Role of Habituation in the Irrelevant Sound Effect: Evidence From the Effects of Token Set Size and Rate of Transition

Transcription

1 Journal of Experimental Psychology: l~au'ning, Memory, and Cognition 1998, Vol. 24, No. 3, Copyright 1998 by the American Psychological Association, Inc /98/$3.00 Role of Habituation in the Irrelevant Sound Effect: Evidence From the Effects of Token Set Size and Rate of Transition S6bastien Tremblay and Dylan M. Jones Cardiff University The disruption of serial recall by irrelevant sound was explored by examining the effect of the number of different tokens (token set size) and by varying the transition rate between different tokens. Two sets of predictions were contrasted. One, based on changing state, posited that a mismatch in physical composition between immediately successive stimuli was the important factor, leading to the prediction that disruption would increase as the token set size increased from 1 to 2 but would show no increase above that. Another, based on habituation, predicted that increasing the set size would increase disruption monotonically, on the grounds that for a given exposure each token would be relatively less habituated. Generally, the results showed the most marked increase in disruption occurred when the token set increased from 1 to 2, giving some support to the changing state hypothesis. The disruptive effect of irrelevant sound on serial recall is well established (CoUe & Welsh, 1976; Jones, Madden, & Miles, 1992; Salam6 & Baddeley, 1982). Even though participants are instructed to ignore them, the presence of spoken words or other sound tokens produces appreciable disruption of serial recall. Recent work has attempted to establish the characteristics of the sound and those of the task that augment or diminish the effect (see Jones, 1995, for a review). The purpose of the present series of studies was to establish the degree to which the effects of irrelevant sounds are mediated either by habituation or by changes in state. Interest in the effect of background sound on performance is longstanding (e.g., Broadbent, 1958). Emphasis at a practical level has been placed on the likely impact of sound on the efficiency at the place of work or learning (e.g., Jones & Broadbent, 1991). Early work concentrated on meaningless sound, the main variable of interest being the effects of intensity, rather than meaning or spectral composition. The main explanatory construct was, therefore, behavioral arousal. However, the discovery that irrelevant verbal material could bring about losses of efficiency on the order of 30% on serial This research was supported by an Overseas Research Studentship (United Kingdom), by the Qutbee government via the funding body Formation de Cbercheurs et Aide ~ la Recherche, by the Defence Evaluation and Research Agency (Air Systems Sector), and by the Economic and Social Research Council of the United Kingdom. We thank Robyn Boyle, Karen Anderson Howes, Bill Macken, Phil Beaman, Jean St-Aubin, and David Alford for critical readings of a draft of this article. We also thank Romin Tafarodi, Jaekie Boivin, and Gordon Harold for statistical suggestions. Nelson Cowan suggested that we undertake Experiment 4. Correspondence concerning this article should be addressed to Stbastien Tremblay or Dylan M. Jones, School of Psychology, Cardiff University, P.O. Box 901, Cardiff CF1 3YG, United Kingdom. Electronic mail may be sent to tremblay@cardiff.ac.uk or to jonesdm@cardiff.ac.uk. recall at relatively low levels of sound intensity (Colic, 1980; Colle & Welsh, 1976) meant that the impact on efficiency was likely to be more widespread than was thought hitherto. Moreover, the fact that early studies indicated that the effect was confined to speech meant that emphasis was placed on constructs involving cognitive representation within memory (Cowan, 1988, 1995; Jones & Morris, 1992; Massaro & Cowan, 1993). Latterly, the finding that nonspeech sounds also produce the effect (Jones & Macken, 1993, 1997a; Jones, Macken, & Murray, 1993; Klatte, Kilcher, & Hellbrtick, 1995) has shifted the emphasis toward the linkage between organizational processes in audition (based on the auditory scene analysis framework of Bregman, 1990) and memory (see Jones, 1993; Jones, Beaman, & Macken, 1996). Indeed, it has been argued that the irrelevant sound paradigm may be used to address key issues in auditory attention that had been explored by the classical variants of the dichotic listening paradigm (Beaman & Jones, 1997). The research has implications, therefore, for a number of domains within psychology, particularly as they relate to the interplay of auditory attention and short-term memory. Several key characteristics of the irrelevant sound effect can be asserted and with a fair degree of confidence: First, and as already noted, intensity does not have an effect, at least within the range of db(a) (CoUe, 1980; Ellermeier & Zimmer, 1997; Jones, Miles, & Page, 1990); second, there is little effect of meaning--that is, meaningless speech is usually as disruptive as meaningful speech (Buchner, Irmen, & Erdfelder, 1996; Colle, 1980; Jones et al., 1990; Salamd & Baddeley, 1982); third, nonspeech sounds can produce as much disruption as speech sounds (Jones & Macken, 1993; Jones & Macken, 1997a; Jones, Macken, & Murray, 1993; Klatte & Hellbrtick, 1993; Klatte et al., 1995); fourth, disruption occurs not at encoding but at a later stage within memory, and therefore the effect is not one on the registration of stimuli but on their maintenance in memory (Baddeley &Salamt, 1986; Jones, 1994; Miles, Jones, & Madden, 1991; Salam6 & Baddeley, 1989); and 659

2 660 TREMBLAY AND JONES fifth, tasks involving rote rehearsal are particularly susceptible to disruption (Beaman & Jones, 1997, in press; Boyle & Coltheart, 1996; Salam6 & Baddeley, 1990; but see LeCompte, 1994, 1996). From the point of view of the current article, a further key characteristic--the changing state effect--is of particular interest. This refers to the well-established phenomenon that a sound sequence made by repeating a token produces small effects that are usually nonsignificant (see below for further discussion), whereas a sequence consisting of different tokens produces markedly greater disruption (Jones et al., 1992, 1993; LeCompte, 1994, 1995, 1996). These findings are embodied within the changing state hypothesis, which rests on two assumptions. The first is that the disruption of serial recall is the product of a conflict of cues to serial order between the sedation of items in memory and the automatic and preattentive sedation of irrelevant auditory events. The second is that information about sedation of auditory events arises from a mismatch in the physical characteristics between successive stimuli. The greater the mismatch, the stronger the cue to sedation, and in any given period the more frequently the mismatch occurs, the greater the number of cues to sedation (see Beaman & Jones, 1997, in press; Jones & Macken, 1995c), both of which increase the disruption of serial recall. A potential significant weakness in this approach is that some changing state phenomena can be incorporated satisfactorily within an entirely different conceptual framework, that of habituation to the orienting response (OR; see Sokolov, 1963). Classically, the OR is a response to novel or significant stimuli and is associated with a panoply of behaviors, including orientation of the head, behavioral quieting, parasympathetic activity, and preferential processing of the eliciting stimulus. Cowan (1995) proposed that the OR can serve to recruit attention away from the performance of a short-term memory task. Abstractly, the OR framework supposes that, with repeated presentations of a stimulus, a neural model is built up (see Sokolov, 1963). An OR may be elicited when the representation of the new stimulus fails to match a previously established neural model of elements within the stimulus sequence. As the neural model builds up, the discrepancy between the model and a repeated stimulus decreases, and therefore the OR habituates. In this way, the habituation hypothesis could explain the changing state phenomenon; that is, repeated sounds are habituated to very quickly, but those that change are habituated to more slowly, and hence continue to disrupt performance. Within the current experimental series, we make the assumption that there is a lawful relation between the magnitude of the OR and the number of different tokens in the stimulus sequence. If, as Sokolov (1963) proposed, each event contributes to the fabrication of a neural model, then for a given rate of presentation the number of tokens in the irrelevant stream should dictate the rate of habituation. That is, if one event is repeated, then only one neural model will be current, with successive events in the stream having progressively diminished ORs in view of their increasingly good fit to the model. An additional characteristic of the habituation hypothesis as it is used in the current article is that the fabrication of the neural model takes time and, hence, the degree of disruption should diminish with repeated presentation, specifically, that the disruption of serial recall by irrelevant sound shoum under some conditions show a relation to the degree of prior exposure. If one further assumes that several models may be simultaneously in the process of construction, over a given period of exposure each will receive relatively few tokens that give a good fit to the model, and hence ORs will be evoked more frequently. Even if there is only one model, the complexity of which varies as the number of tokens in the irrelevant sequence increases, then the same prediction holds; in this case, with larger numbers of tokens the rate of habituation will be slow because each token will be a relatively poor fit to the neural model. Perforce, this will mean that the recruitment of attentional resources that is the behavioral manifestation of the OR will endure. Experiment 1 In Experiment 1, some of the predictions of the habituation and changing state hypotheses are tested. Each theory may be couched in both strong and weak forms. A strong form of the habituation hypothesis may be taken to predict that disruption of serial recall will be a linearly increasing function of the token set size. That is, the degree of disruption increases by the same degree every time the token set size increases by one. Weak forms might not specify a linear increase, only a monotonic one. A strong version of the changing state hypothesis predicts that disruption will only increase appreciably as the number of tokens increases from one to two, with no increase in disruption as the number in the token set enlarges beyond that. However, the same pattern of results could be predicted by adding an auxiliary assumption to the habituation hypothesis, namely, that the transition from one to two mental models carries with it a disproportionate load for attention. A weaker version of the changing state hypothesis might predict a strong rise for the increase from one to two tokens and a more modest rise thereafter on the grounds that stimulus mismatch is derived from several successive stimuli. Another possibility is that the processing system may look for patterns in the random repetition of irrelevant sounds. Until such a pattern is found, it could be argued, habituation could not fully take place. This carries with it the assumption that it would be much harder to find such a repetition of two sounds than in repetitions of more than two sounds. However, the particular difficulty with two sounds seems rather arbitrary and, moreover, there is some evidence that fixed and random sequences of irrelevant sound have roughly a similar effect (Jones, Madden, & Miles, 1992), a result that suggests that the irrelevant sound effect is insensitive to suprasegmental features. Evidence pivotal to differentiating the two hypotheses should also be provided by the nature of the changes over the experimental session. It seems reasonable from the habituation hypothesis to expect both that the degree of disruption will diminish with repeated exposure and that the rate at which the disruption diminishes depends on the token set size, lower set sizes being associated with faster rates of

3 IRRELEVANT SPEECH AND HABITUATION 661 decline. By contrast, neither the decline nor its interaction with set size is a logical consequence from the standpoint of the changing state hypothesis, because the rate of change from one stimulus to another is all that counts according to the changing state hypothesis. A large pool of irrelevant material was used in this experiment, permitting the variation of items from trial to trial within one condition, to heighten the influence of novelty. The usual method of presenting auditory conditions is to randomize them on a trial-by-trial basis. This is a widely used methodology in studies of the irrelevant sound effect; its adoption in Experiment 1 was intended to safeguard against possible effects of habituation. Because the difference between steady state and changing state conditions was originally observed using these designs, it is then fitting that the framework be tested with these designs also. In the case of Experiment 1, this means that the predicted interaction between token set size and exposure might be attenuated relative to a setting in which trials were blocked by conditions. Method Participants. Twenty-four students, each a native English speaker, volunteered to participate in the experiment in return for course credit. All participants reported normal hearing and normal or corrected-to-normal vision. Apparatus and materials. The memory task took the same general form as in earlier studies (e.g., Jones & Macken, 1993), with sequences of to-be-remembered items--the nine consonants, f, k, l, m, q, r, s, t, and v----presented on the screen of an Apple Macintosh Pefforma 475. The sequences were constructed from random orderings of items, with the constraints that a consonant could not be presented in the same serial position in two consecutive lists and that recognizable words or acronyms would be excluded. Following presentation of the list, there was a delay during which participants were expected to rote rehearse the sequences; at the end of the interval they were instructed to write down the sequences in correct serial order. Irrelevant sound was played during the some of the trials. The main variable of interest was the number of tokens presented during a sequence---qhe token set size--with five levels. Set size was systematically varied between one, two, three, five, and seven tokens. For each of these conditions the tokens were drawn without replacement from a pool of 270 monosyllabic words (picked at random from the word frequency list of Gilhooly & Logic, 1980). All sequences of monosyllabic words were checked to ensure that there were no rhymes, no meaningful sentences, and no tokens starting with the same letter within a trial. In the two-token case, the tokens were alternated. The tokens were spoken in a male voice and recorded digitally to 8-bit resolution at a sampling of 22.5 KHz using Sound Designer II software (Degidesign, Menlo Park, CA), and all trials were stored as sound resources within SuperCard (Version 2.5, Allegiant, San Diego, CA). All words were edited using digital signal-processing techniques to last 400 ms, and the length of the silent gap between them was 100 ms. Thus, for every level of the token set, the tokens were played at a rate of two per second. Experimental design. In addition to five levels of the token set variable, a quiet control condition was included. A repeatedmeasures design was used in which the ordering of trims was prearranged quasirandomly such that they were presented randomiy over a block of 6 successive trials and in 6-trial blocks thereafter, each with a different random sequence. Participants received 15 trials in each condition, for a total of 90 trials. Procedure. Participants were tested individually, seated in a soundproof laboratory, at a distance of approximately 0.5 m from the computer screen. At the outset, participants were given standard instructions on the computer screen, informing them of the nature of the recall task and instructing them to ignore any sounds they might hear; participants were also informed that the sounds would not contain any messages and that they would not be tested on the sounds' contents. The experimental trials were preceded by a short practice session of three trials with no sound. Participants used a mouse to initiate the presentation of each list by clicking on a SuperCard button on the screen, and the consonants were displayed individually at a rate of one per second (on for 0.8 s, off for 0.2 s). When nine consonants had been presented, the word wait flashed for 10 s, during which participants were expected to rehearse covertly. The word recall was then displayed to prompt participants to write down the consonants on a response sheet. The irrelevant sound was played over headphones throughout presentation and rehearsal, and was switched off automatically during recall. The experiment took 60 min. Results The results were scored by a strict serial order criterion: That is, if a response was not in its correct serial position, it was counted as an error. A three-way repeated-measures analysis of variance (ANOVA) was carded out on the error data, with auditory condition (six levels), serial position (nine levels), and trial block (three levels) as variables. The 15 trials of each condition were grouped per block of 5 trials according to their order of presentation. It should be noted that because of the trial-by-trial randomization of the presentation of the auditory conditions, the 5 trials of any one condition were not contiguous. There were significant main effects of the three variables: block, F(2, 46) = 4.70, MSE = 5.44, p <.05; auditory condition, F(5, 115) = 20.30, MSE = 2.51, p <.0001; and serial position, F(8, 184) = 48.86, MSE = 6.70, p < The only interaction to reach significance was that between condition and serial position, F(40, 920) = 2.29, MSE = 0.86, p <.0001 (see Figure 1). No functional significance is attached to this interaction; it more than likely reflects a scalar effect, and furthermore no theory attaches a particular functional role to it. Post hoe pairwise comparisons of means (Neuman-Keuls procedure) showed that there were fewer errors in the quiet condition than in steady state and that there were fewer errors with one token than with two tokens, but above two tokens the number of errors did not increase significantly with the number of tokens. Generally the pattern in terms of errors is as follows: seven tokens ~ live tokens ~ three tokens ~ two tokens > one token > quiet (see Figure 2, in which Cohen's d values are given for adjacent contrasts; also, as elsewhere in the series, error bars represent withinsubjects confidence intervals (~t =.05), calculated using the method of Loftus & Masson, 1994). This pattern of results does not give clear support to the strong form of the habituation hypothesis; clearly, the rise was not linear. Arguably, the weak form was not supported either; although the rise in error above two tokens was

4 662 TREMBLAY AND JONES 80- :t 30 t _~/j~/l~ ---O---1Token 20 V// J ~, 3 Tokens 10 LI,L 5 Tokens Serial Position 7 Tokens Figure 1. The interaction of token set size and serial position in Experiment 1. evident, it was not of sufficient reliability to achieve significance, unlike the rise from one to two tokens. The strong form of the changing state hypothesis was not supported unequivocally; there was a weak trend for tokens above two, but a weaker form--predicting the largest increase as the number of tokens increased from one to two--seemed to be broadly supported by the findings. One possibility seems to be that there was a ceiling effect, but this seems remote given that the overall level of error was approximately 50%. The effect of trial block was examined to test whether the strength of disruption by any of the auditory conditions diminished during the duration of the experiment, that is, to examine whether trial block and auditory condition interacted. This interaction term was not significant, F(2, 230) = 1.08, MSE = 2.43, p >.05. As mentioned above, the randomization of trials precludes a test of this feature of the habituation hypothesis, namely, the interaction of trial block with token set size. Discussion The form of the function relating set size to disruption of serial recall supported the changing state hypothesis, albeit a weak version of it. Evidence from the way in which disruption stayed stable over successive periods of the task also suggests that habituation-like processes were not at work. In Experiment 1, the order of the irrelevant items was fixed, but this should not have influenced the outcome. Suprasegmental features, such as meaning or the order of the tokens, appeared to have little influence on the degree of disruption. For example, Jones et al. (1992) showed that fixed changing state sequences of syllables and randomized sequences of syllables disrupted serial recall performance to a similar degree. Moreover, a change in the order of stimulus presentation failed to evoke an OR, according to one study (Furedy, 1968). One initially puzzling feature of the results is that the effect of one token was significantly greater than quiet. If changing state is the primary determinant, why should a condition in which the same item was repeated--a steady state condition--produce a significant degree of deterioration? Part of the reason for this effect may be that the single token condition was not completely free of variation. Two sources of such variation can be suggested. The first is that at the beginning of each trial there is, of course, change from quiet. This, arguably, may be a change sufficiently large to disrupt performance, and may be especially important in the present case because the last item heard in the previous trial will be different from the first items of the current trial. The other possibility is that the information derived from stimulus mismatch was inherently noisy. However, the effect of a single token was sometimes significant and sometimes not and was typically very small in magnitude, which makes it rather difficult to study (see also LeCompte, 1995). The important issue for the changing state hypothesis is that the effect of more than one token is always appreciably greater than the effect of one token. The disruption of serial recall, independent of the number of tokens in the stream, did not show any attenuation over the three blocks of irrelevant sound trials. Throughout the "O-- Tones (Exp 2: n~48) I Speech (Exp I: n=24) L - 29 T.05 T.]] T i 1.02 T o.l.... ;, l Token Set Size Figure 2. Percentage of serial recall errors under different conditions of token set size in Experiments 1 and 2, pooled over serial positions and blocks. Cohen's d is given for adjacent contrasts within the body of the graph. Exp = experiment.

5 IRRELEVANT SPEECH AND HABITUATION 663 experimental session, the difference between multiple tokens and either a single token or the quiet condition was altogether marked and sustained. In terms of the action both of the token and of the pattern of interference over trial blocks, the evidence seems to favor the changing state hypothesis and is in line with previous work (Hellbrtiek, Kuwano, & Namba, 1996; Jones, Macken, & Mosdell, 1997). However, it is possible to argue that using a large pool of irrelevant stimuli could have concealed an attenuation of the irrelevant sound effect over trials, and that it is unlikely that a participant could habituate to a stimulus sequence when its content varies at every trial. If the mental models take appreciable time to build up, say an interval beyond the time for each trial, there would be little opportunity for each token to contribute to a unique model, and therefore the process would not be susceptible to habituation, Additionally, this effect may have been compounded by the complexity of the stimuli. As Sokolov (1963) suggested, neural models for simple representations, such as those for relatively simple tones, may be constructed more readily than those for complex words. Presumably, the faster a model is constructed, the sooner the process of habituation can get under way. Experiment 2 Experiment 2 used a small fixed set of tone stimuli with a view to extending the scope of the results of Experiment 1. Here, the simpler stimuli and the smaller pool of tokens should contribute to faster habituation. There are a number of studies that have shown an effect of tones and nonverbal stimuli on serial recall (Jones & Macken, 1993; Jones et al., 1993; Klatte et al., 1995); hence, the predictions are the same as for Experiment 1. Method Participants. Forty-eight undergraduate students, each reporting normal heating and normal or corrected-to-normal vision, received either course credit or a small honorarium for participating in the experiment. Apparatus and materials. The memory task was the same as that for Experiment 1. Tones were used instead of words as auditory irrelevant material; 18 different tones were selected quasirandomly from a range of eight octaves. Token set size was varied, using one, two, three, five, or seven tokens. Within each sequence, the tones adjacent in pitch were different by an octave. In the case of more than two tokens, ascending and descending sequences were avoided by randomizing the order of the tones (once randomized, the order remained fixed for the experiment). Five different irrelevant sequences (given here in their randomized order) were contrasted: one token (294 Hz), two tokens (220, 440 Hz), three tokens (349, 698, 175 Hz), five tokens (196, 784, 392, 1568, 98 Hz), and seven tokens (660, 330, 165, 2640, 83, 41, 1320 Hz). Each particular tone sequence was fixed for each condition. Thus, there was no variation of the irrelevant material within a condition. Square wave tones were generated to 8-bit resolution at a sampling of 22.5 KI-Iz using Sound Edit Pro (Version 1.0, Macromind and Paracomp, San Francisco, CA) software. Square waves include the fundamental tone frequency and all of its odd harmonics (Rosen & Howell, 1991) and tend to produce a sound of more strident timbre than pure tones (Handel, 1989). Every tone was edited to last exactly 400 ms, and the length of the silent gap between them was 100 ms. Each tone had a rise and fall time of 20 ms. Thus, as for Experiment 1, the recorded tokens were played at a rate of two per second. A quiet condition was used as a control. The experimental design and procedure were identical to those used in Experiment 1. Results As in Experiment 1, a 6 (auditory condition) x 9 (serial position) X 3 (block) repeated-measures ANOVA was undertaken on the serial recall error data. As before, there was a significant main effect of the three variables: block, F(2, 94) , MSE = 4.70, p <.001; auditory condition, F(5, 235) = 9.75, MSE = 3.15, p <.0001; and serial position, F(8, 376) , MSE = 8.03, p < There was no significant interactions (Fs ~- 1). The Geisser- Greenhouse procedure, which provides a conservative F test (Winer, 1971), was used for this ANOVA because of the low homogeneity of covariance (these stricter Fs were also used in Experiments 4 and 5 for the same reason). The serial position effect took the usual form, with 15.2% errors at the first serial position, with errors peaking at 56.1%, and with errors decreasing to 46.8% at the last serial position. Orthogonal planned comparisons (using Helmert transformations) were performed on the error scores. This procedure compares one level of a repeated-measures variable to the mean of subsequent levels and is a useful device for discovering at which level recall error performance ceases to change. The analysis for Experiment 2 revealed that recall performance in the quiet condition, F(1,235) , p <.001, and in the one-token condition, F(1,235) = 13.25,p <.001, was significantly different from that of the subsequent token conditions. All subsequent comparisons---within the two-, three-, five-, and seven-token conditions---were not significant (p >.1). The pattern of significance resembled that in Experiment 1 (seven tokens ~- five tokens ~= three tokens ~ two tokens > one token > quiet; see Figure 2). For Experiments 1 and 2, trend analyses (orthogonal polynomials) were carded out on the data under the different auditory conditions of the token set size. The function relating token set size and degree of disruption appears to be better described by a composite of trends (see Keppel, 1988). Both Experiments revealed a significant departure from linearity (see Kirk, 1982): Experiment 1, F(1, 92) = 6.84, p <.05, and Experiment 2, F(1, 188) = 3.95, p <.05. As indicated by curve fitting (performed on each curve in Figure 1), the best estimate of the underlying relationship is an inverse (Ay-- 1/x) function (Experiment 1: R 2 =.95; Experiment 2: R 2 =.98). Discussion The pattern found with words drawn from a large corpus (Experiment 1) was repeated with tones drawn from a small corpus. Granted, the effect of stimulus type (tone vs. word) and corpus size are confounded, but the key characteristic of Experiment 2 is the similarity of the shape of the relationship between token set size to the degree of disruption. This

6 664 TREMBLAY AND JONES suggests a functional similarity in the action of tones and speech (see Jones & Macken, 1993; but see LeCompte, Neely, & Wilson, 1997). Just as in Experiment 1, the difference between the effect of the number of tokens remained constant over successive blocks of trials, which can be taken as not supporting the habituation hypothesis. However, as before, it is possible to argue that this comparison is a somewhat unfair test of habituation because auditory conditions were randomized, and hence the opportunity to build up a stable representation of the auditory stimuli was rather limited. Experiment 3 addressed this issue by blocking the auditory conditions. It is important to acknowledge at this point that the design of Experiment 3 represents a departure from the concerns that were the initial motivating force for the current series. One of the main purposes of the series was to understand the body of data that consistently showed a changing state effect. These data were derived from designs in which conditions were randomized as a means of minimizing any possible effect of habituation. A blocked design diverges from this tradition, and the relevance of the result to the original purpose of the series is potentially doubtful. One previous study had examined the effect of blocking auditory conditions and found no evidence of habituation-like diminution in response over 20 trials (Jones et al., 1997). The main purpose of Experiment 3, therefore, was to contrast critical conditions of Experiments 1 and 2 with an extended exposure to the token set but with a design in which conditions are blocked. Experiment 3 In this experiment, three critical auditory conditions from the previous two experiments were examined in relation to error in serial recall: The effects of one, two, and seven tokens were compared with quiet. Given that Jones et al. (1997) found no evidence of habituation with speech over exposure to 20 consecutive trials, the number of trials was increased to 30. Further, to avoid the possible contaminating effects of other conditions in which competing neural models are built up, a between-subjects design was used. The main prediction from the habituation hypothesis is that the degree of disruption should diminish as the exposure increases, and further, that this should interact with token set size so that the rate of decline in the level of disruption will be greater for two tokens than for seven tokens over the 30 trials. Unlike in Experiment 1, but like in Experiment 2, the identity of the tokens remained fixed in each condition. The rationale for this was the same as for Experiment 2, namely that according to the habituation hypothesis it should promote the fabrication of few, relatively enduring neural models. Me~od Participants. Eighty undergraduate students were given either course credit or a small honorarium for their participation in the experiment (20 participants were assigned randomly to each of four conditions). All reported normal hearing and normal or correctedto-normal vision. Apparatus and materials. The memory task was identical to that used in the other experiments of the series. Three conditions containing irrelevant sound, this time speech, were contrasted: a sequence of one token (the word cent), a sequence of two different tokens (the words car and verb), and a sequence of seven different tokens (the words turn, kilt, band, jaw, fruit, rod, and porch). As before, the recorded tokens were played at a rate of two per second. Experimental design. A mixed design was used in which auditory condition represented a between-subjects variable, and block of trials and serial position both represented within-subject variables. Participants received five blocks of six trials of one condition. Four conditions were contrasted: three in which sound was presented (one, two, and seven tokens), and a quiet control. Procedure. This was identical to that used in Experiments 1 and 2, except that participants undertook 30 trials of only one of the four conditions. As in previous experiments, the sound was not played continuously over trials. The procedure took 30 min. Results In this analysis, 30 trials were divided into five blocks of six trials to examine changes over each block. Serial order error data were submitted to a three-variable (4 5 X 9) ANOVA with one between-subjects variable (auditory condition, four levels) and two within-subject variables (block, five levels; serial position, nine levels). Auditory condition produced a significant effect, F(3, 76) = 11.25, MSE = 38.15, p <.0001, as did serial position, F(8, 608) = 87.51, MSE = 2.71, p <.0001, and block, F(4, 304) = 10.07, MSE = 3.71, p < There were no significant interactions (all Fs ~- 1). The effect of serial position took roughly the same form as others in the current series, with 21.7% errors at the first serial position, rising to 60.0% and eventually falling to 47.4% at the last serial position. An orthogonal set of planned comparisons indicated that performance in the two-token condition was worse than in the one-token condition, F(1, 76) = 9.09, p <.01, Cohen's d = 1.02; but comparisons between quiet and one token, F(1, 76) = 0.07,p >.05, Cohen's d =.07, and also between two tokens and seven tokens, were not significant, F(1, 76) = 2.54, p >.05, Cohen's d =.63 (where significance is reached at d =.77). Although the contrast between two tokens and seven tokens presents a medium effect size, the difference is modest relative to the large effect size of the critical contrast between one token and two tokens. This pattern of results (seven tokens ~ two tokens > one token ~- quiet) supports previous results (with the exception that the difference between one token and quiet is nonsignificant). Because there were only three levels of token set size, and because these levels were not equally spaced, no analyses of trend were performed (see Keppel, 1988). A greater exposure to the irrelevant material (30 trials instead of 20), and the fact that conditions were blocked by groups, should have provided more opportunity for habituation, but there was no evidence of an habituation-like effect. Following predictions related to the framework of Sokolov (1963), an interaction between block and condition was expected, but this interaction did not even approach significance (F < 1). The significant effect of block could be explained by a practice effect or an improvement in doing

7 IRRELEVANT SPEECH AND HABITUATION 665 the recall task whatever the condition. Figure 3 shows the performance over time, with no evidence of an habituation effect: Differences between two tokens and quiet and between seven tokens and quiet remained numerically steady over the five blocks of trials. Discussion In terms of the effect over blocks, the results of Experiment 3 converge with the results of Experiments 1 and 2 of the current series and also with results found elsewhere. Hellbriick et al. (1996) found changing state speech disruption within- and between-experimental sessions, thus suggesting a long-term stability of the detrimental effect. Jones et al. (1997) modified a preexposure method previously used by Morris and Jones (1990) and found that after a 20-rain exposure to changing state material, the same material still disrupted serial recall performance. The first three experiments of the series show that the most marked increase in disruption occurred as the number of tokens increased from one to two. Although this is encouraging for the changing state hypothesis, though by no means wholly supportive of it given the small numerical increase in error as the token set rises above two, it seems reasonable at this stage to seek further evidence from a different experimental manipulation. In Experiment 4, therefore, a slightly different approach was adopted. The action of two variables was investigated. The first relates to the rate of transition between different tokens and the second relates to the predictability of the token transitions. Two rates of transition were compared: The token (word) was changed either at a low rate, every three stimuli (e.g., AAABBBCCC tg T I ----4D---- Quiet >--- 1 Token + 2Tokens + 7Tokens T T T_ T I 1! 3 I 4 ; Block (6 trials) Figure 3. Percentage of serial recall errors in relation to successive blocks of trials under auditory conditions in Experiment 3. 1 etc.) or at a high rate, after every stimulus (e.g., ABC etc.). The changing state hypothesis predicts that as the rate of transition increases so does the degree of disruption; on average the degree of mismatch between two successive stimuli is greater in the case of a high transition rate, which will therefore produce a greater degree of disruption. The predictability of token transitions was studied by comparing sequences of three tokens that appeared either in a fixed order (ABCABCABC etc.) or in a random order (ACB- CABABC etc.). According to the changing state hypothesis, two conditions sharing the same rate of transition, irrespective of the overall sequence of the tokens, should produce broadly similar effects on the grounds that the crucial variable is the contrast between successive tokens, not the repetition of the supratoken pattern (a result that has some precedent: Jones, Madden, & Miles, 1992). Experiment 4 In Experiment 4, the effect of rate of transition (contrasting two conditions, high and low rate) and predictability (two conditions, fixed vs. random) were compared. In each case, the auditory sequences were based on a common token set (in terms of both set size and rate of presentation). In addition, two control conditions were included, a steady state condition, in which the same token was repeated, and a quiet condition. Method Participants. Forty undergraduates who received either course credit or a small honorarium participated in the study. They were all native English speakers and reported normal hearing and normal or corrected-to-normal vision. Apparatus and materials. The memory task was the same as that for the three previous experiments. Five conditions were used in total. In three of the conditions, auditory sequences were constructed with a different order of the same three monosyllabic words (turn, kilt, and band). The words were chosen to be phonologically distinct. Rate of transition was manipulated by varying the degree of repetition of words; they were either repeated three times before a change or changed from token to token. The predictability of the sequence was changed by having either a fixed sequence of words or by having a quasirandom sequence (the sequence was constrained so that no two successive tokens were identical). Two control conditions were used: a steady state sequence with one word (/aw) repeated and a quiet condition. Auditory stimuli were recorded and presented as in Experiment 1. Experimental design. A repeated measures design was used, in which the presentation of conditions was blocked in groups of 15 trials and the order of conditions was dictated by the order given in a Latin square. Participants received 15 trials in each condition, for a total of 75 trials. Procedure. This was identical to that used in Experiments 1 and 2, except that there were five conditions and trials were blocked. For each participant, the experiment took 50 rain. Results and Discussion The means of number recalled correct as a function of the auditory conditions are shown in Table 1. In numerical

8 666 TREMBLAY AND JONES Table 1 Mean Percentage of Errors Under Conditions of Experiment 4 Condition M SE Quiet control Steady state Low transition High transition (fixed) High transition (random) terms, the experiment resulted in the following pattern: high transition > low transition > steady state. However, some critical tests of these differences were not statistically significant. As revealed by a 5 (auditory condition) x 9 (serial position) 3 (blocks of trials) repeated-measures ANOVA, all three main effects were significant: auditory conditions, F(4, 156) = 7.03, MSE = 5.43,p <.001; serial position, F(8, 312) = 71.19, MSE = 4.96, p <.0001; and block, F(2, 78) = 4.36, MSE = 3.25, p <.05, but no interaction was significant (Fs ~ 1). The serial position effect was similar to that found elsewhere, with 16.2% errors in the first serial position, rising to 61.7% errors later in the list and falling to 51.1% at the final serial position. According to planned comparisons within the auditory conditions variable, the effect of predictability was small and nonsignificant. Indeed, the effect of fixed and random sequences on errors appears to be nearly identical, F(1, 156) = 0.72, p =.79, Cohen's d =.04. Both the fixed and random conditions were significantly different from the steady state condition, F(1,156) = 10.52,p <.001, Coben's d =.52. As in many irrelevant speech experiments, the contrast between quiet and steady state conditions failed to produce a significant difference, F(1,156) = 3.30, p =.07, Cohen's d =.29. The effect of rate of transition, pivotal to the predictions of the changing state hypothesis, was revealed by comparing the high transition conditions (fixed or random) or steady state sequence conditions with the low transition condition. The order of means shows an increasing degree of error going from steady to low transition to high transition. However, only the contrast between low transition versus steady state was significant, F(1, 156) = 4.20, p <.05; the contrast of low transition with high transition (random and fixed pooled scores) was not significant, F(1, 156) = 0.82, p =.36. The overall ordering of conditions in Experiment 4 was as predicted by the changing state hypothesis: A small effect of steady state compared to quiet, an increase in errors for conditions in which the rate of transition of tokens was low, and errors increasing still further for high transition sequences, but increasing the unpredictability of these high transition sequences did not further augment the disruption. However, critical tests of significancewbetween low and high transition sequences--did not lend sufficient weight for this pattern of results to be taken as support for the changing state hypothesis. Clearly, the results of Experiment 4 are as equivocal with respect to the changing state hypothesis as they are to the habituation hypothesis. In Experiment 5, further convergent evidence was sought for the idea that transitions within the auditory stream are the important determinant of disruption by irrelevant sound. Instead of manipulating the number of transitions by modifying the auditory sequence, the number of tokens per unit of time was manipulated. Experiment 5 For an auditory sequence in which there is change, the degree of disruption is dependent on the word dose, in other words, on the number of tokens presented during the course of a trial (Bridges & Jones, 1996). According to the changing state hypothesis, the more occasions on which there is a change between two successive tokens, the greater the disruptive effect of irrelevant sound; that is, more cues to seriation with high dose results in greater disruption of serial recall. However, the opposite result can be predicted from the standpoint of habituation. Simply, the more times a stimulus is repeated, the quicker the OR should habituate and, hence, the disruption of serial recall should be reduced in high dose conditions. To test this prediction, we examined in Experiment 5 the interaction of word dose and the token set size in a sequence of sound. In essence, the 2 2 interaction of dose (high vs. low) and token set (two or six tokens) is a pivotal element of the analysis. It is possible to suggest that the habituation hypothesis will predict a significant interaction. It might be that the disruptive effect of irrelevant sound will be greater for the low dose at the slowest rate and, correspondingly, it will be lower for the highest dose at the fastest rate. By contrast, the changing state hypothesis can be taken to predict no significant interaction: There will be a main effect of dose, and no effect of token set size (remembering the precedent from earlier experiments in the series contrasting two- and six-token sets). In addition to the conditions comprising the critical 2 2 interaction, two control conditions (quiet and single token, but only at high dose to reduce the overall numbers of treatments to comfortable levels for participants) were also used. Method Participants. Twenty-four native English speakers volunteered to take part in the study in return for a small honorarium. All participants reported normal hearing and normal or corrected-tonormal vision. Apparatus and materials. The memory task was the same as that in previous experiments. Two changing state speech sequences were used, namely, a sequence of two different tokens (the words car and verb) and a sequence of six different tokens (the words turn, kilt, band, jaw, fruit, and rod). The variable, number of tokens in a stream (two or six tokens), was combined with the word dose (high or low) variable. All words were edited, using digital signal-processing techniques, to last 400 ms, but the length of the gap between them was varied: 100 ms for the high-dose sequences and 600 ms for the low-dose sequences. Thus, for the two high-dose conditions the recorded tokens (two and six different tokens) were played at a rate of two per second, and for the two low-dose conditions the

9 IRRELEVANT SPEECH AND HABITUATION 667 recorded tokens were played at a rate of one per second. The control conditions consisted of a quiet condition and also a steady state condition with one word (cent) repeatedly played at a rate of two tokens per second (the high-dose rate). Experimental design. A repeated-measures design was used, in which the presentation of trials was blocked. Six auditory conditions of 15 blocked trials were contrasted: quiet, one token at high dose, two tokens at low dose, two tokens at high dose, six tokens at low dose, and six tokens at high dose. The order of conditions was randomized between subjects in such a way that each condition was presented at each position the same number of times. Participants undertook 90 trials in all. Procedure. This was identical to that used in Experiments 1 and 2, except that there were six conditions and trials were blocked. The experiment lasted 60 rain , 30, [] 2 Tokens Results As in previous experiments, participants' responses were scored according to the strict serial recall criterion. An overall analysis incorporating all of the auditory conditions was computed first. This involved a 3 (block) 6 (auditory condition) 9 (serial position) repeated-measures ANOVA. There were significant effects of auditory condition, F(5, 115) = 13.23, MSE = 5.25, p <.0001, and serial position, F(8, 184) = 35.91, MSE = 3.62,p <.0001, and a modestly significant effect of block, F(2, 46) = 4.99, MSE = 2.29, p <.05. Except for the significant interaction between auditory condition and serial position, F(40, 920) = 1.98, MSE = 0.85, p <.05 (see Figure 4), there were no significant interactions (Fs ). As revealed by a test of contrast with quiet, the one-token condition showed a Quiet + I Token 70-2 Tokens - Low dose ~ 2 Tokens - High dose 6O 6 Tokens - Low dose --~ 6 Tokens - High dose 20, Low Dose Auditory Conditions High Dose Figure 5. Percentage of serial recall errors under dose and token set size conditions in Experiment 5, pooled over serial position and blocks. numerically very small and nonsignificant disruptive effect on serial recall, F(1, 23) = 1.72, MSE = 2.59, p >.05, Cohen's d =.26. The data were then submitted to a 2 (number of tokens: two and six) 2 (word dose: low and high) repeatedmeasures ANOVA. A significant effect of word dose was obtained, F(1, 23) = 13.62, MSE = 51.41, p <.005, Coben's d =.75, and the effect of the number of tokens was nonsignificant, F(1, 23) = 0.68, MSE = 58.77, p >.05, Cohen's d =.17. Of particular importance, there was no interaction between the number of tokens and the word dose (F < 1; see Figure 5). Discussion lo Serial Position Figure 4. The interaction of serial position and auditory conditions in Experiment 5. The results of Experiment 5 reinforce the conclusion that word dose is an influential determinant of the irrelevant sound effect (Bridges & Jones, 1996). This in itself constitutes evidence that is difficult to explain by the version of the habituation hypothesis used here; as the rate goes up, the fabrication of the neural model should be accelerated and, hence, the OR should become progressively diminished. This result can be predicted by the changing state hypothesis; however, more frequent changes between tokens provide more cues to seriation and, hence, stronger interference with serial recall (see Bridges & Jones, 1996, for a discussion). A second feature of the data--the failure of word dose to interact with set size--is also difficult to explain with the habituation hypothesis. It was expected that the rate of habituation would be greater when there were fewer different tokens (when the set size was small) and that this trend would be more marked when the dose was higher simply on

10 668 TREMBLAY AND JONES the basis that there were more tokens of a particular class at the higher dose to help fashion neural models. Therefore, these findings generally give some support to the changing state hypothesis: A change between two successive sound units appears to be the main precondition for the disruptive effect of sound on serial recall; moreover, the more changes that occur during the time course of a trial, the higher the degree of disruption, these effects being independent of each other. As in Experiment 3, the contrast between one-token and quiet conditions was numerically very small and statistically nonsignificant. These results might be taken to suggest (at least tentatively) that significant differences between the one-token and the quiet conditions are characteristic of randomized designs, and not of blocked designs. What this means in more abstract terms is not clear, but one common denominator for a randomized design in contrast to a blocked design is that the auditory stimuli at the end of one trial are different than those at the beginning of the next. So, for a one-token condition in a randomized design, the first few stimuli represent a contrast to the last auditory stimuli that were presented, and in a blocked design they do not. In future studies, it may be useful to pursue these ideas beyond the level of conjecture. At present they serve as interesting observations that are somewhat tangential to the main thrust of this article. General Discussion We may summarize the key features of the experimental series as follows. Experiment 1 showed that disruption of serial recall increased as the number of tokens in the irrelevant stream increased from one to two, but as the number of tokens increased above that, to three, then to five and to seven, there was a small numerical increase in disruption that was nonsignificant. When performance was charted in terms of the successive trials in each condition, this effect of tokens in relation to the quiet and the one-token condition remained stable. Experiment 2 used a small set of tones to test the generality of the effect found in Experiment 1, and the effects in form (but not in magnitude) were almost identical. The results of Experiments 1 and 2 are generally, but not completely, in line with the changing state hypothesis. The largest increase in disruption occurred when the set size went from one to two; there was a small rise in disruption thereafter, as token set size increased, but the rise was nonsignificant. Even though the pattern of statistical significance is in line with the changing state hypothesis in interpreting the outcome of Experiments 1 and 2, we place greater emphasis on the shape of the function. The fact that we obtained the same shape of function for tones and for speech we take as an illustration of the functional equivalence of these two types of stimuli (Jones & Macken, 1993). In addition, we draw reassurance from the fact that the tones shared the same shape of function, while being at a lower overall level than the speech tokens, insofar as they logically could not both be a result of a ceiling effect Experiment 3 useda design in which the experimental conditions were blocked; this was designed to increase the likelihood that habituation would occur, but again in their general form the outcome was the same as in Experiments 1 and 2. In contrast, Experiment 4 offered little support for the changing state hypothesis. As the rate of transition was varied, the degree of disruption did not vary significantly. Only the prediction that fixed and random sequences yield identical degrees of disruption is borne out by the data, a finding to which the habituation approach does not directly speak. Much further work is required to reconcile these data with the others in the series, possibly by using the parametric approach to construct some kind of function following the general approach of Experiments 1 and 2 of the current series. As they stand, the results of Experiment 4 detract from the overall coherence of the current series, but they constitute a salutary sign that further work is required on refining the analysis of the rates of transition. Finally, in Experiment 5, the interplay of two variables-- the dose and the token set size within the auditory sequence-- was investigated. It was suggested that from the habituation viewpoint, the two variables should interact, with less disruption of serial recall when the dose was high and the token set was small. By contrast, the changing state hypothesis predicted that there should be no interaction, only a main effect of dose and no effect of token set (remembering that in Experiment 5 the crucial test of the hypothesis two tokens were contrasted with six tokens). The results were in line with the changing state hypothesis. In none of the experiments in the current series is there any evidence of a change in the magnitude of the difference between the conditions as a function of the number of trials undertaken. This is true whether conditions were blocked or randomized. Generally, therefore, there seems to be no evidence of diminution of the irrelevant sound response over time. When the effect of irrelevant sound is charted on a trial-by-trial basis, a similar picture emerges (Hellbriick et al., 1996), which suggests that the process of pooling the results of trials, which has been the common procedure in the current series, has not concealed a very short-lived habituation effect. Whether diminution over time is a critical prediction of the habituation hypothesis is difficult to judge given that classic experiments on habituation of the OR use stimuli that occur far more infrequently than is typical of the irrelevant sound paradigm. If nothing else, however, the results over blocks show that the effect is not evanescent, which in turn means that the irrelevant sound effect may not be an epiphenomenon of laboratory life and therefore will have implications for practical settings. The main impact of the current findings is to provide some support for the changing state hypothesis, but not unequivocal support in view of the outcome of Experiment 4. Just why the effect of rate of transition failed to produce the expected outcome is not clear, and as it stands the result can be regarded as being moot, in view of the fact that it also failed to give unequivocal support for the habituation hypothesis. On the basis of the current series, further work in clarifying the effect of token set size and particularly transition rate seems warranted. In addition, further refinement of habituation theory is also needed. Jones et al. (1997) have already discussed the main difficulties with the applica-

11 IRRELEVANT SPEECH AND HABITUATION 669 tion of habituation-based theories to the irrelevant sound paradigm; they are, therefore, only summarized here. Apart from the problems with the clarity of articulation of the theory (see Gati & Ben Shakhar, 1990, for a critique), there are doubts based on empirical evidence as to whether habituation of the OR occurs when the participant is undertaking a demanding information-processing task. Most observations of the OR have been undertaken in settings in which the participants have been listening passively. Studies of the OR in settings in which attention is directed away from the sound to some information-processing task have yielded equivocal conclusions. Indeed, Ohman (1979) concluded that "habituation to the OR... requires central processing capacity [so that] little habituation of the OR would occur in situations involving heavy processing demands because of subsidiary tasks" (p. 466). The controversy over habituation of unattended stimuli is particularly acute in studies of evoked potentials. Typically, participants engage in a reading task while recordings are made of the evoked responses of the brain to irrelevant auditory stimuli. Research has been focused on the component known as "mismatch negativity," which is evoked to a stimulus distinct in some acoustic feature from a stimulus or stimulus sequence that preceded it (Lyytinen, Blomberg, & Nii~t~nen, 1992; N~i~t~en, 1990, 1991, 1992), usually a few seconds before (MtLntysalo & N~t~inen, 1987). The similarities of this domain of research to that of irrelevant sound are striking, both empirically and conceptually, but for the purposes of the present discussion it is important to note that here too it has been suggested that habituation without attention does not occur (Trejo, Ryan-Jones, & Kramer, 1992; Woldorff et al., 1993), although it should be noted also that this the issue is controversial, at least for pitch (Alho, Woods, & Algazi, 1994). The impact of the current findings on theory is not confined to habituation and to the changing state hypothesis, however; the results also run counter to the application of temporal distinctiveness theory to the irrelevant sound effect (Jones & Macken, 1997b; LeCompte, 1996). The temporal distinctiveness account is based on the idea that retrieval of an item depends on the search set within which it is contained (see Glenberg, 1987; Glenberg & Swanson, 1986). If the item shares the search set with others, such as might result from irrelevant sounds at the time of encoding of a to-be-remembered item, then it becomes more difficult to retrieve. However, to explain the greater disruption by changing as opposed to steady state sequences, the theory has to incorporate the auxiliary assumption that "repetitions of the same item (or different items that share many characteristics) will overload the temporal search set, but this overload will be limited by the large redundancy inherent in that repetition" (LeCompte, 1996, p. 1163). One prediction that should follow from the redundancy assumption is that of a monotonic increase in disruption with an increase in set size, an expectation that is not strongly supported by the results of the current series. Little has yet been made about the mechanisms relating the stimulus mismatch to the resulting disruption of serial recall. Essentially, the disruption of serial recall is argued to arise from a conflict of order cues. These cues are derived from two sources: one stemming from the stimulus mismatch of sounds in the auditory domain, and the other from the linkages between items in the to-be-remembered stream. The effect stems not from a similarity of content of the irrelevant and to-be-remembered information (as Salam6 & Baddeley, 1982, 1989, have suggested); rather, it stems from similarities in process, related to the representation of order. That the effect is not due to the similarity of content has been demonstrated in a variety of ways; for example, the similarity explanation is contraindicated by the mere fact that significant degrees of disruption are produced by an array of nonspeech sounds such as tones (Macken & Jones, 1995), pitch glides (Jones et al., 1993), or bursts of noise (Jones & Macken, 1997a) with minimal similarity to the syllables of the memory task. The same conclusion can be reached if the similarity of the content of the auditory sequence is manipulated systematically so that it is either similar to the to-beremembered sequence or to other tokens in the auditory sequence; in this case, similarity within the auditory sequence seems to dictate the disruption (Jones & Macken, 1995c). The strong effect of within-stream similarity as opposed to between-stream similarity resonates well with the results of the current series, given that we conclude that similarity within the token set is the primary determinant of disruption. Although it seems plausible to talk about linkages between items in terms of deliberate rehearsal of verbal item sequences in a short-term memory paradigm--indeed, they form a central plank of several models of memory (see, e.g., Murdock, 1993, 1996)--proposing a kindred mechanism for generating linkages amongst unattended auditory stimuli seems at first to be rather curious. The key to understanding the role of a linkage mechanism for the auditory material lies in the realization that, even when unattended, sounds are organized into sueams on the basis of their physical properties, just as when they are the subject of directed attention (a phenomenon referred to as auditory streaming; see Bregman, 1978, 1990). Several studies have now demonstrated that the degree of disruption by irrelevant sound can be modulated by organizational factors such as the physical location of the sound (see Jones & Macken, 1995a, 1995b). Therefore, part of the action of information processing of the unattended sound is its organization into streams; our speculation is that the process of stimulus mismatch contributes to the formation of auditory streams. The fine-grained detail of the action of such a mechanism has not been worked out, but early speculations suggest that a process of correlation between successive tokens may be responsible. It seems plausible that these specific effects within the irrelevant sound paradigm inform the general understanding of cognitive architecture. Results from the irrelevant sound paradigm are in line with the idea that short-term memory phenomena are best regarded as an activated subset of long-term memory, rather than as a distinct repository to and from which representations are transported (see also Cowan, 1988, 1995). The demonstration that the damage due to irrelevant sound occurs in a postencoding stage testifies as much (Miles et al., 1991). If sounds within the irrelevant sound paradigm are not deliberately attended to, then in

12 670 TREMBLAY AND JONES some sense they are simply registered, and this could conventionally be regarded as a function restricted to the perceptual level of representation. By contrast, the processing undertaken in a serial recall task is under volitional control; moreover, the material is represented at the postcategorical level, and this would be regarded as a memory effect. If we accept this characterization of each factor, then we have evidence of interference between two activities that apparently occur at different stages in the processing chain. One of the main ways in which apparent conflict could be reconciled is by supposing that in fact the distinction between perception and memory is a false one, and that irrelevant sound interferes with the process involved in the activation of representations. 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