Capturing the Suffix: Cognitive Streaming in Immediate Serial Recall

Transcription

1 Journal of Experimental Psychology: Learning, Memory, and Cognition 2002, Vol. 28, No. 1, Copyright 2002 by the American Psychological Association, Inc /02/$5.00 DOI: // Capturing the Suffix: Cognitive Streaming in Immediate Serial Recall Alastair P. Nicholls and Dylan M. Jones Cardiff University Adding an irrelevant item to the end of an auditory to-be-remembered list increases error on the last list items appreciably, known as the suffix effect. The phenomenon of auditory capture (e.g., Bregman & Rudnicky, 1975), namely, the tendency for a sequence of similar items to form a stream that at the same time isolates perceptually dissimilar members of the sequence, is exploited to explore the suffix effect. Irrelevant items interleaved between to-be-remembered items are used to capture the suffix with the aim of reducing its impact. Four experiments illustrate how the properties of the irrelevant sequence promote capture. The results are problematic for models of the suffix that involve masking of the last list item; instead, models based on grouping are favored. If a spoken list is followed by an irrelevant spoken item then serial recall of the last items in the list is impaired. This, the suffix effect, is one of the canonical features of short-term recall (Crowder & Morton, 1969) and has been used to explore the cognitive underpinnings of auditory memory. Usually, auditory serial recall is superior to visual serial recall (but only in that last part of the list, known as recency), and a suffix on an auditory list diminishes this advantage. Theoretical interest has centered on explaining the action of the suffix and relating it to the functional differentiation of sensory modalities. Perhaps the most pervasive view is that the suffix overwrites (or masks) similar acoustic elements in the terminal item (e.g., Crowder & Morton, 1969; Nairne, 1990; Surprenant, LeCompte, & Neath, 2000). An alternative view is that similarity of the suffix to the terminal item promotes grouping of the suffix with the list and this modifies the encoding of order (e.g., Frankish, 1989; Frick, 1988b; Kahneman, 1973). This article advocates the second view. The experiments reported here were designed to show that the impact of the suffix may be altered appreciably by factors that influence perceptual grouping of sequences into streams. Interleaved with to-be-remembered items was a sequence of irrelevant events whose content and timing were designed to form a stream separate from the to-be-remembered stream and one that captured the suffix. The experiments were undertaken in the light of our claim that the mere presence of the suffix immediately following the ultimate list item does not constitute a sufficient condition for a suffix effect; the suffix has to also be a part of the perceptual stream formed by the to-beremembered list. Alastair P. Nicholls and Dylan M. Jones, School of Psychology, Cardiff University, Cardiff, United Kingdom. Alastair P. Nicholls was supported by studentships from the United Kingdom s Biotechnology and Biological Sciences Research Council and the Thomas Wilson Educational Trust. Dylan M. Jones is an Adjunct Professor at the Department of Psychology, University of Western Australia, Perth, Australia. We thank Fabrice Parmentier, Sébastien Tremblay, and Clive Frankish for useful discussions during the revision of this article. Correspondence concerning this article should be addressed to Dylan M. Jones, School of Psychology, Cardiff University, P.O. Box 901, Cardiff CF10 3YG, United Kingdom. jonesdm@cardiff.ac.uk or nichollsap@cardiff.ac.uk In general, a masking account assumes that a suffix has its greatest impact on recency when acoustic features such as voice and location of the terminal item overlap with those of the suffix. In line with such a view, many of the phenomena associated with the suffix effect show it to be dependent on the acoustic similarity between the to-be-remembered sequence and suffix. For instance, the effect is attenuated when the suffix and to-be-remembered sequence differ in spatial location (e.g., Frick, 1988b; Morton, Crowder, & Prussin, 1971), voice (e.g., Greenberg & Engle, 1983; Greene, 1991), and rhythm (e.g., Crowder, 1971; Frankish & Turner, 1984). The earliest masking account located the suffix effect within a precategorical acoustic store (PAS; Crowder & Morton, 1969). The PAS was described as an auxiliary but passive store prior to the postcategorical short-term memory. Verbal acoustic information enters the PAS automatically, either decaying or being masked by subsequently presented items. Thus, a suffix has its impact on recency by masking the representation of the terminal item in the PAS. The PAS accounts successfully for most of the phenomena associated with the suffix effect. Moreover, PAS is only one of many theories that utilize the phenomenon of masking to explain the effect of a suffix on recency (e.g., Burgess & Hitch, 1999; Crowder, 1978, 1982; Greenberg & Engle, 1983; Hitch, 1975; Nairne, 1988, 1990; Neath, 1999, 2000; Page & Norris, 1998; Surprenant et al., 2000). The grouping account suggests that recency occurs because the terminal item occupies a distinctive position at the boundary of the to-be-remembered sequence. Items occupying boundary positions are more likely than items within the group to be recalled because of fewer opportunities for transposition (e.g., Harris, 1989; Henson, Norris, Page, & Baddeley, 1996; Lee & Estes, 1981; Ryan, See Frick, 1988a, 1989, for possible additional factors). A suffix acoustically similar to the to-be-remembered sequence is subject to grouping processes that act to integrate it perceptually with the to-be-remembered sequence. It is now the suffix that occupies the distinctive boundary position usually occupied by the terminal list item and the result is the suffix effect. However, when the suffix is acoustically different, the terminal item of the tobe-remembered sequence signals the boundary of the to-beremembered sequence. Although not as salient as silence, a suffix that is acoustically different from the to-be-remembered sequence serves the same functional purpose, that is, to indicate perceptually 12

2 CAPTURING THE SUFFIX 13 the termination or boundary of the to-be-remembered sequence and to improve recall of the terminal item (e.g., Frankish, 1985; Frick, 1988b; Kahneman, 1973). The generality of grouping within serial recall is evident with a range of other serial recall phenomena. For example, presenting lists in groups of three or four items improves immediate ordered recall. The same properties that contribute toward the attenuation of the suffix effect appear also to influence grouping. Features such as temporal pauses (e.g., Ryan, 1969), speaker identity (e.g., Frankish, 1989; LeCompte & Watkins, 1993), voice pitch (e.g., Frankish, 1995), prosodic stress (Reeves, Schmauder, & Morris, 2000), and spatial location (e.g., Frick, 1989) promote groupings within a to-be-remembered sequence. Grouping substantially improves recall for items at the boundary of groups as demonstrated by serial position curves that show several small scalloped functions with enhanced recall corresponding to group boundaries. Grouping studies also demonstrate that items are transposed infrequently across the boundaries of groups (e.g., Frick, 1989; LeCompte & Watkins, 1993; Ryan, 1969). Phenomena associated with grouping within lists are not addressed adequately by masking accounts. For example, it is unlikely that a short pause of1sorless between groups could isolate the last item in one group from the masking effect of the first item in the next group. Further and more specifically, the PAS account (e.g. Crowder & Morton, 1969) cannot explain the modality effect under grouped presentation. The advantage in recall of grouped auditory sequences over grouped visual sequences would require the PAS to retain the whole sequence, counter to the original claim that the PAS is of limited capacity (e.g., Frankish, 1985, Experiment 2). The conflict between grouping and masking accounts is not yet fully resolved, despite research spanning several decades. The approach adopted in the present article was to manipulate the perceptual organization of the short-term serial recall task. In every experiment where a suffix was presented, its relation to the terminal item of the list was fixed (and hence the likelihood of masking remained fixed also). Perceptual organization was manipulated to increase the likelihood that the suffix belongs by either grouping to the list or an auditory irrelevant sequence. The logic underpinning the series is that if the impact of the suffix was diminished by being grouped to the irrelevant sequence while the likelihood of masking action was unchanged then masking is an unlikely explanation for the suffix effect. The experiments exploit an established phenomenon in the study of perceptual organization of sound known as auditory capture. This may be illustrated with a study by Bregman and Rudnicky (1975) that examined memory for the order of two tones. Participants heard initially two target tones of slightly different pitch presented in succession quickly (for example, A B represented on the left of Figure 1), followed, after a short interval, by the same two tones (test tones) in either same or reversed order. Participants were asked to indicate whether the test tones were presented in the same order as target tones. When the test tones were presented in isolation (Figure 1, Panel A) the participant could discriminate their order relatively easily. However, if the presentation of the test tones was flanked on both sides by a single tone (flanker tone; F), close in pitch to the test tones (see Figure 1, Panel B), memory for the order of the tones was severely impaired. In addition, this impairment could be reduced selectively by the presentation of further captor tones (C) placed before and after the Figure 1. A schematic illustration of the stimuli used in Bregman and Rudnicky (1975). Participants discriminated the order of the tones, A and B, which were presented either in isolation (Panel A), in the presence of flanker tones (F; Panel B), or embedded in a presentation of flanker tones and captor tones (C; Panel C). presentation of flanker and test tones (as illustrated in Figure 1, Panel C). Task performance was improved substantially when the captors were similar in pitch to the flankers. In every case of this illustration the physical relation of A to B was unchanged as was the potential for their mutual masking. However, by altering the context for their appearance with supplementary stimuli, whose power to mask is also minimal, perception (and performance) was changed appreciably. The phenomenon of capture illustrates the dynamic nature of representations in auditory memory, that is, the belongingness of the F tone had been altered, and the perceived auditory forms were changed (Bregman, 1990, p. 15). The general conclusion from this demonstration is that an event may be either incorporated within or isolated from a sequence by manipulating its relation to the surrounding stimuli. The current experiments attempted to manipulate the suffix effect by capturing the suffix from the to-be-remembered sequence, in much the same way that performance was improved in the Bregman and Rudnicky (1975) study when the flanker tones were perceptually removed from the sequence of target tones. Capture was effected through irrelevant sounds similar in acoustic characteristics to the suffix interleaved between the tobe-remembered items. Experiment 1 The series of experiments started by examining whether repeating an irrelevant item after each to-be-remembered item (the last being the suffix) produces an attenuation of the suffix effect compared with when the list is accompanied by the suffix alone. The series continued by determining whether changing the pitch of successive irrelevant items could determine the extent to which the suffix effect is attenuated. Although the main purpose of Experiment 1 was to demonstrate the capture of the suffix, the study of auditory stream biasing was a subsidiary goal. This refers to the facilitatory effect that repeating an item in an introductory period has on subsequent segregation of a sequence in which that same item is alternated with a different item (e.g., Beauvois & Meddis, 1991, 1997; Bregman, 1990). For example, after a period, a sequence of high (H) and low (L) rapidly alternating tones will be perceived

3 14 NICHOLLS AND JONES as two streams of sound, one consisting of high tones, the other consisting of low tones (e.g., Van Noorden, 1975). However, if the HLHL sequence is preceded by an introductory period of low tones (LLL), the fission of the high and low tones into separate streams is perceived more readily (e.g., Beauvois & Meddis, 1997). The introductory period serves to establish a stream prior to the alternating sequence and facilitates the grouping processes that act to segregate items in the interpolated sequence. In the same manner, Experiment 1 attempted to augment the segregation of the to-be-remembered and irrelevant sequences by exposing participants to an introductory period of irrelevant items prior to the onset of the interleaved sequence. All participants engaged in a serial recall task under four conditions: a no-suffix condition, in which eight to-be-remembered items were presented; a suffix-only condition (the word zero spoken in the same voice as the to-be-remembered list); and two interleaved conditions. Both interleaved conditions ( no-intro and intro ) presented the same irrelevant item (the word zero ) after each to-be-remembered item, including the last. The last irrelevant item the suffix was presented in the same manner, in relation to the terminal item, as the suffix in the suffix-only condition. However, of the two interleaved conditions, only the intro condition presented an introductory period of irrelevant items that preceded the onset of the to-be-remembered list, timed to be in synchrony with irrelevant items in the interpolated phase of the sequence (see Figure 2). The key issue explored in Experiment 1 was the capture of the suffix by the irrelevant sequence (Hitch, 1975; Watkins & Sechler, 1989, see also below). Additionally, interest centered on the role of the introductory sequence in promoting the streaming (or capture) of the suffix. Participants Method Thirty undergraduates from the School of Psychology at Cardiff University were given course credits for their participation. All were native English speakers reporting normal hearing and corrected or normal vision. Figure 2. A schematic representation of the sequencing of stimuli and their relative timing used in Experiment 1. The closed circles represent the irrelevant item zero, whereas the open squares each represent to-beremembered (TBR) items. The onset of irrelevant items occurred 500 ms after the onset of TBR items. The duration of each item in the sequence was 250 ms. Apparatus and Materials To-be-remembered items were chosen from the digits 1 9, and the word zero was used as the suffix. A female speaker recorded all stimuli in a monotone voice with a fundamental frequency of approximately 265 Hz. All items were sampled with 16-bit resolution, at a sampling rate of 48 khz, using Sound Designer II software (Digidesign Inc., Menlo Park, CA). All items were then digitally compressed to 250 ms using the same software, without altering acoustic features such as pitch. To-be-remembered sequences consisted of eight digits ordered quasirandomly with the precaution that no number was repeated within a trial and that consecutive numbers did not follow a pattern already familiar to the participants (e.g., 2, 4, 6; or consecutive digits). In all conditions the digits were presented at a rate of one per second. Four experimental conditions were prepared: no-suffix, suffix-only, and two interleaved conditions the intro and no-intro conditions (see Figure 2 for a schematic illustration). To-be-remembered items were presented at a rate of one per second. Each event, whether a to-be-ignored zero or a to-be-remembered digit, lasted 250 ms. For the suffix-only condition the irrelevant item was presented 500 ms after the onset of the terminal to-be-remembered item. For both interleaved conditions the irrelevant item occurred 500 ms after the onset of each to-be-remembered item, including the last. In the intro condition seven irrelevant items preceded the list onset, each with an interstimulus interval of 750 ms and in tempo with the interpolated irrelevant items (and ultimately with the suffix). All conditions began with a brief warning tone 3.5 s before the onset of the list. In all but the intro condition this meant that there was a period of silence before the list presentation began. Design Four conditions were contrasted in a repeated measures design: nosuffix, suffix-only, no-intro, intro. Over the course of 60 trials, participants completed 15 trials under each condition. Conditions were presented in a quasi-random fashion that prevented the same condition being presented more than twice in succession. The same order of trials was presented to all participants. Procedure Participants were tested individually in a soundproof laboratory. Trials were presented over headphones set to a level of roughly 60dB(A). Participants were instructed to ignore any spoken zero presented during a trial. In addition, participants were made aware that in every trial there would be a fixed delay between the start of the trial and the onset of the to-beremembered list and that this delay would be filled either by repetition of the irrelevant event or with a period of silence. Throughout the experiment, participants sat in front of a computer monitor that served to prompt serial recall: The screen flashed three times in quick succession, 1,500 ms after the offset of the terminal to be remembered, regardless of whether a suffix had been presented. Participants were required to recall in strict serial order from left to right on the response blank and were asked to guess the identity of any item they had failed to retrieve before going on to recall the next item in the sequence. They were instructed not to alter a response once it had been made. Written rather than typed recall was required because the latter has been shown to reduce sensitivity to recency effects (Penney & Blackwood, 1989). Participants initiated successive trials. Prior to the trial proper, participants undertook one practice trial in each of the conditions. Participants, with their consent, were monitored by closed-circuit television to ensure compliance with serial-order instructions. Results and Discussion Scoring Procedure Although the effect of a suffix is not confined to the final items of a to-be-remembered list (e.g., Greenberg & Engle, 1983), dis-

4 CAPTURING THE SUFFIX 15 ruption to recency is offered universally as a defining characteristic of the suffix effect. Not all researchers have adopted the same method for measuring recency, however, sometimes making comparisons across studies rather difficult. Generally, measures fall into one of three general categories: absolute, relative, and transformed. In the current series of experiments, we attempted to gauge the suffix effect by adopting one measure of each type: (a) absolute measure, which takes the accuracy with which terminal items are recalled; (b) relative measure, which is based on the change in recall between terminal position and preterminal position; and (c) normalized measure, which is calculated by expressing correct recall at the terminal position as a proportion of the sum of all correctly recalled items across all serial positions. Although it has been argued that the functional dissociation of recency and prerecency is specious (Bloom & Watkins, 1999), in the current case it seems likely that separating the irrelevant from the relevant items will have repercussions on prerecency performance. Arguably, therefore, a relative measure judging the improvement between the last and penultimate serial positions should be the most appropriate. All responses were marked to a strict serial-order criterion, that is, a correct response was recorded only if the correct digit appeared in the correct serial position. Recency measures will be reported first and then a general analysis that takes into account performance within each condition across all serial positions is reported. Recency Measures Figure 3 illustrates the results using the three recency measures. In summary, the suffix, when presented without irrelevant items, had the usual effect of reducing recency, moreover, recency was restored to some degree when the suffix was part of an irrelevant sequence. Although the results are in line with predictions, the restoration of recency by capture was not complete. Absolute measure. A one-factor repeated measures analysis of variance (ANOVA) on the percentage of items correctly recalled at the terminal position demonstrated a significant main effect of condition, F(3, 87) 62.25, MSE , p.01. The intro (63.78% correct) and no-intro (61.56%) conditions produced similar levels of performance. Planned comparisons showed this slight numerical difference to be nonsignificant (F 1). Performance in the suffix-only condition (54.89%) was significantly less than either the intro, F(1, 87) 9.14, p.01, or no-intro, F(1, 87) 5.14, p.05, conditions. It should be noted, however, that neither interleaved condition approached similar levels of performance displayed by the no-suffix condition (92.35%), each being significantly different from the no-suffix condition: no suffix versus intro, F(1, 87) 92.13, p.01; no suffix versus no-intro, F(1, 87) , p.01. Relative measure. A one-factor repeated measure ANOVA showed that there was a significant main effect of condition, F(3, 87) 6.97, MSE , p.01. There were no significant differences between the interleaved conditions on this measure (F 1). Further, relative error in both the intro (21.3%) and no-intro (25.1%) conditions was greater than that in the suffix-only condition (13.1%), F(1, 87) 4.24, p.05, and F(1, 87) 9.04, p.01, respectively. Furthermore, the closest approximation to the no-suffix condition is the no-intro condition (with a relative score of 30.9%), a level of performance not significantly different from the no-suffix condition, F(1, 87) 2.10, p.15. Normalized measure. On this measure, a one-factor repeated measures ANOVA produced a significant effect of condition, F(3, 87) 18.39, MSE 0.001, p.01. Again the suffix and no-suffix conditions were markedly different, both intro and nointro conditions restored recency. Planned comparisons showed that the difference between interleaved conditions was nonsignificant, F(1, 87) 1.78, p.19, and that both the intro (12.6%) and no-intro (13.5%) conditions were significantly better than the suffix-only condition (10.7%), F(1, 87) 7.46, p.01, and F(1, 87) 16.54, p.01, respectively. However, neither the interleaved nor suffix-only conditions attained levels of recency present in the no-suffix condition (15.8%): no-suffix versus intro F(1, 87) 20.85, p.01; no-suffix versus no-intro, F(1, 87) 10.44, p.01; no-suffix versus suffix-only, F(1, 87) 53.26, p.01. Generally speaking, there is evidence that interpolating irrelevant items could capture the suffix. Moreover, adding an introductory sequence did not further promote this capture of the suffix. Figure 3. Outcome of the three recency measures from Experiment 1. The left axis corresponds to the absolute measure and the right axis represents relative and normalized scores expressed in terms of percentage. Standard error bars are shown.

5 16 NICHOLLS AND JONES Serial Position Analysis Figure 4 illustrates the percentage of correct recall in each condition as a function of serial position. These data were analyzed using a 4 (intro, no-intro, suffix-only, and no-suffix condition) 8 (serial position) repeated measures ANOVA. There was a significant main effect of condition, F(3, 84) 35.21, MSE , p.01, and serial position, F(7, 196) 73.61, MSE , p.01. The interaction between the condition and serial position was also significant, F(21, 588) 8.51, MSE , p.01. Planned comparisons between conditions showed that overall recall in the no-suffix condition was superior to both the two interleaved conditions and the suffix-only condition ( p.01). The intro condition was not significantly different in overall recall from the suffix-only condition (F 1), although both these conditions demonstrated superior overall recall to the no-intro condition, F(1, 87) 13.08, p.01, and F(1, 87) 13.64, p.01, respectively. Overall, performance was depressed in the prerecency portion by the presence of interleaved items. This was particularly true for the no-intro condition (the intro condition being only somewhat worse than the suffix-only condition). The effect of the introductory sequence seemed to have been most marked in prerecency rather than in recency. Indeed, presence of a lead-in sequence seemed not to have been beneficial in promoting capture of the suffix over and above the action of the interleaved sequence. The results are in line with an auditory streaming analysis of the so-called sandwich effect. This refers to the impact of interleaving auditory irrelevant with auditory to-be-remembered items in serial short-term recall. Although some have claimed that the effect is inconsequential (Baddeley, Papagno, & Andrade, 1993), recent findings suggest that the effect can be marked (Nicholls & Jones, in press). Critically, the degree of interference between relevant and irrelevant events can be predicted by the likelihood that the interleaved sequences can be partitioned by the principles of auditory streaming. The results in the prerecency region of Experiment 1 tended to confirm this auditory streaming analysis. Bias toward streaming out the interleaved items was more marked when a lead-in sequence was provided. Figure 4. Percentage of correctly recalled items as a function of serial position and condition in Experiment 1. From a grouping perspective, presenting an introductory period of irrelevant tokens may have induced a stream bias that served to augment the grouping processes (e.g., Beauvois & Meddis, 1997; Bregman, 1990). That is, the lead-in period served to establish an irrelevant stream prior to the alternating sequence and facilitated the grouping processes that acted to partition relevant and irrelevant items. In terms of the suffix effect, the results of Experiment 1 provide evidence against the masking view and in favor of grouping. Only the degree of advantage due to capture is perhaps disappointing, falling short as it does of restoring recency completely, but perhaps this is because the conditions of streaming were less than perfect. Generally speaking, streaming is inherently variable, but even so it might be expected to restore recency to a level more closely approximating that of no-suffix conditions. One well-established way to promote streaming is by increasing the rate of presentation (see Bregman, 1990). In Experiment 2, a slightly faster rate of presentation was used to promote streaming and, as a result, the restoration of recency was expected to be more fulsome. Experiment 2 The purpose of Experiment 2 was to examine whether an interleaved sequence of irrelevant items that steadily increase in pitch could attenuate the suffix effect. Thus, from a grouping perspective, a sequence of sounds steadily rising in pitch should show streaming (and hence capture) also. The use of changing irrelevant sequences was desirable from another standpoint, that of testing alternative interpretations of the findings of Experiment 1. In Experiment 1, all irrelevant items were at the same pitch. However, the principles of streaming do not demand that constituent items in the irrelevant sequence be identical. Streaming occurs through organizational processes that link together successive items that display similar acoustic properties, such as pitch or some other Gestalt principles like good continuation (Bregman, 1990; Darwin, Pattison, & Gardner, 1989; Heisse & Miller, 1951). Successive items with pitch rising in frequency can also form a stream. Varying and unchanging sequences can be contrasted to test the possibility that habituation rather than streaming is responsible for the effect of increasing recency in the interleaved conditions. From the habituation standpoint the impact of the suffix will be reduced because its novelty and, hence, its attention-grabbing capacity is diminished by prior presentation in the irrelevant sequence. In the habituation framework it is assumed that the suffix disrupts processing of the to-be-remembered items because it causes an orientating response thereby drawing attention away from the last list items (e.g., Sokolov, 1963; see Cowan, 1995, for a discussion). However, if participants are given prior exposure to the suffix token, the suffix will have less impact as a result of adaptation or habituation (Watkins & Sechler, 1989). The results of Experiment 1 could be interpreted in these terms: Habituation to interleaved items reduced the impact of the suffix. Two main hypotheses follow from a habituation account. First, habituation to a suffix should be most pronounced when the acoustic characteristics of all irrelevant items remain unchanged. Second, there should be a lawful relation between exposure and attentional orienting; greater exposure to repeated tokens should reduce further the impact of the suffix. In fact, this second prediction from the habituation account has already been put to the test in Experiment 1. The intro condition of Experiment 1 should have

6 CAPTURING THE SUFFIX 17 shown more recency than the no-intro condition (the suffix being relatively more habituated), but the results tend to show the opposite. Arguably, this contrast is confounded insofar as location of the irrelevant items (being prelist as well as intralist in the intro condition but only being intralist in the other conditions) is confounded with number of tokens. In Experiment 2 participants engaged in a serial recall task under four conditions. Three of the conditions were the same as the no-suffix, suffix-only, and intro conditions from Experiment 1. A fourth condition had the irrelevant items whose number and timing were the same as that for the intro condition. However, the pitch of the sequence took the form of an initial seven-item sequence of intro pitch but markedly lower than the pitch of the to-beremembered list, followed by a staircase of seven successive rises of pitch interleaved between to-be-remembered items culminating at the suffix. The pitch of the suffix was identical to that of the to-be-remembered list (see Figure 5). Participants Method Thirty undergraduate students from the School of Psychology at Cardiff University took part in the study. All were native English speakers who reported normal hearing and corrected or normal vision. None of the participants had taken part in Experiment 1. Apparatus and Materials The same digitally sampled digits and suffix ( zero ) used in Experiment 1 were adopted for Experiment 2. All items were digitally compressed to 300 ms using Sound Designer II software (Digidesign Inc., Menlo Park, CA). Although the procedure for assembling to-be-remembered lists was identical to that used in Experiment 1, the timing was different. In Experiment 2 a faster rate of presentation was adopted in an attempt to promote streaming. Onsets of items were 600 ms apart (they were 1 s apart in Experiment 1). The irrelevant items in all conditions were separated from one another (offset to onset) by an interval of 300 ms. There were four conditions: no-suffix condition, suffix-only condition, and two interleaved conditions intro and ascend (see Figure 5). The onset of the suffix in the suffix-only condition occurred 300 ms after the onset of the terminal item. All irrelevant items in the intro condition were presented at the same pitch as the to-be-remembered items. As with the intro condition from Experiment 1, there was also a lead-in period prior to the onset of the to-be-remembered list in which seven irrelevant items were presented. Figure 5 illustrates schematically the relative timing and pitch difference between items in all four conditions. The ascend condition began with seven tokens repeated at the same pitch, 5.25 semitones below the pitch of the list. Commencing at the first interpolated token, the pitch increased 0.75 semitones with every successive token, culminating in the suffix item (which was at the same pitch as the to-be-remembered list). Sound Designer II software (Digidesign Inc., Menlo Park, CA) was used to change the pitch of irrelevant items without adversely influencing other acoustic features or the duration of the token. As in Experiment 1, the no-suffix and suffix-only trials commenced with a short tone and period of silence that matched the duration of the introductory period from within the interleaved conditions. Design and Procedure The design and procedure used in Experiment 1 was adopted for Experiment 2. Participants undertook 15 trials in each of four conditions (no-suffix, suffix-only, intro, and ascend). The same quasi-random arrangement of trials was presented to all participants. Recency Analysis Results and Discussion Overall, the results were comparable to those in Experiment 1 (see Figure 6). Again the irrelevant sequence captured the suffix but this time more emphatically. Indeed, performance in the intro condition now approached that in the no-suffix condition (at least with the relative and normalized measures). However, the recency present in recall of the two interleaved sequences was not equivalent, at least numerically speaking: The intro condition demonstrated more pronounced recency than the ascend condition. This is what might have been expected from the habituation standpoint. Absolute measure. A one-factor repeated measures ANOVA on the percentage of items recalled correctly at the terminal serial position produced a significant effect of condition, F(3, 87) 42.67, MSE , p.01. Planned comparisons indicate that both the intro (64.2% of terminal items recalled correctly) and ascend (57.1%) conditions showed significantly greater recency than the suffix-only condition (49.6%), F(1, 87) 14.98, p.01 and F(1, 87) 4.46, p.05, respectively. Further, there was a trend for the intro condition to show greater recency than the ascend condition, although this improvement proved to be not nonsignificant, F(1, 87) 3.10, p.05. No condition in which a suffix item was presented attained recency to the level exhibited in the no-suffix condition (90.2%): no-suffix Figure 5. A schematic representation of the sequencing and relative timing of stimuli used in Experiment 2. The onset of irrelevant items occurred 300 ms after the onset of to-be-remembered (TBR) items. The duration of each item in the sequence was 300 ms.

7 18 NICHOLLS AND JONES Figure 6. Outcome of the three measures of recency used in Experiment 2. The left axis corresponds to the absolute measure and the right axis represents relative and normalized scores expressed in terms of percentages. Standard error bars are shown. versus intro F(1, 87) 46.28, p.01; no-suffix versus ascend, F(1, 87) 73.31, p.01; no-suffix versus suffix-only, F(1, 87) , p.01. Relative measure. A one-factor repeated measures ANOVA produced a main effect of condition, F(3, 87) 7.77, MSE , p.01. Planned comparisons revealed that the difference between scores in the interleaved conditions was nonsignificant, F(1, 87) 1.48, p.23. Further, as in the absolute measure, greater recency was established in both the intro (26.7%) and ascend (21.6%) conditions than in the suffix-only condition (10.9%), F(1, 87) 14.10, p.01, and F(1, 87) 6.44, p.05, respectively. In contrast to the absolute measure, the intro condition attained a similar level of recency to the no-suffix condition (29.8%), F(1, 87) 0.55, p.46. However, the ascend condition was significantly different from the no-suffix condition, F(1, 87) 3.83, p.05. Normalized measure. A one-factor repeated measures ANOVA using normalized recency scores revealed a significant main effect of condition, F(3, 87) 17.15, MSE 0.001, p.01. Planned comparisons showed no significant difference between the interleaved conditions, F(1, 87) 1.98, p.16. Both the intro (13.6%) and ascend (12.4%) conditions attained superior recency in comparison to the suffix-only condition (9.6%), F(1, 87) 23.60, p.01, and F(1, 87) 11.91, p.01, respectively. However, neither the intro, F(1, 87) 4.46, p.05, nor the ascend conditions, F(1, 87) 12.38, p.01, reached a similar level of recency demonstrated by the no-suffix condition (15.4%). line with the idea that auditory stream biasing is promoted by the introductory sequence (cf. Nicholls & Jones, in press). A 4 (intro, ascend, suffix-only, and no-suffix conditions) 8 (serial position) repeated measures ANOVA on percentage recalled correctly demonstrated a significant main effect of auditory condition, F(3, 87) 35.05, MSE , p.01, and serial position, F(7, 203) 89.21, MSE , p.01. The interaction between the two factors was also significant, F(21, 609) 9.22, MSE , p.01. Planned comparisons between average recall in each condition showed no difference between intro and suffix-only conditions, F 1, and intro and ascend conditions, F(1, 87) 2.62, p.05, but a significant difference between ascend and suffix-only conditions, with suffixonly demonstrating better recall overall, F(1, 87) 6.48, p.05. The result of Experiment 2 can be taken as consistent with habituation caused by the interleaved irrelevant material, insofar as the sequence repeated in pitch is more effective at promoting recency than one ascending in pitch. One way of reconciling the results with the streaming hypothesis is to suppose that the streaming was less than perfect in the ascend condition. Arguably, Serial Position Analysis Figure 7 shows the percentage of items correctly recalled in their correct serial position as a function of condition. Again there was differentiation in the prerecency portion of the list: The presence of interleaved items depressed recall, and the effect of the suffix extended well into the prerecency region. There was convergence of the suffix, ascend, and random conditions at the penultimate serial position, but there was divergence at the terminal serial position. In the prerecency portion of the serial position curve, the effect of interleaved items was rather modest. This is in Figure 7. Percentage of items correctly recalled in position as a function of serial position and condition (Experiment 2).

8 CAPTURING THE SUFFIX 19 Experiment 2 failed in its goal of differentiating habituation from streaming explanations. The results indicated that stimuli very distinct from the to-be-remembered sequence can nevertheless produce the elevation of recency. An alternative possibility is that common timing is a sufficient basis for differentiating the two sequences. That is, a periodic attentional pulse serves to group events into a stream (Baddeley et al., 1993; Hamilton & Hockey, 1974). From this attentional pulse standpoint the effectiveness of partitioning should be independent of the pitch relations between the irrelevant and to-be-remembered events. This prediction was in line with the findings from Experiment 2 in which both an interleaved ascending sequence and one repeated in pitch demonstrated the capacity to attenuate the suffix effect. Experiment 3 Experiment 3 attempted to clarify the outcome of Experiment 2. Two new conditions are fashioned with the aim of differentiating the streaming, habituation, and attentional pulse accounts. In one condition, a long series of irrelevant tokens steeply ascending in pitch was used (ascend condition). In the other condition, the same tokens were ordered more or less randomly (see Figure 8). On the pulsing account we predict no difference between the random and ascend conditions because they both share the same timing. Both the streaming and habituation accounts predict that ascend will restore recency more than random. Given the steep staircase in the ascend condition the habituation hypothesis suggests a rather modest impact of the suffix. From the streaming standpoint it was predicted that the staircase would strongly bind with the suffix, hence the restoration of recency would be particularly marked. Participants took part in four conditions: no-suffix condition, suffix-only condition, and two interleaved conditions ascend and random. Participants Method Thirty undergraduate students from Cardiff University participated in the experiment in return for course credit. All participants were native English speakers and reported normal hearing and normal or corrected vision. None of the participants had taken part in Experiments 1 and 2. Apparatus and Materials Experiment 3 used the same lists as Experiment 2. Fourteen versions were made of the token zero, each at a different pitch from the original. The tokens were transformed to values of pitch in 0.6-semitone steps within the range of semitones below their original pitch (the voice was the same as that used in Experiments 1 and 2). Two types of irrelevant sequences were assembled. Both sequences were drawn from the same set of 14 pitch-shifted irrelevant items. In the ascend condition, the irrelevant tokens were arranged in an ascending series terminating with a 15th irrelevant token at the same pitch as the list, the suffix. In the random condition, the pitch-shifted tokens were arranged in random order, with the constraint that there were no obviously predictable sequences. This sequence was fixed throughout the experiment. Design and Procedure The design was the same as that used in Experiment 2. The same general procedure used in previous experiments of the current series was adopted here. Results and Discussion Recency Analysis The results of the recency analysis can be summarized as follows. Using the relative measure, the ascending condition showed a restoration of recency that was without parallel thus far in the current series. Performance was equivalent to the no-suffix condition. Granted, the suffix effect was itself smaller in magnitude than in earlier studies, but it is still of the order of a 20% change in error; nevertheless, the ascending sequence captured the suffix fully. In addition, there was clear evidence of the failure of the random sequence to capture the suffix. Levels of performance were virtually identical in the random suffix-only conditions. As Figure 9 illustrates, for conditions including a suffix, all three measures of recency among suffixed conditions showed the same trend. Absolute measure. A one-way repeated measures ANOVA on the percentage of items correctly recalled at the terminal position revealed a significant main effect of condition, F(3, 87) 41.85, MSE , p.01. Planned comparisons showed that the two interleaved conditions differed significantly from each other, with Figure 8. A schematic representation of the interleaved conditions used in Experiment 3. The onset of irrelevant items occurred 300 ms following the onset of to-be-remembered (TBR) items. Every item in the sequence was 300 ms in duration. Vertical displacement represents relative difference in stimulus pitch.

9 20 NICHOLLS AND JONES Figure 9. Outcome of the three measures of recency used in the analysis of data from Experiment 3. The left axis represents percentage recalled on the absolute measure and the right axis represents percentage recalled on the relative and normalized measures. Standard error bars are shown. the ascend condition (74.67% of items correctly recalled in the terminal position) producing more recency than the random condition (60.44%), F(1, 87) 18.41, p.01. There was no significant difference between random and suffix-only conditions (62.22%), F 1, but there was a significant difference between ascend and suffix-only conditions, F(1, 87) 14.10, p.01. No condition with a suffix present attained a similar recency score to that demonstrated in the no-suffix condition (93.33%): no-suffix versus ascend, F(1, 87) 31.72, p.01; no-suffix versus random, F(1, 87) 98.47, p.01; no-suffix versus suffix-only, F(1, 87) 88.12, p.01. Relative measure. A one-factor repeated measures ANOVA on the relative recency scores produced a significant effect of condition, F(3, 29) 4.70, MSE , p.01. Planned comparisons showed that the ascend condition (32.4%) produced more recency than both the random (23.3%), F(1, 87) 6.48, p.05, and suffix-only (21.3%) conditions, F(1, 87) 9.64, p.01, and further, the ascend condition attained a similar level of recency to the no-suffix condition (30.9%), F 1. The random and suffix-only conditions were not statistically different, F 1. Normalized measure. A one-factor repeated measures ANOVA revealed a significant main effect of condition, F(3, 29) 13.47, MSE 0.001, p.01. Planned comparisons showed that the ascend condition (14.9%) demonstrated a greater recency score than both the random (12.7%), F(1, 87) 9.21, p.01, and suffix-only (11.7%) conditions, F(1, 87) 20.70, p.01. The ascend condition was not significantly different than the no-suffix (15.6%) condition (F 1). In addition, the random and suffix-only conditions demonstrated no significant difference between their respective recency scores, F(1, 87) 2.30, MSE 0.001, p.13. Serial Position Analysis Figure 10 illustrates correct recall in each condition as a function of serial position. The suffix-only condition produced effects in the prerecency region, as did the presence of interleaved irrelevant material. In prerecency, random sequences were only marginally more damaging than an ascending sequence. There was some convergence in all but the no-suffix condition at Serial Position 7. A 4 (auditory condition) 8 (serial position) repeated measures ANOVA on percentage of items recalled correctly established a main effect for both auditory condition, F(3, 87) 35.10, MSE , p.01, and serial position, F(7, 203) 79.78, MSE , p.01. The interaction between these two factors was also significant, F(21, 609) 7.29, MSE , p.01. Planned comparisons on the average percentage of digits correctly recalled illustrate that the ascend condition (64.25%) showed superior recall over the random condition (59.89%), F(1, 87) 6.81, p.05, while being not significantly different to the suffix-only condition (65.19%), p.57. Performance in the no-suffix condition (at 76.33%) was superior to all suffixed conditions: no-suffix versus ascend, F(1, 87) 52.25, p.01; no-suffix versus random, F(1, 87) 96.77, p.01; no-suffix versus suffix-only, F(1, 87) 96.77, MSE , p.01. Experiment 3 added substantially to our understanding of the process of capture and helped to further adjudicate the competing theoretical accounts. Like the other experiments in the series, there was clear evidence against the idea that the suffix masks the last item in the list. Now, the attentional pulsing account (Baddeley et al., 1993) seems ruled out by the data of Experiment 3; the failure of the random condition to produce roughly the same level of performance as the ascend condition is testimony to that. The results also militate against a strong version of the habituation hypothesis. Given the reasonably marked differences within the ascending series, one might expect what habituation there was to be relatively modest. The very strong tendency for the ascend condition to restore recency fully argues against this, however. The fulsome degree of restoration of recency found here is very much in contrast with the much more modest degree of restoration found in the intro condition of Experiment 1, where the number of irrelevant tokens was the same and the identity of the tokens was unchanging. Experiment 4 Experiment 4 was an attempt to capture the suffix fully, so that complete restoration of recency was displayed without equivocation. That is, the goal was to raise recency performance using streaming techniques to a level directly matching that in the

10 CAPTURING THE SUFFIX 21 evident in prerecency than in earlier experiments of the series. In addition, Experiment 4 used a different single-syllable suffix ( go ) in an attempt to increase the generality of the effect. Use of a shorter token meant also that the rate of presentation could be increased further without stimulus overlap than with the longer two-syllable zero used in the earlier experiments of the series. Participants took part in four conditions: no-suffix condition, suffix-only condition, and two interleaved conditions (now called irrelevant conditions because not all the events are interleaved with the to-be-remembered sequence) one with a suffix (irrelevant suffix) and the other with no suffix (irrelevant-only). Participants Method Figure 10. Percentage of items correctly recalled in position as a function of serial position and condition (Experiment 3). no-suffix condition in all three measures of recency. The means by which this may be achieved are already evident: increasing the rate of presentation while keeping the pitch of the interpolated irrelevant sequence steady. In Experiment 4 the rate of presentation of the irrelevant sequence was increased markedly, to over four per second. This meant also that to keep an even tempo in the irrelevant sequence (another factor that promotes stream formation) there will be a degree of overlap of relevant and irrelevant stimuli (see Figure 11). This was a characteristic that earlier experiments of the current series had tried to avoid to minimize the effect of masking. Pilot work for Experiment 4 established a rate at which capture was likely but one that also minimized the likelihood of peripheral masking of the to-be-remembered stimuli as a result of their overlapping presentation. Rather than impair encoding of list items, the higher rate of presentation was predicted to promote strong streaming and, because fission should as a result be more complete, the effect of the irrelevant sequence should be less Thirty-four undergraduate students from the School of Psychology at Cardiff University were awarded course credits for their participation. All participants were native English speakers who reported normal hearing. None had taken part in Experiments 1 3. Apparatus and Materials Eight list items were selected without replacement from the digits 1 9, and the word go was used as the irrelevant item. All items were recorded by a female speaker in a monotone voice (with a fundamental frequency of approximately 230 Hz). All items were sampled with a 16-bit resolution, at a sampling rate of 48 khz, using Sound Designer II software (Digidesign Inc., Menlo Park, CA). Using the same software, all to-be-remembered items were compressed digitally to 250 ms. The irrelevant item was compressed to approximately 180 ms. Compressing items in this manner did not change their pitch. To-be-remembered sequences comprised eight digits subject to the same constraints as in earlier experiments of the current series. Digits were presented at a rate of one every 720 ms. Four experimental conditions were used: no-suffix, suffix-only, and two irrelevant-item conditions one with a suffix and one without. Irrelevant sequences consisted of the same word go repeated every 240 ms throughout the trial. For the suffix-only condition the offset of the terminal Figure 11. A schematic representation of the sequencing and relative timing of stimuli used in Experiment 4. To-be-remembered (TBR) items were of 250-ms duration (one item presented every 720 ms) and irrelevant items were of 180-ms duration (one item presented every 240 ms). The suffix occurred 50 ms after the offset of the last item in the list. The introductory period was 10 s long, during which 40 irrelevant items were presented. For ease of exposition, unlike the other schematics in this series, Figure 11 does not preserve the scale of item duration and series length. The circles (irrelevant items) are contained within squares (TBR items) to convey the impression of simultaneity.