 1976b) in all three samples, and (b) extraversion scores were normally distributed in both the sample of Experiments IA and 2 and in the Aston normative sample, it is unlikely that there was any substantial sampling bias with respect to the available student population. (The high negative kurtosis and consequent deviation from normality of the Experiment IB sample reflects the selection of relatively extreme groups in this study). We cannot exclude the possibility that Aston students are more extraverted than those of other British universities but, given the relative uniformity of courses and selection procedures of British universities, it is hard to see why this should be the ease. If it is so, then the personality data should be seen as contrasting extraverts with a mixture of introverts and ambiverts. There are two reasons why this would be a relatively minor weakness of the studies. First, extraverts are normally more sensitive to effects of arousal than are introverts (M. Eysenck, 1982), so a comparison of extraverts and ambiverts will not be greatly dissimilar to a comparison of extraverts and in-

11 160 G. MATTHEWS, R. DAVIES, AND J. LEES Table 7 Distributions of Extraversion in Three Samples of Aston University Undergraduates Sample N M SD Skew Kurtosis K-S Z Deviation Experiments 1A & (ns) 0.61 Experiment 1B (p <.05) 0.49 Normative (ns) 0.66 Note. K-S Z = Kolmogorov-Smirnov test (Z) for deviation from normality of distribution. Deviation = standardized difference between sample mean and appropriate normative mean for British undergraduates (Saville & Biinkhorn, 1976a, 1976b). Extraversion was measured with the Sixteen Personality Factor Questionnaire in Experiments IA and 2, and with the Eysenck Personality Inventory in the other two samples. troverts. Second, neither of the variabilities of the extraversion distributions is significantly smaller than those of the published norms, so that there was no restriction of range of extraversion scores. To summarize, the relationship between arousal and sustained attention performance supports the Humphreys and Revelle 0984) theory, but not the Yerkes-Dodson Law. Extraversion effects were inconsistent across the two studies but were suggestive of higher resource availability in extraverts on the externally paced task and of strategy shifts on the self-paced task. Experiment 2: Visual and Memory Search The aim of Experiment 2 was to test whether the personality and energetic arousal effects found in Experiment I would generalize to a rather different attentional task on which both SIT and STM resource demands could be manipulated. The resource-limitations of visual/memory search have been demonstrated within Shiffrin and Schneider's (1977) theory of automatic and controlled processing in attention. In their experiments, subjects were presented with from one to four target items to memorize on each trial. The number of items, which were alphanumeric characters, is referred to as the memory set size (M). They then searched visual displays for the targets, with displays containing from one to four characters: the number of characters is the frame size (F). In consistently mapped (CM) conditions, in which targets and nontargets were drawn from different sets of items, after extensive practice M and F had only small effects on speed and accuracy of responding. In variably mapped (VM) conditions, in which the targets and distractors for each trial were drawn from the same pool of items, performance dropped sharply as M and F increased. Jonides and Gleitman (1972) showed that a zero slope function relating display size to performance can be obtained with only 18 practice and 108 test trials in which targets and distractors are drawn from different alphanumeric categories. The category distinction is presumably so over-learned that deciding between categories is easily automatized. Shiffrin and Schneider suggested that the controlled processing necessary for search in VM conditions was considerably more capacitylimited than the automatic processing necessary for search in CM conditions. A subsequent series of experiments (Schneider & Fisk, 1982) confirmed this hypothesis by using dual-task ex- periments in which the mapping (CM or VM) of the two search tasks was varied. Three task parameters were manipulated in the search task: M (1 or 4), F (1 or 4), and mapping (CM or VM). In the CM condition, target and distractor sets were of differing alphanumeric category. To test the Humphreys and Revelle (1984) model, it may be assumed that in VM search, increasing F increases demands primarily on SIT resources, whereas increasing M increases demands primarily on STM resources. In this case, the model predicts that in VM search, high arousal should facilitate performance with M = I and F = 4, high arousal should impair performance with M = 4 and F = 1, and there should be an inverted-o relationship between arousal and performance with M = 4 and F = 4. No effects of arousal are predicted in CM search, because automatic processing does not require resources. In contrast, the Yerkes-Dodson Law predicts that arousal should be relatively more facilitative in easier conditions (CM search, and VM search with M = 1, F = 1), whereas arousal should be relatively more detrimental in more difficult conditions (particularly VM search with M = 4, F = 4). Method The subjects, and the general procedure for the experiment are as described under Experiment IA. In the following analyses, the energetic arousal measure used was that taken immediately prior to search task performance. Task stimuli and responses. The search task comprised eight separate blocks of 50 trials, with each block corresponding to one of the eight task conditions. Each trial was initiated with the presentation of the word "TARGET(S);' beneath which were the items of the memory set, for 1500 ms for M = 1 conditions, and 2500 ms for M = 4 conditions. Then a fixation dot was presented, for 400 ms, followed by a search display of four positions arranged in a rectangle with the fixation dot at its center, which on 50% of the trials included a target item, for 180 ms. The final stimuli presented were four random dot masks presented in the same positions, for 40 ms. Stimuli were viewed from a distance ofl meter. The MODE 4 characters subtended visual angles of 0.22 horizontally and 0.24 vertically. In the search display, the distance from the fixation dot to the center of each character was Within each block of trials, 25 of the search displays contained a single target item, and 25 contained no target items. Other characters presented in search displays constituted distractors, In M = 1 conditions, one character was presented as a target; in M = 4 conditions, four characters were presented as targets. In F = 1 conditions the search display contained one character and three random dot patterns of the same size as a character. In F = 4 conditions, the search display con-

12 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 161 tained four characters. In the four VM conditions, both targets and distractors for each trial were randomly selected from the set of consonants B, C, D, G, M, T, V, and X. For 50 subjects, CM targets were drawn from the consonants F, H, J, K, L, N, P, and S, and CM distractors from the digits 2-9 inclusive. For the other 50 subjects, CM targets were drawn from the set of digits, and CM distractors from the set of consonants. Subjects responded by using a response box with two keys, pressing the right-hand key ifa target was present, the left-hand key if no target was present. The search task was self-paced. Following respouse, the next trial was initiated immediately, subject to a minimum interval of 1000 ms between offset of the final random dot patterns and onset of the display of target items, to reduce variability in rate of work. Missed targets, false positives, from which P(A) and log/3 values were calculated, and mean RTs for correct positive and negative trials were recorded. Because of the relatively small number of trials per condition, split-half reliability coefficients were calculated, as follows. For each subject and each block of 50 trials, each of the 50 trials was assigned to one of two groups of 25 trials, using an ABIIAAIIliA... sequence, so that separate estimates of the response measures could be obtained for each of the two groups of trials, A and B. Performance on the A trials was then correlated with performance on the B trials, and corrected for length of sequence to give the reliability coefficient. Procedure and design. Subjects performed either the four CM blocks followed by the four VM blocks, or the VM blocks followed by the CM blocks. Prior to the search task, subjects were given 50 practice trials in a CM condition, with M = 2 and F = 2. One aim of this practice was to reduce the role of controlled processing in the letter-digit discriminations necessary for later CM conditions. By the time subjects had completed the first CM block, they would have performed a similar number of trials (100) to the subjects in the whole of the Jonides and Gleitman (1972) experiment. Search data were analyzed as a 2 x 2 X 2 2 X 2 x 2 (Time of Day x Extraversionx Energetic Arousal x Mapping Memory Set Size x Frame Size) factorial design. Results Table 8 gives descriptive statistics and corrected split-half reliability coefficients for RTs of correct responses for positive and negative trials. Reliabilities for P(A) values were also calculated, and were considerably lower: for CM conditions, and for VM conditions. Correlations between P(A) values and mean RT for hits in each condition were small in magnitude (range of rs: through +.20), even allowing for some attenuation due to unreliability, Table 8 shows that the pattern of correct mean RTs to targets and nontargets for the eight task conditions generally conforms to that expected from previous research on search tasks. Effects of task parameters on RT were considerably stronger in the VM than the CM conditions. The strong effect of M in the CM conditions clearly failed to meet the zero slope criterion. As in Schneider and Shiffrin's (1977) studies, performance in the VM M = 4, F = 1 condition was slower than that in the VM M = 1, F = 4 condition. Regression model ANOVAS were used to test effects of time of day, extraversion, energetic arousal, mapping, M, and F on three dependent measures: P(A), log B, and mean RT of correct detections. Cell means for these ANOVAS are shown in Table 9. Three subjects failed to detect sufficient targets to give an accurate estimate of the mean RT for hits in at least one condition, so the analysis of these data was based on a sample size of 97. Effects on sensitivity Significant effects were primarily those of task factors. The effects of mapping, M, F, Mapping x Table 8 Descriptive Statistics and Corrected Split-Half Reliabilities for Correct Reaction Times in Positive and Negative Trials of Visual~Memory Search Task (Experiment 2) Positive trial Negative trial Condition M SD Rel. M SD Rel. Consis~ntmappingcondRion M= 1, F = M= I,F = M= 4, F= M= 4, F = Varied mappingcondkion M= 1, F = M= I,F = M= 4, F = , M= 4, F = 4 1, , Note. Rel. = split-half reliability, corrected by the Spearman-Brown formula. M = memory load; F = frame size. F, Mapping x M, M x F, and Mapping M X F all reached significance at p <.001. The Energetic Arousal x F interaction was the only effect involving between-subjects factors to reach significance, F(l, 92) = 4.2, p <.05: High arousal increased sensitivity in the F = 4 conditions, without much affecting it in the F = l conditions. The Extroversion x Energetic Arousal interaction was almost significant, F(l, 92) = 3.73, p =.057: only introverts benefited from high arousal. Effects on criterion. Two effects of task factors reached significance: the main effect off, F(I, 92) = 26.27, p <.001, and the Mapping F interaction, F(1, 92) = 8.77, p <.01. Subjects adopted a more stringent criterion in the CM F = 4 conditions. There were several significant effects involving the betweensubjects factors, though most were of little theoretical significance. The Time of Day x M F interaction, F(I, 92) = 4.57, p <.05, was associated with an increase in log ~/in the evening in the M = l, F = 4 conditions. The Arousal Time of Day x M interaction, F(l, 92) = 4.23, p <.05, resulted from a weak tendency for/3 to increase in low arousal subjects in the evening in M = l conditions, with the opposite effect in M = 4 conditions. The Extraversionx F interaction, F(l, 92) = 7.56, p <.01, showed that extraverts adopted a more stringent criterion than introverts in F = 4 conditions. Of more theoretical interest was the significant Extraversionx Arousal X Time of Day interaction, F(l, 92) = 4.08, p <.05, associated with a tendency for high arousal to lower/3 in extraverts and raise/3 in introverts in the morning, and for this interaction to reverse in the evening. A comparable effect was found with the undegraded stimuli in Experiment lb. Figure 5 shows log fl averaged across the eight task conditions as a function of time of day, extraversion, and arousal. Effects on RT of correct detectians. All the main and interactive effects involving the task factors mapping, M, and F were only significant at p <.00 I. The only effect involving the between-subjects factors to reach significance was the Mapping x M F X Energetic Arousal interaction, F(I, 89) = 13.53, p <

13 162 G. MATTHEWS, R. DAVIES, AND J. LEES Table 9 Search Task Performance as a Function of Time of Day, Extraversion, Arousal Mapping, M, and F (Experiment 2) A.M. EM. log log Group P(A) 3 RT P(A) 3 Extraverts: High arousal (PC= 14) (PC= 13) Consistent mapping M= I,F = M= 1, F= M= 4, F= M= 4, F = Varied mapping M= I,F = M= I,F= M= 4, F = M= 4, F = , Extraverts: Low arousal (N = 12 *) (N = 11") Consistent mapping M= 1, F = M= 1, F = M= 4, F = M= 4, F = Varied mapping M= l, F = M= 1, F = M= 4, F = l , M= 4, F= , Introve~s: High arousal (N= 11) Consistent mapping M= 1, F= M= l, F = M= 4, F= M= 4, F = Varied mapping M= 1, F= l M= 1, F= M= 4, F = M= 4, F= ,138 Introverts: Low arousal (N= 13) Consistent mapping M= 1, F = M= 1, F = M= 4, F= M= 4, F = Varied mapping M= 1, F= M= I,F = M= 4, F= il 883 M= 4, F= ,194 (N = 9 ) RT Note. RT = mean reaction time for correct detections (ms); M = memory load; F = frame size. "RT data are missing for 1 subject in this group..001, which is shown graphically, averaged across time of day, in Figure 6. Because of their theoretical importance, analyses of the simple effects of arousal on RT within each of the eight task conditions were performed. Only in the VM M = 1, F = 4 condition did the effect of arousal reach significance, F(I, 95) = 13.89, p <.001, search was faster in subjects high in arousal. Curvilinear effects of energetic arousal on correct detection RT. To test predictions from the Yerkes--Dodson Law, an analysis of trend was performed, as for the sustained attention task. Mean RT for correct detections was taken as the dependent variable because of its greater reliability and relevance to theory than P(A) for this task. A number of interactive effects involving arousal polynomial terms and task parameters reached significance, indicating that the trend of the arousal- RT regression depended on task condition. Several linear ef fects reached significance, as did the cubic Arousal M F, 720 F(l,93)=4.01,p<.O5, andarousal MappingxMXF, F(l, ) = 5.50, p <.05, interactions. No effects involving the quadratic term reached significance. The strongest cubic regres sion was found in the VM F = 1, M = 4 condition, as shown in 844 Figure 7. With respect to speed of response, this regression 1,157 cannot be described as an inverted IJ because the slowest group was intermediate in arousal level. Effects of impulsivity and sociability on correct detection RT. The correct detection RT data were analyzed with extraversion 583 replaced by each of the two EPI subfactors in turn. No signifi- 631 cant main or interactive effects of impulsivity were found, and 712 only one significant effect of sociability: the M F Time of 817 Day Sociability interaction,/7(1, 89) = 4.42, p <.05. There 583 was a tendency for high sociables to be faster in the morning, 681 low sociables to be faster in the evening, particularly in the M = 893 1,094 1, F = 1 conditions. As before, analysis ofimpulsivity and sociability data added little of theoretical relevance to the extraversion data. Sex differences. No overall or parameter-dependent sex differences were found for any of the dependent variables. Discussion The search task data differed from those of Schneider and Shiffrin (1977) in that task parameters (particularly M) affected CM search, though such parameter effects were weaker than ,159 those in VM search. Possibly subjects were still tending to perform controlled search of the memory set, even though the task could be performed without such a strategy. Performance of the (N = 17) CM tasks here may correspond to Schneider's (1985) Phase processing, in which weak automatic processes control re sponding mainly where many controlled-processing compari sons between memory and display items must be made, as in the M = 4, F = 4 conditions. RT differences between VM and CM search were most marked in the conditions here. Overall, therefore, task parameters effects were consistent with a re source interpretation, though the CM manipulation is best in ,134 terpreted as one in which the influence of controlled process- ing, and hence of resources, is reduced but not eliminated. The principal finding was the strong but specific facilitative effect of arousal on pure VM visual search, which is exactly what the Humphreys and Revelle (1984) model predicts. As in Experiment 1, arousal appears to be positively associated with SIT resources. However, the predicted effects of arousal in the VM M = 4 conditions were not supported. Increasing memory load eliminated the linear regression of performance on arousal rather than inducing negative or inverted-li regressions.

14 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES L AM PM o o= J o q.a I0 g' e- l L 0.25 L ~ 0.15 S 0. I0-0.05s Extraverts Introverts I I I I I LOW High LOW High Energetic arousal level Figure 5. Mean log/~, averaged across the eight search task conditions, as a function of time of day, extraversion, and energetic arousal. Again, the arousal data did not appear to be compatible with the Yerkes--Dodson Law. It must be assumed that performance in the VM M = l, F = 4 condition varies along the ascending limb of the hypothetical inverted U, to explain the facilitative effect of arousal. If so, arousal should also facilitate performance in the other conditions of similar or lesser difficulty, mainly the other M = 1 conditions. In fact, arousal was not related to performance at all in these conditions. Likewise, in more difficult conditions, notably the VM M = 4, F = 4 condition, arousal should be related to performance negatively, or by an inverted U. As Figure 7 shows, there was no trace of such an effect. High arousal... Lowarousal t- O CM v./. M=4 W I000- p/p t 0 (J L. o t ll M=4 e- o w r M-I" o... ~ 0 M " I 0" o------"---'- 500 I I I I I f ffi I F=4 F- I F=4 Figure 6. Mean reaction time (RT) for correct detections in search task as a function of consistent or varied mapping (CM or VM), memory set size (M), frame size (F), and energetic arousal (Experiment 2).

15 164 G. MATTHEWS, R. DAVIES, AND J. LEES o = ;00- i ooo "------e M=4, F=4 M=4, F=I task should be simply a function of task difficulty. The Humphreys and Revelle (1984) theory predicts that arousal should be most facilitatory with high T and low ML, but should tend to impair performance with low T and high ML. More generally, the main point of interest was whether arousal would interact with T, a factor related to nonperceptual processing, as it did with stimulus discriminability and display size in Experiment soo i i i i Energetic arousal level M-I,F'4 M=I,F=I Figure 7. Mean reaction time (RT) for correct detections in variably mapped (VM) conditions of search task as a function of memory set size (M), frame size (F), and four levels of energetic arousal (Experiment 2). Extraversion effects on speed and accuracy of performance were weak and did not suggest any strong relationship between extraversion and attentional resources. Furthermore, as with the sustained attention data, there was no simple association between extraversion and response criterion. The only extraversion finding of general interest here was the significant interactive effect of time of day, extraversion, and arousal on log B, shown in Figure 5. The morning data, showing that arousal was positively related to responsiveness in extraverts only, correspond directly to the interactive effect of extraversion and arousal on log B in the control condition of the self-paced sustained attention task. Across the three experiments, extraversion and arousal interactively affected ~/in the two self-paced tasks, but not the externally paced task. This result contrasts with previous studies (e.g. Matthews et al, 1989), showing extraversion and arousal interactively affecting performance efficiency rather than criterion. The extraversion data support Revelle's (1987) contention that it is important to distinguish personality effects on strategy from those on efficiency, although it is difficult to see how either arousal-mediation theory could account for the effect. Method Subjects. Subjects were 16 male and 16 female Aston University undergraduates, aged years. All were unpaid volunteers. Task apparatus, stimuli, and responses. Presentation of the letter transformation task was controlled by a BBC Model B microcomputer linked toa VDU, a microphone, and a timer accurate to 1 ms. Stimuli were presented in the BBC Mode 7 character set. The sensitivity of the microphone could be calibrated to each subject's voice. On each trial there was a preparatory interval of 5s, after which the instruction gdd 1' or ~,DD 4' (depending on transformation difficulty T) was displayed for 3s. Then a string of either 1 or 4 letters (depending on memory load ML) was displayed beneath the gdd' instruction. The latency of the subject's spoken response was timed from this point. The subject's response was recorded by the experimenter, who also monitored any premature accidental activation of the timer. Levels of the two orthogonal transformation difficulty and memory load factors were I and 4 for each, giving four types of transformation problem in total. The principal dependent measure was response latency. Procedure. Subjects participated in a single experimental session, which took place between 9 a.m. and 12 noon. First, pupillometric recordings were taken while subjects practiced the letter transformation task--these data are beyond the scope of this paper. Subjects then completed the full version of the UMACL. Next, subjects performed four blocks of trials of letter transformation, each consisting of sixteen transformations of one of the four problem types. Order of blocks was counterbalanced across the experiment. Finally, subjects again completed the UMACL Results and Discussion Preliminary analyses. Descriptive statistics and alpha coefficients (where calculated) for individual difference variables are shown in Table 10. There was no significant change in arousal Experiment 3: Letter Transformation The final experiment was designed to test the generality of the effect of arousal on resources. The first two experiments showed that arousal facilitated digit discrimination under demanding conditions, and VM search of four-character displays. These data suggest that it is specifically resources required for item identification or encoding whose availability is affected by arousal. In Experiment 3 we therefore investigated a task whose demands were primarily on cognitive transformation and short-term storage of information rather than on perception. Hamilton et al.'s (1977) letter transformation task was used for this purpose, requiring subjects to answer problems of the type "DLU + 4 =? (HPY)" Transformation difficulty (T) and STM load (ML) can be independently manipulated. The Yerkes-Dodson Law predicts that arousal effects on this Table 10 Descriptive Statistics and Alpha Values (Experiment 3) Variable M SD a Energetic Arousal Energetic Arousal Latency (ML = 1, T = 1) Latency (ML = 1, T = 4) Latency (ML = 4, T = 1) Latency (ML = 4, T = 4) Note. Energetic arousal data given for two UMACL (University of Wales Institute of Science and Technology Mood Adjective ChecklisO administrations. ML = memory load; T = transformation difficulty; Latency = latency of correct responses in letter transformation task(s).

16 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 165 Table 11 Log Response Latency as a Function of Energetic Arousal, Transformation Difficulty (T), and Memory Load (ML) Arousal level High Medium Low N ML = 1, T = ML = 1, T = ML = 4, T = ML = 4, T = across the two UMACL administrations, t(31) =.03, and individual differences in arousal were stable (r =.76). Error rates on the letter transformation task were very low. Mean error probability averaged across subjects and all 64 task items was 1.4%. Effects on speed of letter transformation. As the experiment was concerned with arousal only, the primary analysis here tested for both linear and curvilinear effects of arousal on speed of letter transformation. Subjects were assigned to three levels of arousal. Level 1 (lowest arousal) corresponded to level 1 in the analysis of trend in Experiment 1, level 2 to level 2 in Experiment 1, and level 3 to levels 3 and 4, so that there was some comparability of results from the two studies. Data were analyzed as a (Energetic Arousal Memory Load Transformation Difficulty) factorial design; cell means are given in Table I I. Latencies were log transformed because of high positive skew. The main effects of ML and T, and the ML T interaction were all significant at p <.001. The main effect of the linear arousal term also reached significance, F(l, 29) = 6.30, p <.05, and this main effect was not significantly modified by any interactions with task parameters. All main and interactive effects of the quadratic arousal term failed to reach significance. Table 11 shows that increasing arousal had a monotonic, facilitative effect on performance. The Spearman correlation between arousal and total error frequency in all four conditions was -.0 l, which was nonsignificant. There were no significant sex differences in either arousal or performance. These data again showed linear facilitatory effects of energetic arousal on performance, consistent with the hypothesis that arousal increases availability of SIT resources (Humphreys & Revelle, 1984). As before, we found neither the Arousal Difficulty interactions predicted by the Yerkes-Dodson Law, nor the Arousal Memory Load interactions predicted by the multiple resource model. In contrast with effects of noise (Hamilton et al, 1977), alcohol (Hockey, MacLean, & Hamilton, 1981), and anxiety (M. Eysenck, 1985), but like effects of incentive (M. Eysenck, 1982, p. 93), effects of self-report arousal on letter transformation speed do not appear to depend on task characteristics. This finding is consistent with the view that energetic arousal affects a general resource rather than one of the three specific processing stages posited by Hockey et al. (1981). If the arousal effect is mediated by individual differences in resource availability, the absence of Arousal Parameter interactions suggests that the arousal-resource function may have been linear here. The general conclusion to be drawn is that facilitative effects of arousal are not restricted to tasks placing heavy demands on stimulus encoding processes. General Discussion Individual Differences in Arousal Resources and Performance The relationship between arousal and performance is an essential link in the causal chain relating extraversion to performance posited by arousal-mediation theories. The data described previously showed a highly consistent facilitative effect of higher arousal on a range of tasks with differing informationprocessing requirements. Task versions making high demands on attention but not STM always showed the effect. High memory load, letter transformation tasks showed a similar effect, but no simple effect of arousal was found on memory search. The consistency of arousal effects here suggests that a shift in emphasis from specific stressors to the internal states evoked by stressors might well give a simpler and more consistent picture of the effects of arousal on performance. The three experiments provide no direct support for the Yerkes-Dodson Law. In sustained attention, arousal was beneficial to performance of difficult task versions and had little effect on the easiest version, a result almost opposite to prediction. The search task version on which speed was facilitated by arousal was of intermediate difficulty. In other search tasks, RT simply seemed to be insensitive to arousal. Task difficulty did not interact with arousal in the letter transformation task. Furthermore, no evidence for inverted-u shaped regressions of performance on arousal was found across the four tasks. The Humphreys and Revelle (1984) theory received mixed support. The facilitative effects of high arousal were almost exactly as predicted. The generalization of the effects across qualitatively different tasks and the dependence of the arousal effect on task parameters in Experiments IA and 2 provide particularly strong evidence for a resource interpretation of the data. However, the predicted deleterious effects of arousal on task versions making demands on STM but not SIT resources failed to materialize. It might be argued that memory loads of four items made insufficient demands on STM resources to induce arousal effects: Anderson and Revelle (1983) found that caffeine reduced accuracy of visual search for a six-letter but not a two-letter target. Against this argument, the memory manipulations here tended to produce greater performance decrements than the purely attentional manipulations. A previous study (Matthews et al., 1989) failed to find significant linear or curvilinear effects of energetic arousal on two tasks with higher memory loads: six-letter search and ordered recall of a ninedigit string. Other evidence suggests that resources play at most a relatively minor role in individual differences in short-term memory. For example, Baddeley (1986) identified a number of subsystems within short-term memory, whose dependence on resources is likely to be complex. In general, then, the data suggest that the primary effect of increased energetic arousal is to increase the availability of resources for attention or the throughput of information. The conclusion that the Yerkes-Dodson Law is of limited validity depends critically on the validity of the self-report

17 166 G. MATTHEWS, R. DAVIES, AND J. LEES arousal measure. Matthews et al. (1989) argued that several potential threats to validity are unlikely to affect seriously studies of arousal and performance. Self-report measures have been validated against physiological measures of arousal in performance-testing environments. Individual differences in arousal are strong enough to be of comparable magnitude to stressor effects, and, as the present data show, individual differences in arousal are reasonably stable over the time periods involved. Other tasks might show greater conformity to the inverted-u relationship, although the present research program has consistently failed to find inverted Us across a wide range of tasks (Matthews, 1985, 1986, 1989; Matthews et al, 1989). Some further problems of validity do require additional research. Anderson (1987) presented evidence suggesting that group data, like those here, may obscure the curvilinear arousal-performance relationships of individuals. It is difficult, however, to see how our data could have been predicted from aggregating individual arousal-performance curves. In addition, as discussed in the introduction, a plausible theoretical argument can be advanced that self-report arousal measures may be unsuitable for between-subjects comparisons. However, such empirical evidence as there is fails to provide compelling support for this baseline-relative interpretation of self-report arousal scores. First, the positive association between self-report arousal and composite measures of autonomic arousal (Matthews, 1987; Thayer 1967, 1970) suggest that self-report measures are at least as valid as psychophysiological measures for between-subject comparisons of arousal level. Moreover, studies of sustained attention using psychophysiological measures have obtained results comparable to those obtained here with self-report measures: subjects higher in autonomic arousal show higher perceptual sensitivity, particularly toward the end of the task (e.g. Munro, Dawson, Sehell, & Sakai, 1987). Second, the baseline-relative interpretation of arousal scores predicts that over some unspecified time the baseline is likely to shift as the person adapts to the environment concerned. With repeated testing, scores should then regress to the mean as the arousal state elicited by the environment becomes more typical of the person's experience. Two examples illustrate our failure to find such effects in the data sets we have examined, over two different time scales. S. Green (see Matthews et al, 1990) deprived subjects of sleep for two consecutive nights. No adaptation was observed; if anything, energetic arousal tended to become progressively lower with increasing sleep deprivation. Southall (1988) compared arousal levels in nonmedicated depressive patients and matched controls. Although the average duration of depression was 7.6 years, mean level of energetic arousal was 1.53 standard deviations lower in depressed subjects, in spite of the long time interval in which they could have adapted to their chronic mood disorder. In summary, the present data do not conclusively invalidate the Yerkes-Dodson Law because of uncertainties over the role of individual differences in arousal baseline. However, the results are sufficiently strong to indicate that it may be dangerous to assume the general validity of the Yerkes-Dodson Law across tasks and experimental settings. The theoretical shortcomings of the Yerkes-Dodson Law are well known (e.g, M. Eysenck, 1982); more attention should be paid to the circumstances under which it is empirically valid. The Problem of Extraversion As in previous studies (e.g, Matthews, 1985), effects of extraversion on performance did not appear to be contingent on the observed correlation between extraversion and arousal. Neither of the associations between extraversion and arousal predicted by the two arousal-mediation theories was found. It appears that although there are consistent associations between individual differences in arousal and performance, at least some effects of extraversion on performance are not directly mediated by arousal. The present research provides further evidence that the role of individual differences in arousal has been overstated in theoretical accounts of extraversion. It appears instead that extraversion has a higher order effect, modifying the relationship between arousal and attentional processes. In contrast with the arousal findings, effects of extraversion across the first two experiments appeared to vary with task pacing. In the externally paced task, the pattern of interactions suggested that extraverts had more available resources, but in the self-paced tasks extraversion and arousal tended to affect criterion setting interactively. As Revelle (1987) proposed, it appears that extraversion may have several qualitatively different effects on performance. It may be advisable for future research to adopt Hockey and Hamilton's (1983) strategy for stress research and to map out how extraversion affects a variety of theoretically meaningful cognitive measures, prior to detailed theorizing. As far as attentional tasks are concerned, two major types of extraversion effect can be discriminated empirically: simple facilitatory effects of extraversion on demanding tasks, and interactive effects of time of day, extraversion, and arousal. M. Eysenck (1982) reviewed considerable evidence suggesting that extraverts perform better than introverts on a range of relatively demanding tasks (M. Eysenck, 1982). The personality data of Experiment IA, showing smaller perceptual sensitivity decrements in extraverts, thus belong to this category and suggest a positive correlation between extraversion and resource availability. Experiment IA also shows that the effect appears to be insensitive to time of day. The inconsistency of the effect here might reflect a rather weak correlation between extraversion and resources. Alternatively, the lack of such effects on selfpaced tasks might suggest that personality effects on resource availability are more dependent on strategy use than are those of arousal. In contrast, the characteristic triple interaction between time of day, extraversion, and arousal is found most consistently with routine, low-error-rate attentional tasks (Matthews et al, 1989). Our data show that tasks selected to be highly resource-limited do not show this typical interactive effect on speed or efficiency of performance, implying that the effect is probably not mediated by individual differences in resource availability. However, there seems to be a further distinction between effects on sensitivity and effects on criterion and speed--accuracy tradeoff, such as the interactive effects ofextraversion and arousal on log B found with the self-paced search and sustained attention

18 AROUSAL, EXTRAVERSION, AND INDIVIDUAL DIFFERENCES 167 Table 12 Tasks Showing the Characteristic Time of Day Extraversion Arousal Interaction A.M. P.M. Mean error Task rate (%) Eft. Crit. Eft. Crit. Intelligence test items (Matthews, 1985) 36.4 * * Early selective attention * NT NT Late selective attention (Matthews & Chappelow, 1986) * NT NT Serial reaction (Mattbews, Jones, & Chamberlain, 1989) 1.7 * * Lexical decision (Mattbews, Harley, McCarthy, & Jones, 1988) * * Sustained attention, low signal probability (Matthews, 1989) 0.3 * * Visual/memory search * Sustained attention, high signal probability (present data) * NT NT Note. Eft. = effect on sensitivity or speed, not associated with speed--accuracy tradeoff; Crit. = effect on response criterion or on speed--accuracy tradeotf; NT = not tested. All tasks were self-paced, except Matthews and Chappelow (1986), and Matthews, Harley, McCarthy, & Jones. (1988) tasks. * Denotes significant effect in predicted direction. tasks (see Figure 5). Table 12 summarizes those tasks in this research program in which the characteristic effect of extraversion, arousal, and performance has reached significance, either on efficiency or on speed-accuracy tradeoff/eriterion. Tasks insensitive to such effects, or which gave atypical effects, or where efficiency and criterion cannot easily be distinguished, are not included. Table 11 suggests that features of the task and time of day may affect whether an effect on efficiency or on strategy is found. Efficiency effects are found primarily with low-error-rate tasks, plus intelligence test items, which argues against a resource interpretation. Criterion effects, which do not occur in conjunction with efficiency effects, are most prevalent in the morning, irrespective of error rate. One task (Matthews, 1989) shows a criterion effect in the morning but an efficiency effect in the evening. Self-paced tasks show both efficiency and criterion effects, whereas externally paced tasks show efficiency effects only. Self-pacing and increased task difficulty may both be associated with greater demands on strategy selection. Furthermore, there is some evidence of a tendency toward greater strategy use in the morning (Marks & Folkard, 1984). Thus, we tentatively suggest that as the importance of strategy selection increases, the greater the probability of an effect on criterion rather than on efficiency. The data reinforce the importance of analyzing extraversion data for both efficiency and criterion effects, but no evidence was found for the simple negative association between extraversion and B reported in some previous studies (M. Eysenck, 1982). Although, as discussed previously, the Eysenck (1967) theory predicts personality effects on strategy shifts resulting from the need to maintain an optimal level of arousal, it is unclear how such a hypothesis might explain in detail the present, interactive effects of extraversion and arousal on/3. For example, low arousal extraverts in the morning should seek stimulation, according to H. Eysenck (1967), but in fact this group tends to be high in ~ and consequently prone to be reluctant to respond. In summary, extraversion appears to have at least two qualitatively different effects on cognitive functioning, neither of which is directly mediated by individual differences in arousal. First, on some resource-demanding tasks, extraversion is positively correlated with availability of attentional resources. These resources are more reliably increased in subjects high in energetic arousal. Second, interactive effects of extraversion, arousal, and time of day are associated not with resource availability but with the routine encoding of simple stimulus attributes (Mattbews et al, 1989). Whether this effect is expressed as an effect on efficiency of performance or on response criterion depends on the strategic demands of the task. References Anderson, K. J. (1987). Impulsivity and caffeine: A within and between subjects test of the Yerkes-Dodson Law. Paper presented to the Third Meeting of the International Society for the Study of Individual Differences, Toronto, Ontario, Canada. Anderson, K. J, & Revelle, W. (1983). The interactive demands of caffeine, impuisivity and task demands on a visual search task. Personality and Individual Differences, 4, Baddeley, A. (1986). Working memory. Oxford: Oxford University Press. Bakan, R (1959). Extraversion-introversion and improvement in an auditory vigilance task. British Journal of Psychology, 50, Broadbent, D. E. (1971). Decision and stress. London: Academic Press. Carr, G. D. (1969). lntroversion-extraversion and vigilance performance. Unpublished doctoral dissertation, Tufts University, Medford, MA. CatteU, R. B, Eber, H. W, & Tatsuoka, M. M. (1970). Handbook forthe Sixteen Personality Factor Questionnaire. Champaign, IL: IPAT. Craig, A, & Condon, R. (1985). Speed-accuracy trade-offand time of day. Acta Psychologica, 58, Craig, A, Davies, D. R, & Mattbews, G. (1987). Diurnal variation, task characteristics and vigilance performance. Human Factors, 29, Davies, D. R, & Hockey, G. R. J. (1966). The effects of noise and doubling the signal frequency on individual differences in visual vigilance performance. British Journal of Psychology, 57, Davies, D. R, & Parasuraman, R. (1982). The psychology of vigilance. London: Academic Press. Eysenck, H. J. (1967). The biological basis of personality. Springfield, IL: Thomas. Eysenek, H. J. (1981). General features of the model. In H. J. Eysenck (Ed.), A model for personality (pp. 1-37). New York: Springer. Eysenck, H. J. & Eysenck, M. W. (1985). Personality and individual differences: A natural science approach. New York: Plenum Press. Eysenck, H. J., & Eysenck, S. B. G. (1964). The Ey.senck Personality. Inventory. London: University of London Press.

19 168 G. MATTHEWS, R. DAVIES, AND J. LEES Eysenck, M. W. (1982). Attention and arousal: Cognition and performance. New York: Springer. Eysenck, M. W. (1985). Anxiety and cognitive-task performance. Personality and Individual'Differences, 6, Green, D. M., & Swcts, J. A. (1966). Signaldetection theory and psychophysics. New York: Wiley. Hamilton, P., Hockey, G. R. J, & Rejman, M. (1977). The place of the concept of activation in human information processing theory: An integrative approach. In S. Dornic (Ed.), Attention and performance (Vol. 6, pp ). Hillsdale, NJ: Erlbaum. Harkins, S. G., & Green, R. G. (1975). Discriminability and criterion differences between extraverts and introverts during vigilance. Journal of Research in Personality, 9, Hirst, W., & Kalmar, D. (1987). Characterizing attentional resources. Journal of Experimental Psychology. General 116, Hockey, G. R. J., & Hamilton, R (1983). The cognitive patterning of stress states. In G. R. J. Hockey (Ed.), The cogn#ive patterning of stress states. Chichester, England: Wiley. Hockey, G. R. J., MacLean, A., & Hamilton, P. (1981). State changes and the temporal patterning of component resources. In J. Long & A. Baddeley (Eds.), Attention and performance (Vol. 9, pp ). Hillsdale, NJ: Edbaum. Humphreys, M. S., & Revelle, W. (1984). Personality, motivation and performance: A theory of the relationship between individual differences and information processing. PsychologicalReview, 91, Jackson, C. E, & Matthews, G. (1988). The prediction of habitual alcohol use from alcohol related expectancies and personality Alcohol and Alcoholism, 23, Johansson, (3. (1976). Subjective wellbeing and temporal patterns of sympathetic-adrenal medullary activity. Biological Psychology, 4, Jonides, J., & Gleitman, H. (1972). A conceptual category effect in visual search: O as letter or as digit. Perception and Psychophysics, 12, Marks, M., & Folkard, S. (1984). Diurnal rhythms in cognitive performance. In J. Nicholson and H. Beloff(Eds.), Psychology Survey Five (pp ). Leicester: British Psychological Society Matthews, G. (1985). The effects ofextraversion and arousal on intelligence test performance. British Journal of Psychology, 76, Matthews, G. (1986). The interactive effects ofextraversion and arousal on performance: Are creativity tests anomalous? Personality and Individual Differences, 7, Matthews, G. (1987). Personality and multidimensional arousal: A study of two dimensions of extraversion. Personality and Individual Differences, 8, Matthews, G. 0989). Extraversion and levels of control of sustained attention. Acta Psychologica, 70, Matthews, G., & Chappelow, J. E. (1986). Effects ofextraversion and arousal on selective attention. Paper presented to the British Psychological Society Cognitive Section Third Annual Conference, Oxford, England. Matthews, G., Harley, T. A., McCarthy, A, & Jones, J. (1988). Extraversion, self-report arousal, ahd semantic priming of lexical decision. Paper presented to the British Psychological Society Cognitive Section Fifth Annual Conference, Cambridge, England. Matthews, G., Jones, D. M., & Chamberlain, A. G. 0989). Interactive effects of extraversion and attentional task performance: Multiple resources or encoding processes? Journal of Personality and Social Psychology, 56, Matthews, G., Jones, D. M., & Chamberlain, A. G. (1990). Refining the measurement of mood: The UWIST Mood Adjective Checklist. British Journal of Psychology, 81, Munro, L. L., Dawson, M. E., Schell, A. M., & Sakai, L. M. 0987). Electrodermal lability and rapid vigilance decrement in a degraded stimulus continuous performance task. Journal of Psychophysiology, 1, Norman, D. A., & Bobrow, D. G. (1975). On data-limited and resourcelimited processes. Cognitive Psychology, 7, Nuechterlein, K, Parasuraman, R., & Jiang, Q. (1983). Visual vigilance: Image degradation produces rapid sensitivity decrement over time. Science, 220, Parasuraman, R. (1985). Vigilance: A multifactorial approach. In M. I. Posner & O. S. M. Marin (Eds.), Attention and performance (Voi. 11, pp ). Hillsdale, NJ: Erlbaum. Parasuraman, R., Warm, J. S, & Dember, W. N. (1987). Vigilance: Taxonomy and utility In L. Mark, J. S. Warm, & R. L. Huston (Eds.), Ergonomics and human factors: Recent research (pp ). New York: Springer. Revelle, W (1987). Personality and motivation: Sources ofinetticiency in cognitive performance. Journal of Research in Personality, 21, Revelle, W., Humphreys, M. S., Simon, L, & Gilliland, K. (1980). The interactive effect of personality, time of day and caffeine: A test of the arousal model. Journal of Experimental Psychology: General 109, Saville, D., & Blinkhorn, S. (1976a). Undergraduate personality by factored scales: A large scale study on Cattell~ 16PF and the Eysenck Personality Inventory. Windsor: NFER. Saville, D., & Blinkhorn, S. (1976b). British undergraduate norms to the 16PF (Forms A and B). Windsor: NFER. Schneider, W. (1985). Toward a model of attention and the development of automatic processing. In M. I. Posner & O. S. M. Marin (Eds.), Attention and performance (Vol. 11, pp ). Hillsdale, N J: Erlbaum. Schneider, W, & Fisk, A. D. (1982). Concurrent automatic and controlled visual search: Can processing occur without resource cost? Journal of Experimental Psychology: Human Learning and Behaviour, 8, Schneider, W, & Shiffrin, R. M. (1977). Controlled and automatic human information processing: I. Detection, search, and attention. Psychological Review, 84, Shiffrin, R. M., and Schneider, W (1977). Controlled and automatic human information processing: II. Perceptual learning, automatic attending, and a general theory Psychological Review, 84, Southall, A. (1988). Automatic and controlled processes in depression. Unpublished manuscript, Aston University, Birmingham, England. Stelmack, R. M. (1981). The psychopllysiology ofextraversion and neuroticism. In H. J. Eysenck (Ed.), A model for personality (pp ). New York: Springer. Thayer, R. E. (1967). Measurement of activation through self-report. Psychological Reports, 20, Thayer, R. E. (1970). Activation states as assessed by verbal report and four psychophysiological variables. Psychophysiology, 7, Thayer, R. E. (1978). Toward a psychological theory of multidimensional activation (arousal). Motivation and Emotion, 2, Thayer, R. E. (1986). Activation-Deactivation Adjective Checklist: Current overview and structural analysis. Psychological Reports, 58, Wickens, C. D. (1984). Engineering psychology and human performance. Columbus, OH: Merrill. Yerkes, R. M., & Dodson, J. D. (1908). The relation of strength of stimulus to rapidity of habit formation. Journal of Comparative andneurological Psychology, 18, Received December 5, 1988 Revision received November 3, 1989 Accepted January 12, 1990