Intelligence and P3 components of the event-related potential elicited during an auditory discrimination task with masking

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1 Available online at Intelligence 36 (2008) Intelligence and P3 components of the event-related potential elicited during an auditory discrimination task with masking V. De Pascalis, V. Varriale, A. Matteoli Department of Psychology, University of Rome La Sapienza, Italy Received 16 June 2006; received in revised form 7 January 2007; accepted 15 January 2007 Available online 1 March 2007 Abstract The relationship between fluid intelligence (indexed by scores on Raven Progressive Matrices) and auditory discrimination ability was examined by recording event-related potentials from 48 women during the performance of an auditory oddball task with backward masking. High ability (HA) subjects exhibited shorter response times, greater response accuracy, larger P3b amplitude and shorter P3b latency to target stimuli than the lower ability (LA) subjects. There were no differences between high and low ability groups for the P3a component of the ERP. Correlation and multiple regression analyses indicated that frontal P3b latency and, to a smaller extent, P3b amplitude are the best predictors of mental ability. These results are consistent with previous studies reporting a relation between intelligence and ERPs and provide further evidence that individual differences in mental ability are reflected in the P3b component, an electrocortical index of the efficiency of stimulus evaluation processes Elsevier Inc. All rights reserved. Keywords: Mental ability; Fluid intelligence; Raven Progressive Matrices; Event-related potentials; P Introduction During the past decades, significant progress has been made in understanding individual differences in mental ability through the application of cognitive processing models (for a review see Deary & Stough, 1996). This line of research explores the associations between psychometric intelligence scores and basic cognitive processes conceptualized within the context of the informationprocessing tradition. This research established an association between higher mental ability, as measured by mental tests, and faster performance on simple tasks, using inspection time (IT) and response time (RT) measures Corresponding author. Dipartimento di Psicologia, Università di Roma La Sapienza, Italy. Tel.: ; fax: address: v.depascalis@caspur.it (V. De Pascalis). (e.g., Jensen, 1982; Vernon, 1990). IT is defined as the time that is required to accurately discriminate between two stimuli that are easily discriminable at longer exposure duration. Deary and Stough (1996) argued that, of all information processing measures used in this context, IT has been shown to correlate most substantially and consistently with individual differences in the performance on standard tests of psychometric intelligence (see, e.g., Brand & Deary, 1982; Deary, Caryl, Egan, & Wight, 1989; Nettelbeck, 1987; Nettelbeck & Lally, 1976; Vickers, Nettelbeck, & Willson, 1972; Zhang, Caryl, & Deary, 1989). Vickers et al. (1972) defined IT as the minimum exposure duration of a stimulus pair for allowing a discrimination of relative magnitude. Higherability subjects accurately discriminate the stimuli at shorter exposure durations than those of lower mental ability. More specifically, the relation between IT and /$ - see front matter 2007 Elsevier Inc. All rights reserved. doi: /j.intell

2 36 V. De Pascalis et al. / Intelligence 36 (2008) mental ability has been observed to be about r= 0.50 (e.g., Kranzler & Jensen, 1989; Nettelbeck, 1987) orbetter (Bates & Eysenck, 1993; Deary, 1993). A backward-masking procedure was featured in the IT paradigm and used for the study of individual differences in mental ability (Vickers & Smith, 1986). In the backward-masking procedure the target to be discriminated is presented briefly (usually less than 100 ms) and then followed by an overwriting masking stimulus. The time interval between the target stimulus and the mask was used as an index of the time that is required for a target stimulus in sensory memory to be later processed into short-term memory. In the original work it was assumed that the mask interrupts or erases iconic (or echoic) storage and precludes target identification from iconic (or echoic) information after the physical offset of the target. A contrasting, and more accepted view, suggests that at shorter inter-stimulus intervals (ISIs) the masking stimulus degrades the contrast of the target stimulus by means of temporal integration with it (DiLollo, 1980; Eriksen, 1966). Several studies have examined the relation between individual differences in mental ability and processing efficiency by using the backward-masking paradigm and recording event-related brain potentials (ERPs). Unfortunately, these studies failed to observe a consistent pattern of effects (for a critical review, see Stelmack & Beauchamp, 2001). Some methodological limitations of the early studies were overcome by Bazana and Stelmack (2002) who reported that higher ability subjects had both shorter ITs and ERP latencies than lower ability subjects. Both effect sizes for performance and ERP measures were quite robust (ranging from p b0.01 to p b0.001). These effects were interpreted by the authors in terms of greater information processing efficiency in the high ability subjects. ERP studies of mental ability are frequently using the oddball paradigm, in which stimuli are presented with different probabilities \ target stimuli are presented with low probability and standard stimuli are presented with high probability. Target stimuli typically elicit an ERP component that reaches a maximal amplitude at around 300 ms. This component is coined the P3 and is most sensitive to task manipulations affecting stimulus evaluation processes, while task manipulations influencing response-related processes do not seem to affect P3 (e.g., Coles, Smid, Scheffers, & Otten, 1995; Donchin, 1979). In addition to the classical P3 or P3b, having a central/ parietal scalp topography, a P3a component, having a more frontal/central scalp topography, is elicited to infrequent or unexpected stimuli. The P3a component is interpreted in terms of the involuntary and transient allocation of attentional resources to analysis of novel and salient stimuli (Polich & Comerchero, 2003; Polich & Criado, 2006; Simons, Graham, Miles, & Chen, 2001; Squires, Squires, & Hillyard, 1975). A number of studies demonstrated a relation between individual differences and mental ability and P3 latency (e.g., Bazana & Stelmack, 2002; Egan et al., 1994; Fjell & Walhovd, 2001; Jausŏvec & Jausŏvec, 2000; McGarry-Roberts, Stelmack, & Campbell, 1992; O'Donnell, Friedman, Swearer, & Drachman, 1992; Stelmack & Houlihan, 1995; Walhovd & Fjell, 2001, 2002; but see Houlihan, Stelmack & Campbell, 1998, for conflicting findings). The main goal of the present study was to examine the relation between individual differences in mental ability and ERP measures by using a hybrid backward-masking/ auditory-oddball paradigm. This backward-masking procedure, similar to that used in previous studies on mental ability and auditory discrimination ability, requires that both the standard and target tones are followed by the masking stimulus at a specific temporal interval (Bazana & Stelmack, 2002; Raz & Willerman, 1985). The aim of the present investigation was to evaluate the predictive value of both RT and neuroelectric measures (i.e., P3a and P3b peak amplitudes and latencies) on mental ability. Both P3a and P3b measures were used to provide an estimate of the efficiency of perceptual processing that is uncontaminated by response-related processes. The focus was on fluid intelligence, as indexed by scores on the Raven Progressive Matrices (RPM), rather than on verbal intelligence. Most, if not all, theories relating individual differences in mental ability to processing efficiency are concerned with fluid, not verbal, intelligence (see, e.g., Deary & Caryl, 1997; Nettelbeck, 2003). It was anticipated that individuals scoring higher on the RPM, compared to lower scoring individuals, would exhibit shorter RTs, shorter P3a and P3b latencies and higher P3a and P3b amplitudes. These predicted differences between mental ability groups are consistent with the notion that high ability subjects may have greater ease or efficacy in making tonal frequency discrimination and this should be reflected to both the initial analysis (indexed by P3a amplitude) and subsequent processing (indexed by P3b amplitude) of the target stimulus (Bazana & Stelmack, 2002; Chapman, McCrary, & Chapman, 1978; Houlihan et al., 1998; McGarry-Roberts et al., 1992). 2. Method 2.1. Subjects Fifty right-handed women aged years (M =24.2, SD = 2.6), all psychology students, were recruited. All

3 V. De Pascalis et al. / Intelligence 36 (2008) subjects self-reported to be right-handed and were unacquainted with their mental ability. Women who were in their menstrual period were invited for EEG recordings during another occasion. Subjects were admitted to participate in the experiment only if they reported absence of drug abuse, use of medication (e.g., psychoactive drugs, antihistamines) or medical conditions that might interfere with vigilance and task performance (e.g., high blood pressure, diabetes mellitus, heart diseases, asthma, neurological disorders). Subjects performed their tasks during two sessions scheduled two weeks apart. During the first session they performed on the Raven's Standard Progressive Matrices and a speeded performance task. During the second session they performed on the backward-masking/oddball task Assessment of mental ability and processing speed Subjects were presented with an individual form of the Raven's Standard Progressive Matrices (RSPM, Raven, 1954) and, following a brief rest, they completed a trail-making test to provide a score of information processing speed (i.e., the Zahlen Verbindungs Test (ZVT); Oswald & Roth, 1978). Three versions of the ZVT versions 1, 4, and 6 were selected from Vernon's (1993) study. Each ZVT version was administered only in a speeded format that counted the number of items completed in 45 s. Task complexity increased with successive versions of the task. In version 1, subjects were required to connect numbers in a forward fashion (1, 2, 3, 4, and so on); in version 4, number/letter backward (adapted for the Italian alphabet: 21, Z, 20, V, 19, U, and so on); in version 6, odd numbers backward (179, 177, 175, and so on). The RSPM scores (M =50.2, SD=6.9; IQ=117.7, SD=10.6, N =50) were used to create two ability groups. Two participants were eliminated because of greater than 3 SD outlier performance on the tasks, occurring apparently due to insufficient effort during task performance. The mean age of the remaining 48 participants was 24.1 (SD =4.1). The high ability (HA) group (n =24) had scores higher than the median RSPM score (Md=49.5) and the low ability (LA) group (n =24) had scores lower than the median RSPM score (M =56.0, SD=2.9; IQ=126.9, SD=2.2, and M =44.1, SD=4.0; IQ=107.9, SD=6.1, respectively for raw and IQ scores of the HA and LA group). The RSPM scores were roughly normally distributed (skewness= 0.42; kurtosis= 0.68) with a range of The HA and LA groups had a quite similar mean age (M =24.3, SD=2.7, and M =24.1, SD =2.5, respectively) Backward Masking-Oddball Task (BMOT) Subjects were first tested for normal hearing by delivering 8 tones (duration 25 ms, 1000 Hz) in steps of 10 db (SPL), starting from 15 to 85 db in ascending order. Stimuli were generated by using a WaveLab-3.0 system and tested by a Bruel and Kjaer (type 221B) precision sound level meter. All auditory stimuli were presented by using E-prime-1.1 system (Schneider, Eschman, & Zuccolotto, 2002). The BMOT consisted of a series of frequent standard tones (85%) and infrequent target tones (15%). Both standard and target tones were followed by an auditory mask. The tones were presented binaurally through padded headphones. The standard tones had a frequency of 600 Hz and the target tones of 700 Hz. The duration of the standard and target tones was 25 ms. The mask had a frequency of 1000 Hz and a duration of 50 ms. All stimuli had an intensity of 88 db (e.g., Bazana & Stelmack, 2002). The ISI between the offset of the standard or target tone and the onset of the mask was varied at 25, 50, or 150 ms in separate conditions. The intertrial interval (ITI) between the offset of the mask and the onset of the standard or target tone was held constant at 600 ms for each of the ISI conditions. For each of the three ISIs there was one block of 500 trials. Thus each block consisted of 425 standard and 75 target tones, presented in random order. ISI blocks were counterbalanced across subjects. Subjects were instructed to press the space bar of the computer keyboard when they detected a target tone and to refrain from responding to a standard tone. Subjects were told to keep their right-index finger on the space bar and to press it as quickly as possible to the target tone while maintaining accuracy. Before starting BMOT, subjects were given a brief practice session (i.e., a series 150-ms ISI trials) to ensure that they understood the task. They were required to correctly detect five consecutive target tones before continuing with the procedure. Subjects performed their task while sitting in a reclining chair located in a sound and electrically shielded cubicle. They received short rests between successive trial blocks Electrophysiological recordings EEG and electro-ocular (EOG) were acquired continuously and simultaneously with the performance measures by using a 40-channel NuAmps DC amplifier system (Neuroscan Inc.), set at a gain of 200, digitization rate of 500 Hz, and with signals band-limited to 200 Hz (notch filter at 50 Hz). Electrode impedance was lower

4 38 V. De Pascalis et al. / Intelligence 36 (2008) than 4 kω. The horizontal EOG was monitored via a pair of tin electrodes placed 1 cm lateral to the outer canthus of each eye and the vertical EOG was monitored via a separate bipolar montage placed above and below the centre of the left eye. EEG data were recorded from 30 scalp sites (Fp1, Fp2, F7, F8, F3, F4, FT7, FT8, T3, T4, FC3, FC4, C3, C4, CP3, CP4, TP7, TP8, T5, T6, P3, P4, O1, O2, Fz, FCz, Cz, CPz, Pz, Oz) referenced to linked-ear electrodes by using an electrocap (Blom & Anneveldt, 1982) with pure tin electrodes. The ground electrode was located 10 mm anterior to Fz. The EEG was later reconstructed into discrete, single-trial epochs. An epoch began 50 ms prior to stimulus onset in each trial and continued for 850 ms following it. Individual trials were then rejected if containing gross artifacts (any trial in which the EEG amplitude exceeded a maximum amplitude of ±100 μv). Prior to signal averaging, an offline EOG correction routine was used (Gratton, Coles, & Donchin, 1983) Data analysis Several performance measures were calculated: (1) accuracy (i.e., the number of correct target detections), (2) RT or the latency of correct detections (i.e., the interval between the onset of the target stimulus and the depressing of the space bar), (3) standard deviation of RT (SDRT) or response variability, (4) number of false positive responses, (5) RT of false positives; (6) RT-difference scores (i.e., RTcorrect RTfalse positive). The performance measures were submitted to analysis of variance (ANOVA; SAS-8.02, glm procedure) with Mental Ability (i.e., HA vs. LA groups) as betweensubjects factor and ISI (150, 50 vs. 25 ms) as withinsubjects factor. For the EEG analyses, single trials were sorted and averaged according to ISI condition, stimulus type (standard or target), and electrode site. Only ERP responses to target tones for correct trials were included in the average and used for data analyses. Waveforms were scored with a computer-assisted scoring routine (Edit-Scan 4.3-1, Neuroscan Inc., USA) and the values were then visually verified by an experienced observer. The amplitude of the P3a and P3b waves was scored for each participant and each electrode by determining the maximum peak amplitudes between 280 ms and 500 ms for the P3a, and between 330 ms and 650 for the P3b. Amplitudes were referenced to a 50-ms prestimulus baseline. Peak latencies were measured from the onset of the stimulus to the point of the maximum peak amplitude of the P3a and P3b waves. All experimental measures were screened and cleared for outliers and missing data. For the ERP peak measures two separate ANOVAs were carried out. One ANOVA was performed on midline electrodes (Fz, FCz, Cz, CPz, Pz). A second ANOVA was performed to assess anterior vs. posterior effects and left vs. right hemisphere effects. This analysis was focused on scalp quadrants (i.e., left-frontal: Fp1, F3, F7, FC3, FT7; right-frontal: Fp2, F4, F8, FC4, FT8; left-central: C3, T3, CP3; right-central: C4, T4, CP4; left-posterior: T5, TP7, P3, O1; right-posterior: T6, TP8, P4, O2). Quadrant scores for P3a and P3b amplitudes and latencies were derived by taking the mean of the peak amplitudes and latencies for the electrodes within a given quadrant. The ANOVA performed on the quadrant scores included Mental Ability (HA vs. LA) as between-subjects factors and Hemisphere (left vs. right), Electrode Location (Frontal, Central, Posterior) and ISI (150, 50 vs. 25 ms) as within-subjects factors. To prevent the risk of falsely significant results, Table 1 Mean percent correct detections, mean RT (ms) and SDRT (ms) of correct detections, number and RT (ms) of false positives for HA and LA groups at 150-ms, 50-ms, and 25-ms ISIs ISI % Correct RT correct SDRT N false pos RT false pos HA LA HA LA HA LA HA LA HA LA 150-ms (35.4) (20.2) (85.8) (98.8) (28.8) (21.6) (3.9) (4.1) (190.2) (133.9) 50-ms (41.5) (22.5) (90.3) (96.2) (22.1) (21.3) (5.2) (4.0) (181.7) (148.0) 25-ms (35.3) (27.8) (71.1) (85.7) (14.4) (21.1) (5.4) (6.2) (151.3) (102.4) Note. Standard deviations are shown in parentheses. HA=higher ability (N=24); LA=lower ability (N=24); ISI=inter-stimulus interval.

5 V. De Pascalis et al. / Intelligence 36 (2008) as may happen using repeated measures ANOVAs if the sphericity assumption has been violated (Vasey & Thayer, 1987), the Huynh Feldt epsilon correction of significance levels was applied when necessary. Post hoc comparisons of the means were carried out by using a Sheffé test procedure with alpha =0.05 (Kirk, 1968, pp ). 3. Results 3.1. Performance Stimulus discrimination was more efficient in the HA group as compared to the LA group. Table 1 shows the means and standard deviations for the behavioral measures at each ISI for both HA and LA groups Response accuracy Response accuracy increased with ISI, F(2, 84) = 7.58, p b The main effect of Mental Ability was significant, F(1, 42)=4.86, pb0.05 and indicated that the HA group was more accurate than the LA group across all ISIs (see Table 1). The number of false positives was greater for 25 ms ISI, as compared to 50-ms and 150-ms ISIs, F(2, 84)=5.13, p=0.016; 7.2, vs. 4.8 and 4.0, respectively, and fell short of significance for Group, F(1, 42)=2.0, p=0.16 (see Table 1) Response speed Response speed decreased with ISI, F(2, 84) = 16.59, pb0.0001; from 474 ms to 487 ms and 519 ms for the 25-ms, 50-ms, and 150-ms ISI, respectively. The HA group tended to respond faster to the target stimulus than the LA group, F(1, 42)=4.1, pb0.05 (see Table 1). A similar pattern was obtained for the speed of false positives. That is, the speed of false positives decreased with ISI, F(2, 46)=10.67, pb0.0001; from 297 ms to 384 ms, and 483 ms. The effect of Mental Ability reached an acceptable level of significance, F(1, 23) = 5.15, p=0.033, indicating that HA group had slower speed of false positives than the LA one (414 vs. 362 ms). Finally, an analysis performed on the difference between RT of correct responses vs. RT of false positives revealed that this difference decreased with ISI, F(2, 46) = 10.07, p = ; from 167 ms to 109 ms and 39 ms Response variability Response variability of HA subjects was significantly smaller than that of LA subjects, F(1, 43)=7.08, p= A rough similar variability was also found across all three ISIs, F(2, 86)=1.52, p=0.227; 90 ms, 92 ms and 97 ms, for the 25-ms, 50-ms, and 150-ms ISI, respectively (see Table 1) Brain potentials The grand average waveforms associated with the processing of the auditory stimuli are presented in Fig. 1 for target vs. standard stimuli, separately, for each ISI. It can be seen that the processing of the auditory stimuli was associated with a P3 complex consisting of two subcomponents: a P3a that peaked over the central region (i.e., peak latency of 380 ms at Cz), and a P3b that peaked over the parietal region (i.e., peak latency of 489 ms at Pz). The grand mean of ERP waveform to the target and standard tones for the 150-ms ITI is shown in Fig. 1. In this picture it can clearly be seen that P3 wave is larger for the infrequently presented target stimuli than for the frequently presented standard stimuli Midline P3a and P3b amplitudes The ANOVA performed on the midline P3a amplitudes indicated that the amplitude was larger over the central and parietal regions compared to frontal sites, F (4, 172)=31.72, p b0.0001; i.e., 2.8 μv, 3.4 μv, 5.2 μv, 4.1 μv, and 4.5 μv for Fz, FCz, Cz, CPz, and Pz sites, respectively. Importantly, the P3a failed to discriminate between ability groups, F(1, 43)=3.30, p=0.076 and between ISIs, F(2, 72)=1.20, p= Midline P3a amplitude scores across ISIs and electrode locations for HA and LA groups are presented in Table 2. Grand average event-related potential (ERP) waveforms at Fig. 1. Grand average event-related potential waveforms at midline parietal electrode site (Pz) for all participants to standard and target stimulus tones during the discrimination task with a 150-ms intertone interval (N= 48). The P3a and P3b labels indicate these ERP components. On the time scale, the solid block ( ) indicates the onset and duration of the standard and target stimuli. The open block ( ) indicates the onset and duration of the masking stimulus.

6 40 V. De Pascalis et al. / Intelligence 36 (2008) Table 2 Mean P3a and P3b amplitudes (μv) and latencies (ms) for HA and LA groups at 150-ms, 50-ms, and 25-ms ISIs across midline frontal (Fz), fronto-central (FCz), central (Cz), centro-parietal (CPz), and parietal (Pz) electrode locations Electrodes ISI (ms) P3a amplitude (μv) P3b amplitude (μv) HA LA HA LA 150 Fz 3.4 (2.2) 1.9 (0.9) 4.6 (3.2) 2.2 (1.4) FCz 4.5 (2.7) 1.8 (1.5) 5.6 (3.1) 2.5 (1.5) Cz 4.9 (2.0) 2.9 (1.7) 6.0 (3.4) 4.0 (2.4) CPz 5.4 (2.7) 3.3 (1.4) 7.2 (4.3) 5.1 (3.0) Pz 5.9 (2.6) 4.0 (1.5) 7.9 (4.4) 6.5 (3.2) 50 Fz 3.6 (1.1) 2.0 (1.5) 5.3 (3.6) 0.5 (0.4) FCz 4.4 (1.4) 2.7 (2.0) 5.6 (3.8) 1.3 (0.8) Cz 4.9 (1.7) 3.5 (2.2) 7.0 (4.1) 2.3 (1.0) CPz 5.2 (1.8) 4.1 (2.4) 7.9 (4.2) 3.8 (2.0) Pz 5.9 (2.4) 5.1 (2.1) 9.0 (4.2) 5.4 (3.5) 25 Fz 3.6 (1.6) 2.4 (1.0) 3.7 (3.4) 0.5 (0.3) FCz 4.1 (2.0) 3.2 (1.9) 4.9 (3.9) 1.4 (2.1) Cz 4.4 (2.1) 3.7 (2.2) 5.6 (4.1) 2.4 (1.2) CPz 5.0 (2.0) 4.1 (2.4) 6.4 (4.4) 3.2 (2.1) Pz 5.6 (2.2) 5.0 (2.6) 6.6 (4.2) 4.3 (2.3) 3.7. Quadrant P3a and P3b amplitudes The analysis performed on the quadrant scores of P3a amplitude yielded a significant effect of ISI, F(2, 86) = 4.36, p = 0.018, showing smaller P3a amplitudes for 150-ms ISI relative to the 50-ms and 25-ms ISIs; 2.5 μv, 3.3 μv and 3.2 μv, respectively. P3a amplitude was larger over the central and parietal regions compared to frontal sites, F(2, 86)=53.89, pb0.0001; i.e., 1.9 μv, 3.2 μv, and 3.9 μv for frontal, central, and posterior site, respectively. The interaction between Hemisphere and Electrode Location was highly significant, F(2, 86) = 16.62, p b0.0001, but this simple interaction was qualified by the higher-order interaction including ISI factor, F(4, 172) = 3.23, p = Post-hoc multiple comparisons (Sheffé's method) revealed that for 25-ms P3a latency (ms) P3b latency (ms) 150 Fz 410 (50) 409 (48) 501 (52) 522 (64) FCz 407 (49) 409 (47) 495 (48) 518 (67) Cz 406 (50) 408 (44) 502 (44) 518 (68) CPz 407 (52) 406 (41) 499 (44) 513 (64) Pz 404 (54) 401 (44) 497 (43) 513 (56) 50 Fz 377 (47) 382 (43) 464 (49) 505 (73) FCz 382 (42) 379 (43) 482 (64) 503 (76) Cz 389 (42) 388 (38) 478 (59) 517 (75) CPz 380 (49) 382 (34) 479 (58) 518 (75) Pz 385 (53) 384 (31) 475 (54) 506 (67) 25 Fz 369 (44) 374 (25) 464 (63) 544 (65) FCz 365 (41) 372 (25) 472 (71) 536 (57) Cz 374 (38) 378 (21) 472 (65) 532 (54) CPz 372 (35) 370 (20) 472 (65) 529 (50) Pz 373 (26) 374 (20) 468 (66) 527 (48) Note. Standard deviations are shown in parentheses. HA=higher ability (N=24); LA=lower ability (N=24); ISI=inter-stimulus interval. midline parietal site for higher and lower ability groups across ISIs are displayed in Fig. 2. P3b amplitude to the target stimulus was larger for the HA group compared to the LA group across all midline scalp sites, F(1, 43)=20.82, pb (see Table 1). P3b amplitude was more pronounced over Pz as compared to the other recording sites, F(4, 172)=62.99, pb0.0001; 2.8 μv, 3.6 μv, 4.6 μv, 5.6 μv, 6.6 μv, for Fz, FCz, Cz, CPz, and Pz scalp sites, respectively. The main effect of ISI and all interactions failed to reach significance. Fig. 2. Grand average event-related potential (ERP) waveforms at midline parietal electrode site (Pz) for higher ability (HA, N=24) and lower ability (LA, N=24) groups during discrimination tasks with 25-ms, 50-ms, and 150-ms intertone intervals (ITIs). The solid block ( ) indicates the onset and duration of the standard and target stimuli; the open block ( ) indicates the onset and duration of the masking stimulus.

7 V. De Pascalis et al. / Intelligence 36 (2008) and 50-ms ISIs in the left hemisphere there was a less pronounced peak amplitude over the frontal region compared to central and posterior sites, while for 150-ms ISI there were no significant differences across scalp sites; i.e., 25-ms: 2.2 μv, 3.4 μv, and 4.4 μv; 50-ms: 2.1 μv, 3.6 μv, and 4.5 μv; 150-ms: 2.6 μv, 2.8 μv, and 3.4 μv, for frontal, central, and posterior site, respectively. The ANOVA performed on the quadrant scores of P3b amplitude replicated the outcomes of the previous analysis. That is, the ANOVA yielded a significant main effect for Mental Ability, F(1, 41) = 15.97, p = and also evidenced that P3b amplitude was more pronounced over posterior scalp sites, F(2, 82)=83.33, pb0.0001; 2.2 μv, 3.9 μv, and 5.0 μv, for anterior, central, and posterior scalp sites, respectively. Moreover, across frontal and central sites HA group had more pronounced P3b amplitudes as compared to LA group, F(2, 82)=6.64, p=0.0061; i.e., 3.2 μv vs. 1.1 μv, 5.1 μv vs. 2.6 μv, 5.5 μvvs.4.5μv, for HA vs. LA group across frontal, central, and posterior regions, respectively. The main effect of ISI reached the significance, F(2, 82)=3.22, pb0.05 and indicated that P3b peak decreased at shorter ISIs, 3.0 μv, 3.9 μv, and 4.0 μv, respectively for 25-ms, 50-ms, and 150-ms ISIs Midline P3a and P3b latency P3a latency at the midline recording sites was sensitive to ISI since it was significantly shorter at shorter ISIs; i.e., 372 ms, 383 ms, and 407 ms peak latency, for 25-ms, 50- ms, and 150-ms ISI, respectively F(2, 86) =7.84, p = However, P3a latency failed to discriminate between mental ability groups. By contrast, P3b latency at the midline scalp sites was for the HA group shorter than that for the LA group, F(1, 43) = 18.71, p b ; 475 ms vs. 529 ms, respectively. This main effect was qualified by the simple interaction including ISI factor, F (2, 86)=4.23, p= Post-hoc multiple comparisons by Sheffé's test revealed that HA group had significantly Table 3 Pearson correlation matrices between Raven's scores (RSPM), Zahle Verbindungs Test Z21-V20 (ZVT-v4), percent correct detections (%Cor), number of false positive detections (N-false-p), false positive response time (False-p RT), response time (RT), standard deviation of response time (SD-RT), midline P3a latency (P3aL) and amplitude (P3aA) at Fz, Cz, and Pz scalp sites 50 ms ISI 150 ms ISI RSPM ZVT-v4 %Cor N-false-p False-p RT RT SDRT P3aL-Fz P3aL-Cz P3aL-Pz P3aA-Fz P3aA-Cz P3aA-Pz RSPM ZVT-v %Cor N-false-p False-p RT RT SD-RT P3aL-Fz P3aL-Cz P3aL-Pz P3aA-Fz P3aA-Cz P3aA-Pz ms ISI RSPM ZVT-v %Cor N-false-p False-p RT RT SDRT P3aL-Fz P3aL-Cz P3aL-Pz P3aA-Fz P3aA-Cz P3aA-Pz 1 Bold: pb0.01; italic: pb0.05; bold underlined: pb0.001; bold italic: pb Correlations for the 150-ms, 50-ms, and 25-ms ISIs are reported on the upper side, middle side, and bottom side of this table, respectively.

8 42 V. De Pascalis et al. / Intelligence 36 (2008) shorter P3b latencies than the LA group for 25-ms and 50- ms ISIs, while the groups did not differ for the 150-ms ISI; i.e., 458 ms vs. 545 ms (pb0.01), 468 ms vs. 524 ms (pb0.05), and 498 ms vs. 518 ms (pn0.05), for 25-ms, 50-ms, and 150-ms ISIs, respectively Quadrant P3a and P3b latency The ANOVA on the P3a latency quadrant scores failed to show any effect involving Mental Ability although there were significantly shorter latencies for the 25-ms and 50-ms ISIs as compared to 150-ms ISI, F(2, 86) = 10.60, pb0.0001; 384 ms and 389 ms vs. 416 ms, respectively. The analysis on the P3b latency quadrant scores displayed shorter latencies for the HA group compared to the LA group, F(1, 43)=17.65, pb0.0001; 473 ms vs. 525 ms, for HA vs. LA group. In addition, the following effects were significant: Electrode Location, F(2, 86)=5.34, p=0.0156; the simple interaction between Mental Ability and ISI, F(2, 86) =3.26, pb 0.05; the higher-order interaction including Mental Ability, Electrode Location and ISI, F(4, 172)=2.96, p= Posthoc multiple comparisons (Sheffé's method) revealed that in the HA group compared to the LA group there were, for the 25-ms ISI and at a less extent for 50-ms ISIs, shorter P3b latencies over frontal, central, and posterior scalp regions, although the smallest difference between groups was obtained over posterior region; i.e., 25-ms ISI: 457 ms vs. 545 ms, 456 ms vs. 537 ms, 455 ms vs. 527 ms; 50-ms ISI: 469 ms vs. 521 ms, 467 ms vs. 524 ms, 466 ms vs. 519 ms; 150-ms ISI: 490 ms vs. 522 ms, 497 ms vs. 518 ms, 492 ms vs. 513 ms; for HA vs. LA group across frontal, central, and posterior site, respectively Correlation analysis The correlation coefficients between ZVT 1-2-3, ZVT , ZVT Z21-V20, and the RSPM scores were Table 4 Pearson correlation matrices between Raven's scores (RSPM), Zahle Verbindungs Test Z21-V20 (ZVT-v4), percent correct detections (%Cor), number of false positive detections (N-false-p), false positive response time (False-p RT), response time (RT), standard deviation of response time (SD-RT), midline P3b latency (P3bL) and amplitude (P3bA) at Fz, Cz, and Pz scalp sites 50 ms ISI 150 ms ISI RSPM ZVT-v4 %Cor N-false-p False-p RT RT SD-RT P3bL-Fz P3bL-Cz P3bL-Pz P3bA-Fz P3bA-Cz P3bA-Pz RSPM ZVT-v %Cor N-false-p False-p RT RT SD-RT P3bL-Fz P3bL-Cz P3bL-Pz P3bA-Fz P3bA-Cz P3bA-Pz ms ISI fsrspm ZVT-v %Cor N-false-p False-p RT RT SD-RT P3bL-Fz P3bL-Cz P3bL-Pz P3bA-Fz P3bA-Cz P3bA-Pz 1 Bold: pb0.01; italic: pb0.05; bold underlined: pb0.001; bold italic: pb Correlations for the 150-ms, 50-ms, and 25-ms ISIs are reported on the upper side, middle side, and bottom side of this table, respectively.

9 V. De Pascalis et al. / Intelligence 36 (2008) (pb0.05), (p =0.082), and (p b0.01), respectively. The correlations between the RSPM, the ZVT Z21- V20, the BMOT performance measures, and the midline P3a and P3b measures are presented in Tables 3 and 4 to summarize the size of mental ability effects at each ISI. Correlations between RSPM, accuracy, RT, as well as STD-RT scores were significant (pb0.05) and in the expected direction (see Table 3). Moreover, correlations between these BMOT performance measures were all highly significant. The number of false positive responses was also significantly correlated with RSPM solely for the 150-ms ISI (see Table 3). There were no significant relationships between RSPM and P3a measures although P3a amplitude at Fz and Cz sites was positively correlated with RSPM for the 50-ms ISI (see Table 3). The P3b measures yielded a number of highly significant relationships with RSPM for the 50-ms and 25-ms ISIs. In particular, for these ISIs, the RSPM scores were negatively correlated with P3b latency at Fz, Cz, and Pz sites and positively correlated with P3b amplitude (see Table 4). The scatterplots for the most significant correlations between mental ability and P3b measures are reported in Fig. 3. Considering that P3a and P3b components may seem quite similar, correlations were calculated between midline P3a and P3b latencies and between P3a and P3b amplitudes in order to obtain a complete picture and to allow for an assessment of the discrimination of these parameters. These intercorrelations were not very high as expected. The P3a and P3b latencies were not significantly correlated (pn0.05) for the 25-ms and 50-ms ISI. In contrast, for the 150-ms ISI these measures were moderately, but significantly correlated (the lowest r value was between P3a and P3b latencies at Fz and the highest value was between P3a and P3b latencies at Cz: 0.32, and 0.43, respectively). Similarly, the P3a and P3b amplitudes were not significantly correlated for the 50-ms and 25-ms ISIs. However, the most consistent r values were obtained for the 150-ms ISI (0.51, 0.54, and 0.58, respectively between P3a amplitude at Cz and P3b Fig. 3. Scatterplots in the central and left sides are illustrating the relationships between Mental Ability (RSPM) and P3b Latency at Fz, Cz and Pz scalp sites for 50-ms and 25-ms ISIs. In the right side are depicted the relationships between Mental Ability (RSPM) and P3b amplitude at Fz and Cz for 50-ms and at Fz for 25-ms ISIs.

10 44 V. De Pascalis et al. / Intelligence 36 (2008) amplitude at Fz, Cz and Pz sites; 0.44, 0.51, and 0.68, respectively for P3a amplitude at Pz and P3b amplitude at Fz, Cz and Pz sites). Finally, considering that the ZVT-v4 and P3b measures for the 50-ms and 25-ms ISIs were also highly correlated with RSPM, two separate stepwise linear regression models were tested to predict Mental Ability from the ZVT and P3b measures. The first stepwise regression included as predictors the ZVT-v4 and the most correlated P3b measures for the 50-ms ISI (i.e., P3b latency at Fz, Cz, and Pz sites and the P3b amplitude at Fz and Cz sites, see Table 4). This procedure retained the P3b latency at Cz and frontal P3b amplitude as the best predictor of RSPM scores. The midline central P3b latency accounted for the 31.3% of the total variance (F=17.4, p=0.0002) and the frontal P3b amplitude accounted for the 15.2% of the total variance. The adjusted R-square for the total equation was of The second stepwise regression included the ZVT-v4 and the most correlated P3b measures for the 25-ms ISI (i.e., P3b latency at Fz, Cz and Pz sites and the P3b amplitude at Fz site, see Table 4). The single retained predictor was the frontal P3b latency which accounted for the 45.5% of the total variance (F=31.7, pb0.0001). The adjusted R-square for the total equation was of Discussion The performance measures derived from the BOMT were highly sensitive to mental ability, as indexed by the scores on the RSPM. Response accuracy and response speed were both higher for HA compared to LA participants. This pattern of findings is in line with the results obtained previously by Bazana and Stelmack (2002) and others using a backward masking paradigm (Raz & Willerman, 1985; Raz, Willerman, Ingmundson, & Hanlon, 1983). In broad outline, the current findings provide support for the view that performance efficiency, as indexed by accuracy and speed, is an important dimension of mental ability (Deary, 2000). In the present study, brain potentials were recorded during BMOT performance in order to establish whether the relation between performance efficiency and mental ability consists primarily of perceptual vs. responserelated processes. The P3 complex was differentiated into two separate components: the P3a mainly observed over frontal-central electrode sites and the P3b recorded over parietal central electrode sites. The results showed that P3b, but not P3a component, was sensitive to mental ability. That is, P3b amplitude was higher and P3b latency was shorter for the HA group compared to the LA group. Given that the P3 component is thought to reflect stimulus evaluation processes and to be independent of response execution processes (Doucet & Stelmack, 1999), the present P3b findings can be seen as indicative of the greater speed in making frequency discrimination of the HA group. P3a amplitude and P3a latency measures did not discriminate between ability groups, although quadrant P3a amplitude was found smaller at the longest ISI and P3a latency was found shorter at shorter ISIs. It is known that P3a is produced when the attention focus for a primary discrimination task is interrupted by an infrequent nontarget stimulus, which does not have to be perceptually novel (Polich & Comerchero, 2003; Simons et al., 2001; Squires et al., 1975). According to Polich and Criado (2006), if stimulus discrimination demands increase the degree of focal attention required for task performance, the P3a indexes the operation of an automatic attention network that is responsive to stimulus deviance (orienting response). Thus, considering the lacking differences between groups of P3a measures, the accuracy and RT differences between mental ability groups cannot be attributed to individual differences on automatic attention mechanisms for stimulus discrimination. The present findings of a more pronounced P3b amplitude for the HA group relative to the LA group are consistent with Bazana and Stelmack's (2002) findings. Moreover, the lacking differences for the P3a amplitude between mental ability groups also indicated that these differences are not due to orienting mechanisms but rather to mental speed, i.e., the time required to complete an updating of the current stimulus representation in working memory associated with P3b production (Donchin & Coles, 1988). These results are also in line with the larger P3 amplitudes to memory set stimuli obtained in high ability subjects during a Sternberg digit recognition task (Houlihan et al., 1998; McGarry-Roberts et al., 1992). Finally, there are several observations supporting the view, put forth by Deary (1994) and later confirmed by Stelmack and collaborators (Bazana & Stelmack, 2002; Stelmack & Beauchamp, 2005), that accuracy effects are determined by processing speed rather than discrimination ability. First, the shorter and more accurate RT responses and shorter P3b latencies were highly significant effects. Second, the findings that these measures were shorter for shorter ISI contradict the hypothesis of a perceptual discrimination effect, since the value of these measures would be expected to be longer with decrease in ISI. Third, because P3b amplitude decreased with increase in ISI, the larger P3b amplitude for HA than LA could be attributed to the greater discrimination ability

11 V. De Pascalis et al. / Intelligence 36 (2008) for the HA group. However, considering that the ERP waveform was derived only from trials on which a correct response was made, it must be concluded that the level of discrimination for P3b was the same for HA and LA groups. Moreover, it is known that P3b is sensitive to task demands, i.e., it varies directly with easier discriminations (e.g., Pritchard, 1981; Pelosi et al., 1992) and, vice versa, it varies inversely with factors that increase task difficulty such as decreasing stimulus presentation time and degrading the stimulus (Johnson, 1986). Thus, the greater P3b amplitude for HA groups, observed in the present study, is indicative of the greater ease or efficacy of information transmission of the HA group in making the frequency discriminations. According to Rockstroh and collaborators (e.g., Elbert, 1991; Rockstroh, Elbert, Canavan, Lutzenberger, & Birbaumer, 1989), slow positive components of the ERPs indicate a compensatory cortical inhibition of underlying circuitry to prevent for additional information intake from a new sensory input. Thus, a complementary interpretation of the P3b wave findings may lie on the greater capacity for inhibition of task-irrelevant information of the HA compared to the LA group. This interpretation is in line with the inhibition hypothesis of intelligence proposed by Dempster (1991). This author has outlined that intelligent behavior cannot be understood without reference to inhibitory processes and indicated the frontal lobe functions as responsible of both inhibitory as well as excitatory mechanisms. The findings that HA participants had more correct detections, shorter and less variable RTs with the tendency to respond slower to false positive detections, appear consistent with the hypothesis that inhibition is a major dimension underlying individual differences in mental ability. Correlation analysis yielded significant relationships between mental ability and both speed-of-processing and P3b measures. The direction of these relationships was in line with findings displayed by ANOVAs since for the shorter ISIs mental ability was negatively correlated with P3b latency and positively correlated with P3b amplitude at Fz, Cz, and Pz sites (see Table 4 and Fig. 3). On the other side, no robust relationships were found between P3b and behavioral measures for these ISIs. However, for the longest ISI (150-ms) there were significant positive correlations between RT, response variability, and midline P3b latency scores and significant negative correlations between these variables and the proportion of correct responses (see Table 4). Stepwise correlational analyses indicated that P3b latency on central location and frontal P3b amplitude were best predictors of mental ability at the medium ISI (50-ms). Moreover, at the shortest ISI (25-ms) only the frontal P3b latency resulted the best predictor and accounted the 45.5% of the total variance. This last finding indicates that, during tasks requiring higher processing speed, frontal processes accounted for individual differences in mental ability. 5. Conclusion In conclusion, the auditory oddball paradigm with backward masking was a good paradigm in evidencing the greater accuracy and faster speed of processing for HA than LA participants on a tonal frequency discrimination task. ERP analysis of performance demonstrates that both RT and P3b amplitude and latency measures derived solely from trials on which a correct response was made, reliably distinguish differences among individual differing in mental ability. P3b latency findings closely parallel effects observed with RT measures. Frontal P3b latency, and to a less extent, P3b amplitude, resulted the best predictor of mental ability that account for a substantial portion of the variance. The former is inversely related and the latter directly related to general intelligence. The P3b latency results complement the analysis of reaction time and mental ability in suggesting that a substantial portion of the variance in general intelligence is attributable to the speed and efficiency with which these cognitive operations are performed. Overall, the P3b findings of the present study parallels those previously reported by Bazana and Stelmack (2002). 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