D isruption of S hort-term Recogn itio n M em ory for Tones: Stream ing or Interference?

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1 Ó THE QUARTERLY JOU RNAL OF EXPERIM ENTAL PSYCHOLOGY, 1997, 50A (2), 337± 357 D isruption of S hort-term Recogn itio n M em ory for Tones: Stream ing or Interference? D.M. Jones, W.J. M acken, C. Harries University of Wa les, Ca rdiff, U.K. A sequence of auditory stimuli interpolated between the initial presentation of a tone and a com parison tone impairs recognition perfor mance. Notably, the impairment is much less w ith interpolated speech than with tones. Six experiments converge on the conclusion that this pattern of impairm ent is due m ore to the org anization of the interpolated sequence than to its similarity to the to-be-remem bered standard. Factors that contribute to the coherence of the interpolated sequence into a stream distinct from the initial tone are prim ary determinants of the level of impairment. This is dem onstrated by manipulating factors that contribute to the coherence of the interpolated sequence by the action of temporal, spatial, tim bral, and tonal attributes. However, the relative immunity of recognition performance to the interpolation of unprocessed digit sequences is not explained wholly by such coherence. The judgement of the sim ilarity of pitch between two successive test tones is critically dependent on the p resence and nature of irrelevant sound interpolated between the tones. Two interpretations of this effect are contrasted in this p aper: O ne con siders that the disruption is prod uced by interferenceð that is, on the sim ilarity of representations within m emory of the test tones and the irrelevan t ton es; the other supposes that the interference is the product of the degree to which the interpolated material is perceptually integrated w ith the test tones. Th ese issues are addressed using a methodology rst em ployed by D eutsch (197 0). The paradigm is one in which m em ory for the pitch of tones over a period of a few seconds is assessed. S ubjects rst h ear a stand ard tone, followed by a sequence of sounds that they are instructed to ignore, and then, after a short interval, a comparison tone. T hey are asked to judge whether the standard tone and the comparison tone are of the same or different pitch. The m ain variable of interest is the different types of sound in the interpolated sequence. In the study by D eutsch (1970), interp olated spoken digits and interpolated tones produced m arkedly different effects on the pitch judgem ent task: Speech prod uced errors on 2% of trials, but tones produced Requests for reprints should be sent to Dylan Jones, School of Psychology, PO Box 901, University of Wales, Cardiff, CF1 3YG. jonesdm@ cardiff.uk.ac The work reported here was supported by a project grant from the Economic and Social Research Council. Thanks are due to Seth Schwartz for his help in running Experim ent 1. The development of the experim ental series bene ted from discussions with Clive Frankish. Revisions to the manuscript were helped greatly by Robyn Boyle and Karen Howes The Experimental Psychology Society

2 3 3 8 J O N E S, M A C K E N, H A R R I E S errors on 32% of trials. T he lower level of disruption by speech led Deutsch (1970; see also D eutsch, 1984, for a sum mary) to supp ose that recognition m em ory was related to the degree of sim ilarity between the standard tone and the interpolated m aterial. The very marked difference produ ced by speech and n on-speech also suggested to Deutsch (1970) that speech an d non-speech are function ally distin ct w ithin memory. This interp retation was reinforced by Pechmann and M oh r (1992), who viewed their ndings using the Deutsch (1970) paradigm as supporting a m odi ed working-memory model in which non-speech and speech were functionally distinct, stored respectively in a ``tonal loop and an ``articulatory loop. By both these accounts the marked degree of interference by interpolated tones is explained in term s of retroactive interference with the standard tone by the subsequ ent interpolated tones, with the suggestion that this interference is especially low in the case of speech becau se distinctiveness is not m erely structural but may also be functional. The main thrust of the p resent paper is that the results of Deutsch (1970) an d Pechmann and M ohr (1992) can also be explained in term s of a processing system within which representations in memory are organized according to the principles of auditory scene an alysis (B regman, 1990). Such an ap proach focuses on the characteristics of soun ds that lead to their grouping and coh erence. For exam ple, tem poral proxim ity or spectral sim ilarity can be used to organize materials into perceptual stream s corresponding to distinct entities in space and over time. It is argued that such organization is re ected in the represen tation within m em ory, which, in turn, has repercussions for the accessibility of events within and between stream s. Thus, organizational factors will not on ly separate different stream s of sound so that they are distinct in phenomenal term s, but will also d etermine organ ization in memory and consequently determine the ease of retrieval from memory. This object- and stream-formation fram ework has already proved usefu l in the context of interferen ce effects in short-term serial recall (e.g. Jones, M acken, & M u rray, 1993). Th e curren t series of experiments seeks to extend its applicability to the phenom ena of interference with pitch m emory as investigated by Deutsch (1970). From this stream ing standp oint, two factors not controlled in the experimental paradigm used by Deutsch (1970) may have resulted in a misleading picture of how interpolated items interfere with m em ory for pitch. By controlling for these factors in new experiments, results consistent with an auditory scene analysis fram ework and at variance with the interference fram ework should em erge. The rst factor is that in the original paradigm the tim ing of the standard tone in relation to the interpolated sequence of tones en couraged the perception of the standard as an element of the interpolated sequence. Speci cally, the interval between the standard tone and the rst interpolated tone was the sam e as that between successive interpolated tones (300 msec). This timing is likely to lead to all those items (both standard and interpolated) being organized into a single higher-order perceptual u nit. Failure to isolate the standard tone would prejudice the process of m atching it to the com parison tone and therefore lower the recognition rate. This is based on the generalization that the m ore attributes successive stimuli have in common, the more likely they are to be grouped togetherð a phenom enon that leads to dif culty in isolating inform ation about single events within a sequence (see Bregman & Rudn icky, 1975, for an illustration). For exam ple, if a sequence of stimuli share both

3 M E M O R Y F O R T O N E S timing and tim bre, even though they may differ in pitch, the stimuli will be bound together more rmly than if they shared only timing. W ith speech, therefore, the grouping of the standard tone with the interpolated item s will be less compelling, as it is distinct in timbre, though not in tim ing, from the interpolated sequence. T he effect of interpolated tones can therefore be accounted for in part by the dif culty of segregating perceptually the stand ard tone from the tones in the interpolated sequence with which it is grouped. If this account of the greater disruption produced by interpolated tones is correct, increasing the interval between the standard and the rst stimulus of the interpolated set should signi can tly red uce disrup tion. The second confounding factor is that in the com parison of speech with non-speech by Deutsch (1970) the degree of acoustic variation of the interpolated materials was n ot controlled appropriately. As a result, the digit sequence contained items that were not only distinct from the standard tone, but were also m ore distinct spectrally from one another than were the item s within the sequence of interpolated tones. Additionally, in the materials used by Deutsch, the tone sequences varied in frequency over an octave, but the speech stimuli shared a com mon fundam en tal frequ ency. S peech by its nature d iffers acoustically from pure tones; however, given that an important distinction rests on this confounding, it seems reasonable to argue that a more exacting an alysis of the effects of different characteristics of speech is required. Sp eci cally, such a test should match the variation in both speech an d non-speech stimuli along one dim ension w hile controlling all other stimulus dimensions. For example, it might be desirable to m atch the frequency of tones to the fundam ental frequ ency of each speech stimulus. In this w ay the d egree of variation in pitch w ithin the interpolated sequence would be varied to the sam e degree for the d ifferent classes of material. The following sequence of experim ents explores the pattern of interference in pitch memory as investigated by Deutsch (1970) and Pechmann and M ohr (1992) with a view to establish ing the applicability of a stream ing fram ework to the results. E X P E R I M E N T 1 Experiment 1 is a replication (w ith m inor variations) and extension of the main conditions of Deutsch (1970). T he central aim of this experiment was to test two propositions: rst, that the timing of the standard tone in relation to the sequence of interpolated tones will determ ine, in part, the degree of disruption via its effect on grouping; and second, that when the differences within each type of interpolated sequence are made com parable as far as is practicable, sequences of speech when comp ared to sequences of non-speech will prod uce equivalen t degrees of disrup tion of mem ory for tones. W h en both of these factors are controlled, it is argued, a m ore accurate picture of the mechanism s of disruption of memory for pitch will emerge. Two form s of manipulation were exam ined in Experim ent 1. One involved changing the interval between the standard and the rst item in the interpolated sequence. 1 Two conditions were contrasted: In one case, the interval was the same as that separating the 1 Thanks are due to Peter Bailey for pointing to an unpublishe d study reported as a footnote in the paper of Deutsch (1978) in which the results of a similar experiment are describe d brie y.

4 3 4 0 J O N E S, M A C K E N, H A R R I E S item s in the interpolated sequ ence (350 msec), in the other it was appreciably longer (1500 msec). On the basis of the stream ing hypothesis, it was predicted that recognition memory for the standard would be superior when the interval between the standard an d the interpolated list is longer, but that this improvem ent may depend on the role of other factors (including the acou stic com position of the interpolated list). In order to investigate the role of the com position of the interpolated list and its joint action with the standard-list interval, four types of interpolated sequence were used. Two of these were also used in the Deutsch (1970) exp eriment: p ure tones at different frequencies and sequences of random digits. Two further conditions were added: In the rst, a sequence of cello notes having the same pitch as the sequence of pure tones was used to compare the effect of having the interpolated sequence distinct in tim bre from both the standard an d the comparison sounds. Sem al and Dem any (1991) found that manipulating the tim bre of the interpolated series did n ot im prove recognition perform ance; however, in their version of the Deutsch parad igm, the interval between the standard an d the rst item of the interpolated series was longer (at 500 msec) than the interval between m embers of the interpolated set (300 m sec), thus possibly serving as a basis for segregating the standard. Additional cues to segregation based on the timbre of the interpolated set m ay not have been useful under these conditions. This suggests that the role of timbre will be revealed by its interaction with the interval between the standard and the rst item of the interpolated list. A lthough listeners trad e temporal and tim bral inform ation, w ithin limits, it seem s plausible that when tim ing does not serve as a strong basis for segregation (as in the 350-msec interval condition of Experiment 1) the effect of distinctiveness of timbre will be m ad e more likely. A sim ilar logic ap plies to the second additional condition in which a single token of the spoken digit ``one is repeated. Changing the timing between the standard and the interp olated list would only affect conditions in which the interpolated item s were tones, not those comprising speech or cello notes, as the tim bral difference between these interpo lated items and the standard tone means that the standard is not likely to be grouped with them anyway. In Experiment 1 there are slight differences to the procedure adopted by Deutsch (1970) and Pechm ann and M ohr (1992)Ð nam ely that all the sounds were 350 msec long (not 200 msec) and in the interp olated sequence there were nine items, rather than six, separated by 350 m sec, rather than 300 m sec. T hese values were adopted following pilot stu dies and were designed to prevent effects of timing between the stan dard and the interp olated sequ ence being m asked by ceiling effects. These changes are relatively minor, and it seems safe to regard such differences between the studies as unlikely to prejudice the hypotheses u nder test. M e th o d S u b j e c t s M ale and female student volunteers were recruited for the study. E ach reported normal hearing. Subjects were screened for their ability to discriminate between pure tone stimuli a semitone apart. A criterion of 7 correct discriminations in 10 presentations was used. Of the subjects, 8 failed to meet the criterion; 20 subjects undertook the full experimental procedure after passing the test.

5 M E M O R Y F O R T O N E S A u d i t o r y M a t e r i a l s All sounds were recorded and edited digitally to 16-bit resolution and sampled at 48 K Hz. The sounds were tape-recorded onto a high-quality cassette tape recorder for playback during the experim ent. Each trial consisted of a standard pure tone (lasting 350 msec), followed, after an interval of either 350 m sec or 1500 m sec, by an interpolated sequence of nine item s, each of w hich was 350 m sec long and separated from its neighbours by silence of 350 m sec. Following a further interval of 1500 m sec, the com parison tone (lasting 350 msec) was presented. Care was taken to tailor, by digital editing, the rise and fall tim e of the to nes so that no artefactual clicks were prod uced (each tone rose and fell its full amplitude in 50 m sec). An interval of 10 sec followed the comparison tone, during which the subject w as required to m ark the judgem ent ``sam e or ``different on a response blank. On half the trials standard tones and com parison tones were the sam e; on the other half they were a sem itone apart (randomly either higher or lower). Four types of interpolated sequence were assembled: (a) a sequence of pure tones drawn randomly from the set of 12 sem itones in the range C4 (262 H z) to B4 (494 H z); (b) spoken digits, the integers 1 to 9, recorded individually in a m ale voice in a m onotone at a pitch corresponding to C 4 (262 H z) and thereafter assembled using digital editing into a quasi-random sequence in which no digit was repeated; (c) a repeated syllable spoken in a random sequence of fundamental frequencies in the sam e range as for tones; in this case, a single token of the word ``one w as spoken at a pitch corresponding to C4 (262 Hz) and was subsequently transformed using digital signal processing techniques using D igidesign s S ounde dit software so that the sam e token was played at each of the pitches corresponding to those of the pure tones without a change in the duration of the stim ulus; (d) bowed cello notes at fundamental frequencies in the same range as that for tones (taken from com mercially recorded digital samples using Sa mplecell software). The pitches of standard and test tones were chosen from those not already selected as interpolated items. Each of the speech tokens was edited to last 350 m sec (thus m aking them the sam e physical length as the non-speech tokens) by means of a routine in Digidesign s S ounde dit software in which the length of the token could be reduced w ithout any changes in any other feature such as the pitch of the token. Individually, transformed tokens were not perceptibly different from untransfor med tokens. P r o c e d u r e Subjects were given written instructions in w hich they were asked to judge the sim ilarity of the standard and the comparison tones by responding ``sam e or ``different as quickly and accurately as possible. They were asked to ignore the interpolated material and were told explicitly that they would not be tested on its contents. The conditions were presented in random order from trial to trial, and each subject undertook 160 trials in all, 20 in each condition, half the trials having the correct response ``sam e and half having the correct response ``different. R e s u l t s a n d D i s c u s s i o n Percentage correct responses for this and all other experiments in the series are given in Table 1. S tatistical analysis by ANOVA revealed no overall effect of the interval between the standard and the interpolated list, F(1, 19) = 4.02, p >.05. There was however a signi cant effect of interpolated material, F(3, 57) = 28.52, p <.0001, and a signi cant interaction between interpolated m aterial and interval, F(3, 57) = 4.31, p <.01.

6 3 4 2 J O N E S, M A C K E N, H A R R I E S T A B L E 1 S u m m a r y o f R e s u l t s f o r t h e E x p e r i m e n t s i n t h e S e r i e s I nterval 350 msec 1,500 msec Experiment 1 tones digits repeated syllable cello Experiment 2 9-tones ton es 67 Experiment 3 same ear 52 different ear 60 both ears 60 Experiment 4 digits/common F0 82 digits/changing F0 65 one digit/changing F0 74 Experiment 5 cello 62 digits 86 instrument/repeated 66 instrument/changing 56 Experiment 6 tone 63 digits 84 reversed digits 79 glides 73 As D eutsch (1970) and Pechm ann and M ohr (1992) found, the difference between tones and the digit sequence was highly signi cant, both in the 350-m sec and the msec interval conditions: F(1, 57) = 89.44, p <.0001, and F(1, 57) = 23.27, p <.001, with tones showing much greater disruption of recogn ition. H owever, the contrast between the cello sequence and the repeated syllable sequence was clearly non-signi cant, F < 1 in both the 350-msec and the 1500-m sec interval conditions. Interpolated tones led to po orer performance than the repeated syllable sequence for the 350-m sec interval, F(1, 57) = 13.76, p <.001, but not for the 1500-m sec interval, F < 1. T he same pattern was also obtained with the com parison between interp olated tones and interp olated cello: F(1, 57) = 7.74, p <.01 for the 350-msec interval, an d F < 1 for the 1500-m sec interval. In sum, although speech m ade up of a sequence of spoken digits shows the effect found by Deutsch, when speech and non-speech are matched in term s of between-stimulus acoustic change within a sequen ce, the difference between speech and non-speech is n onsigni cant.

7 M E M O R Y F O R T O N E S In relation to the effect of tim ing, the bene cial effect on recognition of extending the interval between the standard and the interpolated sequence from 350 msec to 1500 msec was restricted to the case of interpolated tones, F(1, 19) = 14.45, p < Extending the interval from 350 m sec to 1500 msec had no effect on sequences with interpolated cello, interp olated repeated syllable, or interpolated spoken digits, F(1, 19) = 1.69, p >.05, F(1, 19) = 0.08, p >.05, and F(1, 19) = 0.70, p >.05, respectively. O verall, these results supp ort the stream ing hypothesis and iden tify the role of confounding factors in the original D eutsch (1970) study. T he results also speak to the interplay of tim ing and timbre in stream ing: Only when the standard w as bound perceptually to the interpolated list by timing did the action of tim bre serve to isolate the standard, thereby enhancing recognition perform ance. One possible alternative interpretation is that increasing the tim e interval between the standard and the interpolated sequences might provide subjects with add ition al time to encode more accurately the standard tone before it is degraded by the subsequent interpolated tones, leading to improved p itch recognition. This seem s unlikely. T he interval of 350 m sec is longer than that wh ich usually gives rise to auditory backward recognition masking (e.g. K allm an & M assaro, 1979), and so an exp lanation of the high level of disruption found with interpolated tones in these term s would seem to be im plausible (see also K allm an, Cameron, B eckstead, & Joyce, 1987). F urthermore, the longer interval of 1500 msec is within the time ran ge that gives rise to the perception of single isolated auditory even ts, rather than grouped items (Fraisse, 1978). Overall, the results of Experiment 1 are m ost plausibly interpreted in term s of an auditory scene an alysis approach, within wh ich speech and non-speech h ave similar effects. F irst, there was no difference in terms of the disruption produced by matched speech and non-speech sounds. Second, the effect of the timing of the standard and the interp olated sequence was lim ited to tones, implying that the original nding of Deutsch (1970) was subject to the effect of perceptual grouping. Th e results of Experiment 1 suggest strongly that speech and non-speech sounds are equipotent in their capacity to disrupt m emory for sound, and that speech is not a suf cient condition for low interference. In the next two experim ents, we further explore the role of grouping m anipulations in determining pitch recognition perform an ce. E X P E R I M E N T 2 The effects of timing of the standard tone in Experim ent 1 point to the im portance of the organization of the interpolated list. This indicates the possible value of construing the disruption of pitch mem ory in the Deutsch (1970) paradigm in terms of the processes by which items are organized into higher-order units, or stream s, rather than in terms of a degradation in the representation of the standard tone by retroactive interference due to the similarity of the interpolated list to the standard tone. A further test of this proposition is mounted in Experim ent 2 by manipulating the number of tones in the interpolated sequence. We compare the case in which 9 tones are presented in the interpolated sequence (as in Experim ent 1) with the case w here the number of tones is increased to 17, without changing the interval within which these tones are presented. From the viewpoint of retroactive interference, it would be expected that an increase in the number

8 3 4 4 J O N E S, M A C K E N, H A R R I E S of similar stimuli should lead to an increase in interference, which would be signalled by a decrease in recognition perform ance. B ut if, as is strongly suggested by Experim ent 1, grouping processes in uence the degree of disruption produced by interpolated tones, the effect of nearly dou bling the num ber of interpolated tones (while keeping the total duration of the interpolated sequence constant) will be to isolate p erceptually the interpolated tones from the test tone. A s the num ber of interpolated tones is d oubled, the standard no longer shares the inter-item tim ing of the sequence as a whole. Therefore, the stream ing hypothesis makes the opposite prediction to that of the interference hypothesisð n am ely that n early doubling the number of tones in the interpolated interval will increase the likelihood of correct recognition. M e th o d S u b j e c t s Twenty-one underg raduates, each of whom reported normal hearing and passed the tone test described in Experim ent 1, volunteered to take part and were paid an ho norarium. N ine potential subjects failed the tone test. M a t e r i a l s a n d P r o c e d u r e Only tones were used as interpolated m aterials in this study, but the overall nature of the task and the frequency range of the tones were identical to the corresponding conditions of Experiment 1. The control condition was identical to the 350-msec interpolated (``tones 350 ) tones condition of Experiment 1. In the experim ental condition the test tone was separated by 350 msec from an interpolated set of 17 tones, nearly tw ice the number used in the control condition. In no case was a tone immediately followed by another of the same frequency w ithin the interpolated sequence. Although in the 17-tone condition one to ne nished as another started, the rise and fall tim es of the tones (as in E xperim ent 1), coupled with a change of frequency at each to ne in the sequence, m eant that they were phenomenally separate and distinct. In both conditions the com parison tone occurred, as in Experim ent 1, 1500 m sec after the end of the interpolated list. In each condition, 20 trials were presented, with ``sam e and ``different responses equal in number appearing in random order. The procedure was as in E xperim ent 1. R e s u l t s a n d D i s c u s s i o n Results were analysed in term s of p ercen tage of correct responses. Performance in the pure-tone control condition (60% correct) was com parable to that found in the ``tones 350 con dition of Experiment 1 (in which the level was 57% correct). In addition, recognition perform ance was signi cantly better with 17 interpolated tones (67% correct) than with 9 interpolated tones, F(1, 20) = 6.05, p <.025. The results of Experim ent 2 were directly in line with the stream ing hypothesis and at variance with the retroactive interference hyp othesis: Almost doubling the number of interp olated tones did not increase disruption; rather, it was reduced. The effect was one roughly com parable in m agnitude to that found by extending the interval between the standard and the interpolated tones in Experiment 1. Any account based on retroactive

9 M E M O R Y F O R T O N E S interference by interpolated sounds would be hard-pressed to predict that an increase in the number of interpolated tones would improve recognition of the test tone. Although the results of Experiment 2 support the general thrust of the argum ent developed in Experiment 1, there is residual am biguity about the outcome of Experim ent 2, stemm ing from a confounding of interstimulus interval and the number of interpolated stimuli. As the number of interpolated stimuli was doubled, the interstimulus interval within the interpolated list was reduced from 350 msec to zero, but the interval between the standard and the rst item of the interpolated sequence remained at 350 msec. This am biguity cannot be resolved fully; therefore subsequen t experiments of the current series of experiments seek evidence using other stimulus manipulations that help converge on the stream ing hypothesis. Thus far, two different techniques of manipulating stream ing have provided convergent evidence for the usefulness of the auditory scene analysis fram ework in exam ining disruption of memory for pitch in the Deutsch paradigm. Experiment 3 seeks to extend this theme using another typ e of stream ing manipulation. E X P E R I M E N T 3 In this experiment the role of stream ing processes in determining disruption of pitch memory is exam ined by varying the spatial location from which the interpolated items are presented. This represents an extension of a previous study by Deutsch (1978) in w hich a comparison was m ade between con ditions whereby standard and com parison tones were presented m on aurally, and interpolated tones were presented either to the same ear as standard and comparison tones or to the other ear. Conditions in wh ich the interpolated tones were presented contralaterally to the standard and the com parison led to signi - cantly better performance than cond ition s where the interpolated tones were presented ipsilaterally. This result m ay be seen as supporting the interference view of the impairment of pitch memory, in the sense that the degradation of the representation of the standard tone by the subsequent occurrence of interpolated tones m ay be attenuated or elim inated by assigning those interpolated tones to a different ear-speci c representation. 2 However, another p ossible interpretationð one in line with a stream ing approach Ð is that spatial location serves as a powerful segregation cue to disem bed the stan dard from the early item s in the interpolated sequence. T he conditions used by D eutsch (1978) do not allow discrim ination between these two possibilities. In Experim ent 3, the possibility that the nding of D eutsch (1978) is better conceived of in grouping term s is explored by adding a third condition to her design. As well as presenting interpolated tones both ipsilaterally and contralaterally to the stan dard and comparison tones, we also include a condition in which the interpolated tones are presented to both ears simultaneously. B ecause it is presented binaurally, the interpolated sequ ence will be perceived as originating somewhere in the centre of the head, thus forming a distinct perceptual group relative to the 2 In fact Deutsch (1978) interpreted her results in an attentional rather than in a retroactive interference framework. Her results are compatible with a retroactive interference explanation, however.

10 3 4 6 J O N E S, M A C K E N, H A R R I E S standard. From the point of view of the stream ing hypothesis, therefore, this con dition would be expected to give rise to an improvem ent in recognition equivalent to that found with contralateral presentation of the interpolated tones. From the viewpoint of the earspeci c channel hypothesis, such a condition would be expected to be sim ilar in its effect to ipsilateral presentation, as interpolated tones are still presented to the same ear as the standard and the comparison and therefore enter the sam e ear-speci c channel. M e th o d S u b j e c t s Twenty subjects were selected on the basis that they passed two screening tests. The rst was the pitch discrim ination test used in E xperiments 1 and 2. The second involved testing subjects for their ability to discriminate the location of presented tones. Subjects were played 15 tones: ve to the left ear, ve to the right ear, and ve to both ears at once. They were required to m ake a three-alternative forced-choice decision as to whether to ne s occurred in the left, right, or centre locations. Only subjects w ho scored 13 or m ore correct out of 15 went on to the experim ent proper, and 7 potential subjects were rejected as a result of these two tests. A p p a r a t u s /M a t e r i a l s The 12 pure tones at sem itone intervals of the scale C4 to B4 were generated as described in Experiment 1. T hese tones were assembled to provide three different types of trial sequence. The tem poral features of all three types of sequence were identical, with a standard tone, lasting 350 msec, followed after 350 msec by nine 350-m sec interpolated tones, each separated from its neighb ours by 350 msec. After a silent interval of 1500 msec at the end of the interpolated sequence, a comparison tone, also lasting 350 m sec, was presented. On half the trials, the com parison w as the sam e pitch as the standard; on the other half, it differed by a sem itone (half were higher and half lower than the standard). The frequencies of the to nes in the sequence were selected in the sam e way as the to ne sequences described in Experim ent 1. Three types of sequence were generated. In the same-ear condition, the standard w as presented to a single ear, followed by the interpolated sequence to the same ear, followed by the comparison tone also to the sam e ear. In the different-ear condition, the interpolated tones were presented to a different ear to that of the standard and the com parison tones. In the both-ears condition, standard and comparison tones were both presented monaurally to the same ear, but the interpolated tones were presented stereophonically to both ears. The amplitude of the binaurally presented interpolated sequences w as set in pilot trials such that they were of equivalent loudness to the m onaurally presented tones. Tones presented to a single ear were presented to the left ear on half the trials and to the right ear on the other half of trials. The sequences were stored as ``snd resources on a M acintosh IIcx for playback in a predetermined random order via a Hyperca rd program during the experiment. Subjects wore stereo headpho nes throughout the screening tests and experiment. D e s i g n a n d P r o c e d u r e Subjects were presented with 60 trials in all, 20 for each of the three conditions (sam e-ear, different-ear, both-ears). Conditions changed random ly from trial to trial. In other respects the procedure was the sam e as Experim ent 1.

11 M E M O R Y F O R T O N E S R e s u l t s a n d D i s c u s s i o n Subjects responses were calculated in terms of percentage correct for each condition. M eans were as follows: 52% in the sam e-ear condition; 60% in the d ifferent-ear condition; and 60% in the both-ears condition. T here was a signi cant overall effect of condition, F(2, 38) = 4.82, p <.02. Planned comp arisons revealed signi cantly better performance in both the different-ear and the both-ear conditions relative to the sam e- ear condition, F(1, 38) = 7.00, p <.02, and F(1, 38) = 7.45, p <.01, respectively. T here was no difference between differen t-ear and both-ears conditions, F < 1. These results converge with those of Experiments 1 and 2 in providing further sup port for the streaming hypothesis of interference effects in pitch mem ory. If the marked level of disruption foun d w hen interpolated tones are presented in the sam e spatial location as the standard tone was due to interference w ithin an ear-speci c channel, then the bothears condition should have led to the sam e degree of disruption as the same-ear condition. Instead, the results support the argument that disruption effects in pitch m em ory m ay be viewed usefully in the context of the perceptual processes whereby item s and events are organized into higher-order perceptual groups, or stream s (see, for exam ple, B regm an, 1990). To the extent that such processes allow the standard and the interpolated tones to be perceived as distinct entities, then recognition p erformance will be enhanced. T he results also serve to reinforce those of K allm an et al. (1987), who found the superiority of the d ifferent-ear condition was restricted to a procedure in which con ditions were blocked rather than randomized. The results of the curren t series of experim ents show that the effect also occurs with randomized conditions. Thus far, the results have suggested that the auditory scene analysis fram ework provides a useful w ay of approaching the ndings of Deutsch (1970, 1978) and Pechm an n and M ohr (1992). W ithin the present series, grouping manipulations (by time, by timbre, by location, an d by num ber) have been show n to m od ulate the interference of pitch memory cau sed by interpolated tones. In the three nal experiments we return to the question, rst p osed in Experim ent 1, of why spoken digits produce such low levels of disruption. As h as already been mentioned, such stimuli d iffer from the tone sequences in poten tially critical ways: First, each item in the sequence is spectrally distinct from its neighbours in a way that the tones are not; and second, the spoken digits do not change in overall pitch in the w ay that the tones do. In the following experiments, the in uence that these factors m ay have on modulating the disruptive capacity of interpolated sequences is investigated. E X P E R I M E N T 4 A potentially critical difference betw een the interpolated tones and the digits used by Deutsch (1970) was that whereas the tones varied in frequency from item to item, all digits shared a comm on fundam ental frequency (that is, they were spoken at the sam e overall pitch). In Experim ent 4, we exam ine the possibility that this com mon overall fundamental frequency contributes to the low level of disruption of pitch m emory found with such digit sequences. From the streaming point of view, we m ay hypothesize that a shared fundam ental frequency am ong item s increases the likelihood that such items will

12 3 4 8 J O N E S, M A C K E N, H A R R I E S be grouped together coherently, thus facilitating the perceptual isolation of the standard tone from the rst few items in the interpolated sequence. In Experim ent 4, the effect of interp olated digit sequences that share a comm on fundam ental is compared with sequences that change in fund am ental frequency from item to item. Changing the fundamental frequency should reduce the tendency for items to be grouped together coherently. From the streaming view, changing the fundam ental frequency from digit to digit may red uce the strength with which such items may be grouped together separately from the standard, therefore increasing the level of disruption of pitch mem ory. Th e technique used in this experiment is to change the frequency of a token using digital signal processing techn iqu es allow ing all other characteristics (such as duration and intonation) to rem ain xed while the pitch is changed. Phenomenally the effect of this is to m ake the digits sound as if they are spoken by different voices. In order to exam ine these possibilities, Experim ent 4 em ploys conditions in w hich the content of the interpolated sequences is m anipulated in three ways: (a) the usual spoken digit condition as used in Experiment 1; (b) a condition that involves presentation of the nine spoken digits, but in this case each item is shifted in pitch to 1 of 12 semitone steps; and (c) a single spoken digit (``one ) shifted in pitch in the same way as in Condition (b). Conditions b and c, therefore, share a com mon range of pitches but differ in term s of the variation in tim bre; Condition c has a com mon tim bre throughout the interpolated sequence, but in Condition b the timbre varies. Repeating the sam e speech token, albeit at different pitches, should also increase the likelihood that such a sequence w ill form a more coherent group than sequen ces that have both pitch and tim bral change (as in Cond ition b). Therefore, from the point of view of the grouping hypothesis, we would expect Condition c to produce less disrup tion than Con dition b. Thus, the conditions exam ined in Experiment 4 may be seen within the streaming framework as representing three levels of coherence in the interpolated items, with repeated digits sharing a com mon fundamental being the m ost coherent, and pitch shifted versions of all the digits the least coherent. M e th o d S u b j e c t s Eighteen subjects participated in the experiment for an honorarium. They were selected on the basis that they passed the pitch discrimination test as described in Experiment 1; 4 potential subjects were rejected on this basis. A u d i t o r y M a t e r i a l s Three types of sequence were constructed: (a) digits 1± 9/common F0, identical to those used in the ``digits 350 condition used in Experim ent 1; (b) digits 1± 9/changing F0, in which each of the digits 1 to 9 was digitally processed such that each had a fundamental frequency corresponding to one of the sem itone steps in the octave C4 to B 4; and (c) single digit/changing F0, in which the spoken digit ``one was digitally copied and pitch-shifted to each of the semitone steps of the C4 to B4 octave. The pitch shifting technique was the sam e as that used in Experim ent 1. The overall order of the sequences was the sam e as that used in the 350-m sec conditions of E xperim ent 1.

13 M E M O R Y F O R T O N E S D e s i g n a n d P r o c e d u r e Twenty sequences of each type were constructed and randomly ordered on a digital tape for playback through headphones during the experiment. The general procedure was the same as used in Experiment 1. R e s u l t s a n d D i s c u s s i o n Subjects responses were scored as percentage correct in each condition. T he means for the three conditions were as follows: digits 1± 9/com mon F0, 82% ; digits 1± 9/changing F0, 65% ; and single-digit/changing F 0, 74%. A repeated-measures A NOVA revealed a signi cant m ain effect of condition, F(2, 34) = 18.78, p < Planned com parisons indicated that digits 1± 9/com mon F0 led to better recognition perform ance than either the digits 1± 9/changing F0 condition or the single-digit/chan ging F 0 condition, F(1, 34) = 37.47, p <.0001, and F(1, 34) = 7.85, p <.01, respectively. There was also a signi cant difference between the digits 1± 9/changing F 0 and the single-digit/ changing F 0 conditions, F(1, 34) = 11.02, p <.005, w ith digits 1± 9/changing F0 producing a greater degree of disruption of pitch recognition. These results clearly indicate that one feature of spoken digit sequences giving rise to low levels of disruption is that the items share a comm on fundamental frequency. Changing the F0 breaks up the coherence of such sequences. Presenting such sequences with each item at a different frequency leads to a substantial increase in their disrup tive capacity, to the order of a 17% increase in errors. The results also suggest that this is due, at least in part, to the effect that changing the frequency from item to item has on the strength w ith which such sequences form coherent groups. By repeating the same item at different frequencies (single-digit/changing F 0), performance is im proved relative to the digits 1± 9/changing F0 by approximately 9% Ð an im provement that we suggest is due to the increased likelihood with which sequences may be coherently group ed, facilitating the perceptual isolation of the standard tone. E X P E R I M E N T 5 Experiment 4 focused on som e of the aspects of speech sounds that m ay give rise to low levels of disruption. In Experim ent 5 attention is turned to non-speech sou nds. In particular, questions are posed relating to whether the low level of disruption p roduced by spoken digits can also be prod uced by non-speech sounds that share the characteristics of spectral change from item to item while retaining a com mon fundam ental frequency. W ithin the current series of experiments it has been shown already that speech (repeated syllables) can be made to exhibit effects sim ilar to those of non-speech soun ds (repeated cello notes) in Experiment 1. For analytic purposes, it would be useful also to d em onstrate the complementary effectð that is, to show that n on-speech sounds can produce effects sim ilar to com plex and varying speech. This would show that speech w as not a necessary condition, augm entin g the evidence of earlier experiments in the current series that demonstrated that speech was not a suf cient condition for low levels of disruption.

14 3 5 0 J O N E S, M A C K E N, H A R R I E S In Experim ent 5, two conditions are introduced to examine this possibility. In one, the interp olated sounds are made up of a range of different instruments playing the sam e note, thus varying in timbre but not in pitch. This is contrasted with a condition in which the sam e range of instrum ents play different notes: speci cally, the range of pitch change used in previous experiments in semitone steps w ithin an octave range. Th e comparison of these two conditions should indicate whether change in pitch and tim bral qualities within a sequence of non-speech sounds gives rise to the same type of effect as has been shown with speech sounds in Experiment 4. As control, conditions identical to the ``cello and ``digits (at 350-msec interval between the standard and the interpolated sequence) conditions of Experim ent 1 are also used. In sum, Experim ent 5 poses two questions: D oes varying a com mon tim bre augm ent or reduce the group ing, and does an additional change of pitch reduce coherence still further? M e th o d S u b j e c t s Twenty subjects, screened by a test for tone discrimination (see above), were paid an ho norarium for taking part. Nine potential subjects were rejected on the basis of the pitch discrimination test. P r o c e d u r e The general procedure was identical to that used in E xperim ent 2. Four conditions were used in which interpolated materials were constructed from the following sounds: (a) cello, w hich was identical to the ``cello 350 condition of Experim ent 1; (b) digits, identical to the ``digits 350 condition of E xperiment 1; and two new conditions: (c) an instruments/repeated- pitch condition comprising sequences of instrumentsð cello (bowed), guitar, French horn, saxophone, pipe-org an, ute, piano, glockenspiel, trum petð playing the sam e note (C 4) in random order; and (d) an instruments/changing-pitch condition using the same randomly ordered sounds, but in addition the notes varied in the sam e range as those for ``cello Ð that is, 12 notes in sem itone steps for the octave C4 and above. The instrumental sounds were based on digitized samples of sound effects stored on compact disk (Digidesign Sa mplecell). T hese were subsequently pitch-shifted and edited (as in previous experiments in this series) using Digidesign SoundTools digital signal processing software. T he timing of the stimuli was as in the ``350 conditions of E xperiment 1Ð that is, with the standard spaced 350 msec ahead of the rst interpolated sound and each of the interpolated sounds spaced at 350 msec. Subjects were presented w ith 20 trials per condition, as in previous experiments. R e s u l t s a n d D i s c u s s i o n Results were an alysed in terms of the percentage of correct responses in each condition. The means for the four con ditions were as follows: instruments/repeated-pitch, 66% ; instrum en ts/changing-pitch, 56% ; cello, 62% ; an d spoken digits, 86%. An overall ANOVA showed the effect of the type of interpolated material to be highly signi cant,

15 M E M O R Y F O R T O N E S F(3, 57) = 26.89, p < Planned com parisons indicated, just as shown with spoken digits in Experim ent 4, that changing the pitch from item to item in the instrum ent sequences led to an increase in errors relative to the instruments that share a com mon fundam ental frequency, F(1, 57) = 5.56, p <.03, although the instrum ents/repeatedpitch still produced more errors than the spoken digits condition, F(1, 57) = 34.77, p < The results of Experiment 5 also speak to the m atter of tim bre. The level of performance in the instruments/changing F0 condition was as poor as that found for tones in Experiment 1. T he main difference between this use of instrum ental sound and its use in Experiment 1 is that tim bre changed from one item in the interpolated sequence to the next. Added to the loss of coherence in tim bre is the effect of change of frequency. The substantial differen ce between the disruption produced by the instru ments/ repeated F0 condition and the spoken digit condition indicates that comm on fundam ental frequency and spectral chan ge from item to item in the interpolated sequence are not in themselves suf cient to give rise to the very low levels of disruption of pitch recognition found with digit sequences. Nonetheless, the com parison between instrum en ts/changing F0 and instrum ents/repeated F0 in the present experim ent, as well as the comparison between spoken d igits and pitch shifted digits in Experiment 4, indicates that a shared fundam ental frequency am ong item s leads to a decrease in disruptionð probably, we would argue, because a shared fundam ental frequency am on g item s increases the likelihood that such items will be grouped together coherently. However, the question rem ains as to what other characteristics of spoken digit sequences lead to very low levels of interference. O ne possibility is that even though each spoken digit is spectrally distinct from oth ers, they still share m any attributesð other than fundam ental frequencyð which may also contribute to the likelih ood of their being grouped together. As the digits were all spoken in the sam e voice and therefore share certain acoustic qualities in a way in which different instrument sounds do not, it seems possible that such item s are m ore likely to be grou ped together. Such an explan ation would appear to be plausible in the light of previous results that have highlighted the importance of grouping processes in modulating the disruptive capacity of interpolated soun ds (see Jones & M acken, 1995; Jones, M acken, & M urray, 1993). An alternative explanation m ay be that the sem antic content of the speech serves to bind the items more coherently together. Thus there m ay be some ``top-down processes that serve to integrate the items more coherently into a separate perceptual grou p. We explore these possibilities in the next experim ent. E X P E R I M E N T 6 Experiment 6 evaluates the effect of sequences of non-speech sounds constructed in a way likely to increase their binding together of the items. Coherence within the interpolated sequence was achieved by generating a continuous glide varying randomly in frequen cy. Continuous glides were produced by rst low-pass ltering pink n oise at a very low frequency and then using this randomly varying sign al to drive a voltage-controlled oscillator. T he oscillator then generated a sound with a pitch in proportion to the voltage driving it. Th is glide was then interrupted regularly by short periods of silence. Hence, a