An experimental evaluation of three theories of auditory stream segregation

Transcription

1 Prcption & Psychophysics 1993, 53 (2), An xprimntal valuation of thr thoris of auditory stram sgrgation WENDY L. ROGERS and ALBERT S. BREGMAN McGill Univrsity, Montral, Qubc, Canada Thr thoris of auditory stram sgrgation wr valuatd. In two-part trials, subjcts hard an induction squnc, whos ffcts upon an immdiatly subsqunt tst squnc wr masurd. Th rhythm and total duration of Induction Squnc tons wr varid in two xpri. mnts. Th similarity btwn induction and tst squncs aidd sgrgation, but rhythmic prdictability and longr ton durations did not. Frquncy altrnation during th induction squnc was not ncssary to induc sgrgation in th tst squnc. Furthrmor, priphral procsss inadquatly account for th sgrgation ffcts found. Th data suggst that, onc a distinct prcpt mrgs from an auditory scn, proprtis drivd from th prcpt (particularly changs) ar fd back to control th ongoing analysis of that auditory scn. A nural adaptation to stimuli with constant proprtis may form part of this analysis. Whn two tons of diffrnt frquncis ar altrnatd continuously, th listnr s prcption changs ovr tim. At first, an up-and-down pattrn (high-low-high-low) is hard. If th squnc is prsntd long nough and rapidly nough, and if th tons ar far nough apart in frquncy, th listnr hars th high nots and th low nots sgrgat from on anothr and form two apparntly indpndnt squncs of tons. This splitting phnomnon has bn calld stram sgrgation by Brgman and Campbll (1971), and it has bn studid xtnsivly (Brgman, 1990, chap. 2). Th radinss of an altrnating squnc to form two strams incrass with th rat of ton prsntation and th frquncy sparation btwn th high and low tons, and ths factors can b tradd off on against th othr. Thr ar thr thoris about stram sgrgation. Brgman (1978a, 1990) ss straming of altrnating squncs as an illustration of th partitioning, by th auditory systm, of incoming sounds into groups (calld strams) likly to blong to th sam sound sourc. H hypothsizs two main classs of grouping procsss: simultanous (combining concurrnt sounds, as in haring svral co-occurring pur tons as on complx sound) and squntial (grouping sounds ovr tim, as in linking of tons to form a mlody). Th straming of altrnatingfrquncy squncs is dscribd, by Brgman s thory, as squntial grouping of tons according to th Gstalt Ths xprimnts wr prformd in partial fulfillmnt ofth rquirmnts of W.L.R. s PhD dgr at McGill Univrsity. W thank Pirr Ahad and Rhonda Amsl for assistanc, and thr anonymous rviwrs for suggstions and commnts. Th rsarch was supportd by a grant to A.S.B. by th National Scincs and Enginring Rsarch Council of Canada and by a McGill Major Fllowship to W.L.R. W. L. Rogrs is now at th Douglas Hospital, 6875 LaSall Boulvard, Vrdun, Qubc, Canada H4H 1R3. Rprints ar availabl from A. S. Brgman, Dpamnnt ofpsychology, McGill Univrsity, 1205 Dr. Pnfild Avnu, Montral, Qubc, Canada H3A lbl. principl of proximity, with frquncy proximity tnding to ovrrid tmporal proximity. Th straming, or sgrgation tndncy, builds up ovr a priod of svral sconds, and th tndncy to favor a two-rgion prcpt is lost slowly aftr th tons ar stoppd. Ths tim lags ar thought to b ncssary (Brgman, 1978b) for th stability in th straming procss to b maintaind. Data from on xprimnt (Brgman, 1978b) show that th sgrgation of a rpating squnc bcoms strongr as th numbr of rptitions incrass. Brgman proposd that this could b a simpl cumulativ ffct of ton duration in crtain frquncy rgions, implying that mor stimulation in a frquncy rgion would always incras th sgrgation. Van Noordn (1975) proposd anothr thory: that altrnating-ton squncs activat pitch-motion (frquncyjump) dtctors, which thn bcom adaptd and unabl to follow th up-and-down pattrn. Anstis and Saida (1985), who proposd a thory similar to Van Noordn s, found vidnc consistnt with adaptation of auditory frquncymodulation dtctors: th obsrvd adaptation is spcific to th frquncy rgion of th altrnating tons, incrass ovr tim, and tnds not to rvrs; that is, whn a stram of sounds splits, it tnds not to rturn to th form of a cohrnt stram. Anstis and Saida (1985) rviwd th vidnc for th xistnc of nural frquncy-modulation dtctors, and thy compard th rsults of thir auditory xprimnts to thos from a similar study (Anstis, Giaschi and Cogan, 1985) prtaining to brakdown of apparnt motion in th visual systm, which is blivd to b trackd by nural motion dtctors. An analogy has oftn bn notd (.g., by Brgman & Achim, 1973) btwn straming of altrnating tons and a phnomnon in vision: whn two lights turn on and off in altrnation at crtain rats, th apparnt motion first sn btwn thm dgnrats into flickr. 179 Copyright 1993 Psychonomic Socity, Inc.

2 180 ROGERS AND BREGMAN Anstis and Saida (1985) proposd that similar procsss ar at work in both th visual and th auditory systms whn th lattr attmpt to link up succssiv stimuli ovr tim. With rspct to altrnating tons, th procss that tris to link tons according to frquncy proximity is in conflict with th procss that tris to track th up-anddown pattrn of tons according to tmporal proximity. If th lattr procss shows adaptation, th formr procss will com to dominat. Anstis and Saida (1985) found that adaptation dos not cross th ars. If a monotic squnc is prsntd to on ar until it sgrgats, and if it is thn suddnly switchd to th othr ar, th squnc is hard as intgratd again. Thy concludd that th adaptation was likly to b a priphral procss, occurring bfor th point in th nrvous systm at which information from th two ars coms togthr. Thy also found that adaptation was spcific to th particular rang of frquncis at which it had occurrd. Thrfor thy thought of it as a procss that was local (in th frquncy domain) that is, as a procss that works within small rangs of frquncis rathr than on that could b affctd by any of th frquncis prsnt in th sound mixtur. Howvr, th thory of priphral and local pattrn dtctors is contrary to crtain xprimntal vidnc: th ida that th procss is a local adaptation conflicts with vidnc that it is th rlativ spacing, rathr than th absolut spacing, of tons in frquncy and tim domains that dtrmins straming (Brgman, 1990; McNally & Handl, 1977). Th whol auditory contxt appars to b usd. Also, frquncy-jump dtctors ar not ncssary to xplain som cass of sgrgation of ton squncs. In an xprimnt rportd in 1975, Brgman and Rudnicky causd a pair of lowr tons (X..X) to sgrgat from a pair of highr ons (AB) in a four-ton squnc (XABX) by placing othr low tons (flanking tons, C) both bfor and aftr th original squnc (CCCXABXCC). Th flanking tons wr similar in frquncy to th first and last tons of th original squnc. Although no frquncy jumps wr addd, th xtra tons capturd tons of th sam frquncy rang out of th ovrall squnc. Jons (1976) proposd th third thory of straming: that ofrul-basd prdictability. According to hr thory, auditory stimuli ar rprsntd psychologically on dimnsions (such as pitch, loudnss, and tim), and th auditory systm is dsignd to dtct crtain typs of rgularitis in a squnc of sounds. Ths rgularitis ar thn usd, by an attntional procss, as ruls to prdict subsqunt incoming sounds. Jons also proposd that changs along th othr dimnsions wr rquird to b proportional to changs on th tim dimnsion; for xampl, a rapid squnc of altrnating tons (tons with a small tim gap) would only b followd whn th frquncy sparation btwn squntial tons was small that is, in proportion with th tmporal sparation. Jons s thory is thus consistnt with th fact that th prsntation rat and th siz of th frquncy jump can b tradd off against ach othr. According to Jons s (1976) thory, th rgular spacing of th flanking tons (CCC....CC ) in Brgman and Rudnicky s (1975) study was what allowd straming to occur; th rgular tons st up an xpctancy, and th prdictability of th squnc mad it asir to hold togthr as a stram. Jons, Kidd, andwtzl (1981), using similar stimuli, found that whn th flanking tons had a diffrnt rhythm from th othr tons, th flanking tons wr lss ffctivat capturing othr tons into a stram. Howvr, Frnch-St. Gorg and Brgman (1989), using squncs with random variations in frquncy or timing or both, showd that nithr prdictability of frquncy nor that of timing was ncssary for straming of ton squncs into two frquncy rangs. Yt it was admittd by th authors that thir subjcts may not hav bn abl to prdict th mor rgular squncs wll nough to provid an adquat contrast to th unprdictabl conditions of th xprimnt. Jons s (1976) thory dos not xplain why rpating cycls, of tons altrnating btwn two frquncis (vry prdictabl squncs), ar hard as on stram for svral sconds bfor thy split. Such rptitiv stimuli bcom mor prdictabl ovr tim and ought to b incrasingly rsistant to straming, according to Jons s thory. Th vidnc is conflicting, or not availabl, for many issus rlatd to th thoris about straming. With rspct to Brgman s (1990, 1978b) thory, is thr a cumulativ mchanism (activatd by tons in a givn frquncy rgion) that bcoms a strongr influnc on grouping as th cumulativ ton-duration incrass? Rgarding th frquncyjump dtctor thory, ar frquncy-jump dtctors ncssary for straming? Ar straming procsss priphral, or do thy rquir information from both ars? If frquncyjump dtctors xist, do thy caus straming by bcoming adaptd? Finally, concrning Jons s thory, whn all othr proprtis of a squnc ar th sam, do a prdictabl and an unprdictabl squnc hav diffrnt influncs on th cumulativ ffcts of straming? GENERAL METHOD Th sam basic mthod was usd in thr xprimnts, to tst th thoris. In ach trial, an induction ton squnc, immdiatly followd by a tst squnc, was prsntd. Th boundary btwn th two squncs was not signald to th subjcts. Th induction squncs wr varid, and thir ffcts on a constant tst squnc wr masurd. Th subjct ratd th dgr ofsgrgation of th tst squnc at its nd, and this was usd as a masur of th ffctivnss of th induction squnc at building up a sgrgativ tndncy. This paradigm providd a way to vary th proprtis of th tons prcding th tst squnc in ordr to dtrmin which proprtis aid or rsist sgrgation. A tst squnc with wll-known proprtis was chosn for us with all trials: th two-frquncy gallop squnc dvlopd by Van Noordn (1975), shown in Figur 1. This pattrn of thr quallngth nots and a silnc has a rhythm that changs noticably whn

3 AUDITORY STREAM SEGREGATION 181 F q U 0 C y 400 msc Tim Figur 1. Two 400-msc cycls of Van Noordn s gallop squnc. Each rctangl rprsnts on pur ton. Th pattrn compriss a high not, low not, high not, and silnc. Conncting lins rprsnt possibl forms of prcptual grouping. it sgrgats into two strams, thus aiding th subjct in making a rating of sgrgation. Whn hard as a singl stram, it is hard as an up-and-down pattrn, as indicatd by th solid lins, with a galloping rhythm. Whn th squnc splits into two strams, as shown by th dashd lins, th sparat high- and low-pitchd rhythms bcom isochronous. Th subjcts wr askd to listn for a gallop. Van Noordn (1975) found that attntional factors ar important whn masuring straming ffcts: it is almost always asy to listn for and har out th individual strams, but th ability to maintain prcption of th upand-down gallop pattrn is highly snsitiv to th ton rptition rat and th siz of th frquncy sparation. Thrfor, in ordr to study what maks a cohrnt squnc split and in ordr to obtain a rang of ratings in ths xprimnts, it was ncssary that th subjcts try to har a gallop in th tst squnc. Th xprimntal dsign was similar for all th xprimnts. In ach, thr wr two xprimntal factors: (1) th siz of th frquncy sparation btwn high and low tons of th tst squnc, and (2) th typ of induction squnc. Th frquncy sparation in th tst squnc was varid for two rasons. On was to chck that subjcts in th xprimnt rplicatd th wll-known ffct of gratr straming with largr frquncy sparations. According to Van Noordn s (1975) data, it should b asy for subjcts to har a gallop (in th control condition) whn th tst squnc tons ar 5 smitons apart, and difficult or impossibl whn th gap is abov 12 smitons (an octav). Ths two xtrm valus wr chosn as lvls of th frquncy-sparation factor. Thy srvd also as a rmindr for th subjct, during th xprimnt, about th xtrm nds of th rating scal. Thus a statistically significant main ffct was always xpctd for frquncy sparation, and this is alludd to only brifly in th rsults. A scond rason for varying th frquncy sparation of th tst squnc tons was to dal with individual diffrncs by providing at last on sparation for ach subjct that was ambiguous nough to b affctd by th induction squnc. Th wid frquncy sparation ncssary to guarant straming may sm surprising, givn Van Noordn s (1975) data onth cohrnc boundary. Howvr, his subjcts wr xprincd at listning to altrnatingfrquncy squncs. In our laboratory it has bn found that naiv listnrs (th vast majority of our subjcts) can hold togthr squncs of tons that hav a larg frquncy gap; thr appars to b a strong ffct ofxprinc on th sgrgation ofsuch squncs. Th smiton (frquncy ratio of 1.06:1) was usd as th masur of frquncy proximity. Smitons rprsnt frquncy proximity on a log frquncy scal, and Millr and His (1950) dmonstratd that proximity rlatd to straming is linarly rlatd to frquncy sparation on a log frquncy scal. Soft whit nois, of th sam duration as th induction squncs, was chosn as a control for th lattr bcaus it has qual intnsity ovr th audibl frquncy rang, and bcaus it has bn shown to b quivalnt to silnc with rspct to th cumulation of straming ffcts (Brgman, 1978b). It is prfrabl to silnc bcaus it cus th subjct that a trial is in progrss. Th purpos of th control was to provid a contrast with xprimntal induction squncs by inducing th minimum possibl amount of sgrgation. Plannd comparisons wr usd to tst th hypothss of two xprimnts. For unxpctd xprimntal rsults, post hoc tsts wr usd in addition to plannd comparisons. Rpatd masurs wr usd in all of th xprimnts. Dspit th randomization of th ordr of prsntation of stimuli, th assumptions of homognity of covarianc wr not mt. Individual diffrncs in rspons pattrns, a common caus of nonhomognous covarianc, hav bn found for many xprimnts in our laboratory. Th Grnhous-Gissr procdur (Grnhous & Gissr, 1959), in which smallr dgrs of frdom ar usd in th F tst, is rcommndd whn thr is not homognity of covarianc; it provids a vry consrvativ F tst (Winr, 1971). W usd this approach for a priori hypothss. For post hoc tsts, Myrs (1979) has cautiond against th us of th Tuky HSD tst with rpatd masurs whn assumptions of homognity of covarianc ar violatd. Thrfor, post hoc contrast ffcts wr tstd with Bonfrroni-adjustd t tsts. This tchniqu was slctd bcaus it holds th xprimntwis alpha lvl constant and is robust to th violation of sphricity assumptions (Maxwll, 1980). OVERVIEW OF EXPERIMENTS Thr wr thr goals in Exprimnt 1: (1) to tst whthr straming can b inducd by a ton squnc that has no frquncy transitions, (2) to tst whthr prdictability aids cumulativ ffcts in straming, and (3) to tst whthr th total ton duration is a factor in th cumulativ ffcts in straming. Thus, th xprimnt addrssd a part of ach of th major thoris dscribd. Exprimnt 2 was dsignd so that th rlationship btwn cumulativ ton duration and straming could b xamindmor closly, bcaus th first xprimnt did not giv conclusiv rsults. Exprimnt 3 was dsignd to tst a part of th frquncy-jump dtctor thory: whthr or not priphral information is sufficint to xplain th induction of straming. EXPERIMENT 1 Th purpos in th first xprimnt was (1) to tst whthr a cohrnt gallop squnc can b causd to sgrgat by prcding it with a squnc of tons of only a singl frquncy (showing that th fatiguing of jump dtctors is not ncssary to xplain straming); (2) to compar th tndncis of a prdictabl vrsus an unprdictabl squnc, with othrwis similar proprtis, to induc sgrgation; and (3) to discovr whthr or not incrasing th dnsity of stimulation at on frquncy rsults in mor ffctiv induction of straming in tst squncs containing tons of th sam frquncy.

4 182 ROGERS AND BREGMAN Mthod Subjcts Th subjcts wr 21 adults who rportd normal haring, rcruitd from among studnts at McGill Univrsity. Thy wr paid for participating. On subjct s rsults wr discardd bcaus h disclosd a haring loss only aftr participating in th xprimnt. No scrning tsts of haring wr conductd. Stimuli All stimuli wr diotic. Each trial consistd of a 4.8-sc induction squnc, immdiatly followd by a 1.2-sc tst squnc, with a subsqunt 6.4-sc silnc for making a rspons. All tons wr pur tons, lasting 100 msc (with xcptions to b dscribd blow), whos statd durations includd both a l0-msc quartrsin-wav onst and dcay. All induction-squnc tons, and th uppr tons of th tst squncs, wr 524 Hz (on octav abov middl C). Th tst squncs wr thr 400-msc cycls of twofrquncy gallop pattrns. Thr wr four lvls of frquncy sparation btwn th high and low tons of th tst squnc: 5, 9, 11, and 18 smitons. Thr wr four induction squnc typs, ach containing twlv 400-msc cycls of sound. Part of a trial, showing th nd of th induction squnc and th bginning of th tst squnc for a sampl of ach of th four induction conditions, is shown in Figur 2. Condition I (th high-dnsity condition) was chosn to tst whthr incrasing th total duration of tons, and shortning th silncs, in th uppr frquncy rgion would incras th induction of sgrgation. It containd four qual-lngth tons pr 400-msc cycl. Th dnsity of high tons was thus twic that of th tst squnc. It was xpctd, according to Brgman s hypothsis of cumulativ frquncy bias, that this induction squnc would b mor ffctiv than squncs with lowr ton durations at inducing sgrgation of th tst squnc. Two othr conditions wr dsignd to compar th ffctivnss of prdictabl and unprdictabl squncs, with othrwis similar proprtis, at inducing straming of th tst squnc. Induction squncs for both ths conditions containd two tons pr 400-msc cycl. Th tons of Induction Condition 2 (th prdictabl condition) matchd th high tons of th tst squnc in tmpo Condition Numbr 1 Last 2 cycls of Induction Squnc I 2 ~ 3 4 ~ Tim U First 2 cycls of Tst Squnc. I ~ U * I. ~ III. Figur 2. Part of a trial for Exprimnt 1. Each dark rctangl rprsnts on pur ton, and th larg stripd rctangl rprsnts whit nois. A vrtical solid lin indicats th (unsignald) boundary btwn induction and tst squncs, and th vrtical dashd lins indicat cycl boundaris. and lngth. Th tons of Induction Condition 3 (th unprdictabl condition) wr unprdictabl in tmpo and lngth but had th sam avrag ton dnsity as did th prdictabl squnc. This is diffrnt from Frnch-St. Gorg and Brgman s (1989) xprimnt, in which th most rgular stimulus was a rpating squnc of ight tons of diffrnt frquncis. Th rgular squnc in th currnt xprimnt was vry simpl (with only on frquncy and an isochronous rhythm). Thrfor, it was unlikly that a prdictiv mchanism would b unabl to crat th appropriat ruls to dscrib it during th4.8 sc of th induction squnc. Furthrmor, this xprimnt was usd to tst th ffct of an induction squnc upon a subsqunt (diffrnt) squnc, whras Frnch-St.Gorg and Brgman usd th sam squncs for both induction and tsting. Th unprdictabl squnc was constructd by combining two diffrnt typs of randomization. Halfof th 400-msc priods containd loo-msc tons that could appar at th start of any of th four quartr parts of a 400-msc intrval (for ach quartr part, a random dcision was mad with rspct to whthr a not would appar). Th othr half wr constructd so that tons altrnatd with silncs, but th lngths of tons and silncs varid randomly from 60 to 140 msc. Cycls of th two typs wr altrnatd to mak th stimulus. Ovrall, th squnc was dsignd so that th avrag lngth of a ton was 100 msc and th avrag numbr of tons pr 400-msc cycl was two. Thr was on such squnc constructd and usd for all subjcts. Th subjcts rportd that it soundd lik Mors cod. Th stimulus was constructd with th us of th two diffrnt mthods bcaus it is not known whthr th concpt of irrgular timing or irrgular not lngth is mor important to th concpt of irrgular rhythm. Thrfor, both typs of irrgularity wr includd. Condition 4, th control condition, consistd of whit nois attnuatd to 52 db SPL. Th lvl of tons was calibratd to masur 70 db SPL (A wighting) in ach ar. Th frquncy rang of stimuli for this xprimnt ( Hz) was chosn in a rlativly flat part of th qual-loudnss contours (Moor, 1982). Procdur Training. Each subjct was first shown a diagram of th gallop pattrn, and th xprimntr tappdout th rhythm. An audio dmonstration of th gallop pattrn (thr contiguous cycls of it), at fiv diffrnt starting frquncis and frquncy sparations btwn high and low tons, was givn. Th subjct was thn qustiond to nsur his/hr undrstanding of th gallop prcpt. Aftr bing shown how to mak a rspons on th computr, having rad th instruction sht, and having bn givn a chanc to ask furthr qustions, th subjct did fiv practic trials. Tsting. Th subjct was askd to listn to th tons of ach trial and torat whthr or not a gallop was hard at th nd of th trial. Rating was don on a kyboard in th intrtrial silnc. Th rating scal wnt from 1 to 10, with a choic in th 1-5 rang indicating a gallop prcpt and a choic in th 6-10 rang indicating som prcptothr than a gallop. Choics closr to th middl of th scal indicatd lss confidnc in th rating, whras aior a 10 indicatd high confidnc in th rating. All subjcts wr tstd undr all conditions, with thr rplications of ach condition. Th conditions wr randomizd within blocks of trials containing on instanc of ach condition. Thr such randomizd blocks wr cratd, and th subjcts wr assignd squntially to on of th six possibl prsntation ordrs of th blocks. Th xprimnt took plac in on sssion. Apparatus Stimuli wr mad with th MITSYN sound procssing systm (Hnk, 1980) runningon a PDP-I 1/34 minicomputr with th RSX- I IM oprating systm. Th stimuli wr output by a sparat computr program. Th sampling rat was 10 khz, and a Rockland 851 anti-imaging low-pass filtr, with a roll-off of 48 db/octav, was

5 AUDITORY STREAM SEGREGATION 183 st to 4800 Hz. Th signals wr amplifid with a Sony TA-F444ES amplifir and snt to an Industrial Acoustics audiomtric tsting room (Modl 1202). Th signals wr playd ovr Tktronics TDH- 49P stro hadphons. Th lvls wr calibratd with a Gnral Radio Company sound-lvl mtr (Typ 1551-C), with a flat-plat hadphon couplr (A wighting). Rsults For ach subjct, th rsults of th thr trials of ach condition wr avragd, yilding on masur of inducd sgrgation pr condition for us as th dpndnt variabl. Figur 3 shows th avrag sgrgation inducd in th tst squnc by ach of th four induction conditions at ach frquncy sparation. A 4 (frquncy sparation) x 4 (induction condition) rpatd masurs analysis of varianc (ANOVA) was conductd, using th Grnhous Gissr corrction. Rliabl main ffcts for both induction condition [F(1,19) = O,p <.001] and frquncy sparation [F(1,19) = , p <.001] wr found. Thr was no significant intraction btwn th two factors [F(1,19) = 1.166, p >.25]. Th mans for frquncy sparations of 5, 9, 11, and 18 smitons, rspctivly, wr 2.9, 4.4, 4.6, and 6.3. Th mans for th highdnsity, prdictabl, unprdictabl, and control conditions wr (rspctivly) 4.9, 5.4, 5.4, and 2.5. Two orthogonal plannd comparisons wr don among induction condition typs. Th prdictabl and th unprdictabl conditions (with ton dnsitis of two tons pr 400-msc cycl) wr compard to dtrmin whthr 10 MAX 9 S 8 r 6 g a 5 t j4 0 n3 2 MIN Frquncy sparation (smitons) INDUCTION ~y~le ~ 4 tons + 2 tons~rgular * 2 tons~irrgular B- control (wh. nois) Figur 3. Rsults from Exprimnt 1. Inducd sgrgation as a function of tstsqunc frquncy sparation, for ach of th four induction conditions. rgularity of rhythm had an ffct. This was not significant ~F(1,l9) =.O04,p =.953]. Th high-dnsity condition, with four induction tons pr cycl, was compard with th avrag of th conditions with only two induction tons pr cycl (prdictabl and unprdictabl conditions combind). Thr was a rliabl diffrnc [F(1,19) = 4.582, p <.05], with th two-ton conditions mor ffctiv at inducing sgrgation. DISCUSSION Th main ffct of frquncy sparation confirmd that th data conform to known principls of stram sgrgation. Th thr xprimntal induction squncs inducd sgrgation of th tst squnc significantly bttr than did th control squncs, although thr wr no frquncy changs in any of thm. This dmonstrats that th habituation of frquncy-jump dtctors is not ncssary for xplaining th accumulation of sgrgation tndncis of tons altrnating btwn two frquncis. W did not find that a prdictabl induction squnc was any mor or lss succssful at augmnting sgrgation of th tst squnc than an unprdictabl on, whn th two induction squncs wr qual in frquncy composition and avrag ton dnsity. Bcaus both ths induction squncs had th sam ton dnsity as did th high tons of th tst squnc, th ton dnsity of th tst squnc was prdictabl from th induction squnc. Howvr, th prdictability was not rul-basd as in th thory of Jons (1976); that is, no rul could b usd to captur tons by prdicting whn th nxt on would occur, or how long it would last, bcaus ths proprtis wr random. Th four-ton-pr-cyclinduction squnc was lss ffctiv at augmnting sgrgation than was ithr of th two-ton-pr-cycl induction squncs. This is an intrsting finding, bcaus it shows that mor stimulation in a frquncy rgion is not ncssarily bur from th point of viw of capturing a stram of subsqunt tons from that rgion. EXPERIMENT 2 Th xprimntjust dscribd dos not support th ida that having mor tons in a frquncy rgion will ncssarily caus bttr capturing of tons of th sam frquncy. Yt with so fw conditions in th xprimnt, th rlationship btwn th total duration of th induction tons and th amount of inducd straming could not b dtrmind. Th following xprimnt was dsignd to lucidat this rlationship. Brgman s viw of a cumulativ procss in stram sgrgation prdictd a frquncy bias basd on th cumulativ amount of stimulation in on frquncy rgion. Anothr altrnativ, suggstd from th rsults of Exprimnt 1 abov, was that straming is basd on cumulativ information about th proprtis of th induction squnc, and that similarity btwn th induction and tst squncs would b th basis for grouping. In th prsnt xprimnt, all induction squncs wr

6 184 ROGERS AND BREGMAN mad irrgular to avoid th issu of prdictability. Jons s (1976) thory is thrfor not rlvant to this xprimnt. Mthod Only dtails that ar diffrnt from th prvious xprimnt will b dscribd. Subjcts Th subjcts wr 16 graduats and undrgraduats at McGill or Concordia Univrsitis. Stimuli Th tst squncs wr thr rptitions of a 400-msc two-ton gallop squnc whos lmnts wr pur tons of 80-msc duration (with linar onst and offst of 10 msc ach). Th diffrnc in frquncy btwn th high and low tons was 5, 9, 11, or 14 smitons. Th high tons wr always 1048 Hz. Thr wr six induction squncs, ach 4.8 sc long (twlv 400-msc cycls), containing only tons of on frquncy, th sam frquncy as that of th high gallop tons. Ths tons filld 0%, 20%, 40%, 60%, 80%, or 100% of th induction squnc. (For a diagram of on sampl cycl for ach of th induction squnc conditions, s Figur 4.) Th 0% condition, in which thr was no ton (control condition), was filld with soft whit nois. Th middl conditions (i.., in which th dnsity was not on of th xtrms of 0% or 100%) containd tons that wr intrmittnt and sparatd by silncs. Thir avrag lngth was 80 msc and thir dnsity was 1, 2, 3, or 4 tons pr 400-msc priod. Th actual ton lngth and spacing within ach priod wr random. Th stimulus in th 100% condition was a long ton, 4.8 sc in duration. Ths stimuli wr constructd by a computr program, as follows: for ach 400-msc cycl of th stimulus, ton lngths wr randomly chosn for th appropriat numbr of tons. Th tim rmaining inth cycl was allocatd to silncs, whos lngths wr randomly chosn, and which wr intrlavd with th tons. On squnc of 12 cycls was cratd for ach of th 20%, 40%, 60%, and 80% conditions, and all subjcts hard th sam squncs. Th subjcts rportd that th stimuli for ths middl conditions soundd lik Mors cod. Ton Duration Induction Squnc cycl (CONTROL> 100% 20% 40% 60% 80% Figur 4. On 400-msc sampl from th 4.8-sc induction conditions in Exprimnt 2. Th total ton duration is xprssd as a prcntag of th total tim of th induction squnc. Th larg stripd rctangl indicats whit nois. Th black rctangls rprsnt pur tons. Th lvl of th intrmittnt tons was 62.5 db SPL. Th continuous ton of th 100% condition was rducd to 59.5 db bcaus pilot studis showd it to b unplasantly loud to subjcts. Th lvl of th whit nois was 47 db SPL. Th lvls of th frquncy-sparation factor in th tst squnc wr chosn from pilot study data so that subjcts wr crtain to xprinc th full rang of ratings, from no sgrgation to complt sgrgation. Th lvls ofth ton dnsity factor wr chosn to covr vnly th rang btwn no ton and continuous ton in th induction squnc. Th frquncy rang ( Hz) was in a rlativly flat portion of th qual-loudnss curvs. Procdur Training. Th subjcts wr prsntd with a mnu on th computr scrn, which allowd thm to slct a sampl of a sgrgatd two-ton squnc, a cohrnt gallop, a fixd-ordr squnc of all th induction squncs, or a sampl trial. Thy wr rquird to listn to ach at last onc, but thy could choos xtra rptitions. Thn thy wr givn 12 trials, in which thy practicd giving ratings of sgrgation of th tst squnc. As a rsult, thy knw what kind of sounds to xpct and wr familiar with giving rsponss on th computr kyboard. Task. Th subjcts wr rquird to rat, on a 7-point scal, th dgr to which thy hard a gallop squnc clarly at th nd of th trial. A rating of 1 indicatd crtainty that no gallop was prsnt, and a rating of 7 indicatd that a gallop was clarly and asily hard. Th subjcts wr instructd to try to prciv a gallop. At last 5 sc of silnc, th subjct s rspons tim plus 5 sc, sparatd th trials. Th xprimnt took plac in on sssion. All subjcts wr tstd undr all conditions, with thr rplications of ach. Th conditions wr randomizd within blocks of trials containing on instanc of ach condition, and th randomization was diffrnt for ach subjct. Apparatus Th sampling rat usd in this xprimnt was 20 khz, and th Rockland 851 low-pass filtr was st to 10 khz. Sound was prsntd ovr Sony MDR-V7 hadphons. Th lvls wr masurd with A wighting, both on a continuous ton of 1048 Hz and on whit nois. Rsults Th subjcts scors, whr low numbrs indicatd sgrgation, wr rcodd (by invrting th numbrs of th scal) so that th y-axis on th graph of rsults would b comparabl to that in th figur for Exprimnt 1. This rcding was prmissibl bcaus, although th numbrs on th rating scal wr diffrnt, th task was idntical in both xprimnts; th scors masurd th amount of sgrgation whn th subjct was trying to hold th tst squnc togthr as a cohrnt squnc. For ach subjct, th rsults of th thr rplications of ach condition wr avragd, yilding on masur of sgrgation of th tst squnc pr condition for us as th dpndnt variabl. A 4 (frquncy sparation) x 6 (total ton duration during induction) rpatd masurs ANOVA was conductd, using th Grnhous-Gissr (1959) corrction. Rliabl main ffcts for both ton duration [F(1,15) = 54.08, p <.01] and frquncy sparation [F(l,15) = 36.93, p <.011 wr found. Thr was no rliabl intraction btwn th two factors [F(l,15) = 2.56, p >.1]. Th mans for frquncy sparations of

7 5, 9, 11, and 14 smitons, rspctivly, wr 2.4, 3.3, 3.5, and 4.2. Th mans for th control (0%), 20%, 40%, 60%, 80%, and 100% ton-duration conditions wr, rspctivly, 1.7, 3.1, 4.2,4.8,4.7, and 1.4. Th rsults ar shown in Figur 5, with sgrgation plottd against frquncy sparation for ach total ton duration. Longr ton durations did not always incras sgrgation. Surprisingly, th induction squnc in which th ton was always prsnt (100% condition) inducd no masurabl incras in sgrgation; that is, it had no mor ffct than th control condition, in which no ton was prsnt. Thus thr was not a monotonic rlationship btwn ton duration and sgrgation. Howvr, as th numbr of ton onsts incrasd, th sgrgation of th Tst gallop incrasd up to th point at which th tononst dnsitis of induction and tst squncs wr qual (3 onsts pr400-msc cycl, for a total of 36 onsts in th induction); byond that point thr was no chang. A Bonfrroni-adjustd t tst of all pairs of mans was don at th.05 lvl. All induction conditions xcpt for th control condition gav rliably mor sgrgation than did th continuous-ton condition. Th 20% condition (on induction ton pr cycl) was significantly diffrnt from all th othr conditions: it inducd lss sgrgation than did th 40%, 60%, and 80% conditions, but mor than th control or th 100% condition. Th thr middldnsity conditions (40%, 60%, and 80%) wr not rliably diffrnt from ach othr. Thr was no significant diffrnc btwn th control and th continuous-ton condition. 7 MAX 6 S gs r g 4 a i3 0 n 2 M1N Frquncy sparation (smitons) 0 (control) ~ 20 % Induction tons ~ i- 40 ~ Figur 5. Rsults ofexprimnt 2. Inducd sgrgation as a function of tst squnc frquncy sparation, shown for ach lvl of th ton-duration factor. Th total numbr of ton onsts in ach induction squncis shown on th graph bsid th appropriatlin. AUDITORY STREAM SEGREGATION 185 Discussion Th main ffct of frquncy sparation confirmd that th data ar rsponsiv to factors known to affct sgrgation. Brgman s hypothsis about cumulativ frquncy bias was disconfirmd; it is clar that th total duration of tons of on frquncy dos not prdict th induction ffct on subsqunt two-frquncy squncs. Instad, th scond altrnativ for th cumulativ ffcts of straming was supportd; th most induction occurrd whn thr was a rough matching of proprtis btwn th induction and tst squncs. Th numbr of ton onsts (vnt dnsity) appars to b important. This support for squntial grouping by similarity is an intrsting finding which is discussd mor fully blow. Of spcial intrst was th failur of th continuous ton to induc sgrgation. Th nhancmnt ffct (Summrfild, Haggard, Fostr, & Gray, 1984), which would prdict nhancd prcption of frquncis othr than that of th continuous ton and a rsulting sgrgation of th tst squnc, was not oprating in this cas. Examination of th data for continuous-ton and control conditions, for ach lvl ofth frquncy-sparation factor, rvals a consistnt pattrn: thr is lss sgrgation inducd by th continuous ton than by th control. Howvr, th numrical diffrnc was slight, prhaps bcaus of a floor ffct. Thus, although th continuous ton may for som rason hav nhancd th cohrnc of th tst squnc, rathr than mrly failing to sgrgat it, a post hoc tst may not hav bn powrful nough to dtct a diffrnc. Svral subjcts mntiond th monotony (xprincd as vry tdious) of th continuous-ton condition, which was not prsnt in th control bcaus whit nois is always changing. It is possibl that stimuli with 36 ons ts unchanging proprtis caus habituation of som compa- 48 ona:ts rator mchanism, which thn lads auditory attntion prfrntially toward stimuli with othr proprtis. Th 24 onsts subjcts in th xprimnt, waring hadphons and sitting in a soundproof booth, had no othr sounds to turn 12 onsts thir attntion to, and thy found this unnatural xprinc unplasant. Thir consistntly lowr sgrgation scors for th tst squnc aftr th continuous-ton condition may hav rflctd a rcovry from habituation onc th monotonous stimulus ndd. Control A mchanism that kps track of stram proprtis, for 1 onst calculation of similarity, might dal with th squncs of this xprimnt as follows. W assum that th dfault intrprtation is th prcption of a singl stram. Aftr th silnc that prcds th trial, tons start to arriv. Initially thy ar prcivd as a singl stram. Thir proprtis, such as th man frquncy and th frquncy rang, as wll as th onst rat (or ton dnsity), ar computd. During th induction squnc, thr is no rason to form mor than on stram, sinc thr is only a singl frquncy and th statistical proprtis of its changs rmain within th initial stimats. Aftr th tst squnc bgins, two changs will b noticd: th prsnc of a nw frquncy and possibly a diffrnt ton-onst dnsity. To th dgr that such proprtis ar changd, th

8 186 ROGERS AND BREGMAN nw tons will b jctd to form a stram of thir own, sparat from th first. A lack of chang should rsult in th gratst maintnanc of th original stram, on of high-frquncy tons only. A larg chang should rsult in all th tst squnc tons, vn th high ons, forming a nw stram. Initially this nw stram will b unsgrgatd, until vidnc for a bimodal distribution of frquncis has bn accumulatd. Th data ar consistnt with this hypothsizd stram-tracking mchanism, in which th rcnt proprtis of xisting strams ar an important input to th auditory scn analysis procss. This mchanism may hav its basis in adaptation to stimuli that ar not changing. Haftr and Bull (1990) hav found that th auditory systm shows rapid adaptation to intraural tim diffrnc information, but that th adaptation rvrss quickly whn thris a chang inth stimulus proprtis. A similar procss may work for stram sgrgation, with unchanging information givn littl attntion onc rgistrd. EXPERIMENT 3 Th prvious xprimnts confirmd that th adaptation of frquncy-jump dtctors is not ncssary to xplain straming, but that dos not man that such dtctors do not xist or that thy do not contribut to straming. If thy xist, howvr, it would b surprising if thy wr priphral to th point of binaural fusion. Exprimnts don by Stigr and Brgman (1982) indicat that th procsss of squntial stram sgrgation and binaural fusion ach influnc th othr that is, that no on procss maks a prmptiv dcision about what maks a stram. Smith, Hausfld, Powr, and Gorta (1982) confirmd that straming involvs comptition among proximitis in frquncy, spatial location, loudnss, and pitch. Monaural information alon is not nough to xplain ths rsults. On finding from Anstis and Saida s (1985) study could b intrprtd as proof that stram sgrgation is carrid out by a priphral procss: squncs of altrnating tons, prsntd to on ar until straming had takn plac, wr immdiatly prcivd as cohrnt again whn th prsntation was suddnly switchd to th othr ar. This rsult could qually wll b xplaind by th thory that w prsntd in th discussion of Exprimnt 2; th tons prsntd to th scond ar had a compltly diffrnt spatial location and might thrfor b tratd as a nw stram. In Exprimnt 3, w lookd for a cntral procss for th origin of straming. Tst squncs (gallop cycls) wr prsntd to th right ar only. Induction squncs for th xprimntal conditions wr constructd so that a gallop was also prsnt in th right ar. Howvr, in som conditions, th gallop was prcptually camouflagd or movd in spac by mans of contralatral sounds, acting according to known auditory grouping procsss. A priphral jump-dtctor mchanism would b xpctd to induc qual amounts of right-ar straming on th tst squnc in all conditions, sinc a gallop was always prsnt at that ar in both th induction and th tst squncs. A cntral mchanism would b rvald by diffrncs among th ffcts of th induction squncs. Mthod Only dtails that ar diffrnt from thos of th prvious xprimnt will b dscribd. Subjcts Thr wr 15 subjcts, ach of whom had participatd in Exprimnt 2. Stimuli Th induction squncs wr 8 sc long, 20 cycls of400 msc ach. Not that ths wr much longr than th induction squncs of prvious xprimnts. It was important to nsur that, if frquncy-jump dtctors wr involvd in sgrgation, thy would hav adquat tim to bcom adaptd. Th tst squncs wr prsntd to th right ar only. Th right ar was chosn arbitrarily, bcaus Anstis and Saida (1985) had alrady shown that th ffct of switching a stram of sgrgatd altrnating tons from on ar to th othr is symmtrical. Th uppr gallop tons wr 1048 Hz. All tons of both squncs wr of 50-msc duration, including l0-msc linar onsts and offsts. Th frquncy diffrncs btwn high and low tons of th gallop wr 8, 11, 13, and 16 smitons. Ths lvls wr chosn, from pilot study data, so that th subjcts wr crtain to xprinc th full rang of ratings from no sgrgation to complt sgrgation of th tst squnc. Fiv conditions for th induction squnc wr chosn in th following mannr: Two xtrm conditions wr providd such that on(th control) would hav no ffct on th tst squnc and on (th monotic gallop) would hav a larg ffct. Excpt for th control, all conditions prsntd a gallop to th right ar. S Tabl 1. On condition, th disguisd gallop, was dsignd so that th rightar gallop was prcptually hiddn by tons in th lft ar. Th thr rpating lft-ar tons wr chosn in light of prcptual grouping principls. Th first and third tons of ach cycl wr prsntd clos in frquncy to th high right-ar tons and just aftr thm in tim. Th middl ton of ach cycl was on musical fifth blow th low right-ar ton (frquncy ratio of 2:3) and was prsntd simultanously with th lattr (s Figur 6). Th high lft-ar tons groupd squntially with th high right-ar tons, and th low lft-ar tons groupd simultanously with th low rightar tons. Th stimulus was hard as an upward trill altrnating with a low complx ton. That is, a gallop was not prcivd vn though all th rquird tons, and only thos, wr rgistrd at th right ar. Finally, two xprimntal induction conditions in which th gallop was prcptibl wr chosn. In on, th nois-plus-gallop condition, whit nois was prsntd to th lft ar throughout th induction squnc. In th scond, th diotic gallop condition, tons idntical to th right-ar tons wr prsntd to th lft ar. An incidntal by-product of th dsign was th spatial shifting of th gallop in th lattr two conditions. Th whit nois, by mans of Tabl 1 Conditions for Exprimnt 3 Induction Typ Lft Ear Right Ear Control Whit nois Whit nois Disguisd gallop Distractor tons Gallop Diotic gallop Gallop Gallop Nois plus gallop Whit nois Gallop Monotic gallop Silnc Gallop

9 AUDITORY STREAM SEGREGATION 187 L 0 9 F r q u n C y Tim (msc) E1 Figur 6. Exprimnt 3, disguisd gallop stimulus. Right-ar tons (gallop) ar dnotd by black rctangls. Lft-ar tons (distractors) ar dnotd by whit rctangls. All tons ar of 50-msc duration. Th rst of th 400-msccycl (btwngallops) is filld with silnc. contralatral induction (Warrn, 1982, pp ), causd a slight shift in th apparnt spatial location of th right-ar gallop. Th prsntation of concurrnt gallop tons to th lft ar, in th diotic gallop condition, also shiftd th gallop prcpt to th cntr. Th lft-ar stimuli thus altrd th prcivd proprtis of th rightar induction gallop to varying dgrs in th xprimntal conditions: compltly (th disguisd gallop), partially (th nois-plusgallop and diotic gallop), or not at all (th monotic gallop). Each subjctwas prsntd with all possibl combinations of th two factors: induction condition (fiv lvls) and frquncy sparation of th tst squnc gallop (four lvls). Each block of trials, with on occurrnc of ach condition, was randomizd, and thr wr four blocks. W xpctd that th disguisd gallop condition, bcaus its rightar gallop did not form a sparat stram during th induction squnc, would induc no mor sgrgation than th control condition, and that th xprimntal conditions (i.., noncontrol conditions) would diffr from ach othr. Procdur Training. Subjcts wr prsntd with a mnu on th computr scrn, which allowd thm to slct a sampl ofa sgrgatd twofrquncy squnc, a cohrnt gallop, two cycls from ach of th induction squncs (in a fixd ordr), or a sampl trial. Thy wr rquird to listn to ach of thosat last onc, but thy could choos xtra rptitions. Thn thy wr givn 12 practic trials. Task. Th task was idntical to that of Exprimnt 2, dscribd abov. Apparatus This was th sam as that for Exprimnt 2, xcpt that th loudnss lvls wr st at 65 db SPL. For calibration, a pur ton of 1048 Hz was usd. Rsults For ach subjct, th rsults of th four trials of ach condition wr avragd, yilding on masur of sgrgation of th tst squnc pr condition for us as th dpndnt variabl. All th rportd mans rfr to th rcodd valus (with numbrs on th rating scal invrtd, as in Exprimnt 2), whos high numbrs indicat sgrgation. A4(frquncy sparation) x 5 (induction condition) rpatd masurs ANOVA was conductd, using th Grnhous-Gissr (1959) corrction. Rliabl main ffcts for both frquncy sparation [F(l, 14) = 9.530, p <.01] and induction condition [F(l,l4) = , p <.01] wr found. Thr was no significant intraction btwn th two factors [F(l,14) = l.2l3,p >.1]. Th mans for frquncy sparations of 8, 11, 13, and 18 smitons, rspctivly, wr 2.5, 2.7, 3.1, and 3.5. Th mans for th control, disguisd gallop, diotic gallop, nois-plus-gallop, and monotic gallop wr, rspctivly, 1.8, 1.9,2.4, 3.l,and 5.4. Th rsultsar shown in Figur 7. Th subjcts scors wr rcodd as in Exprimnt 2, so that th y-axis on this graph would b comparabl to that in th figurs for th othr xprimnts. A plannd comparison btwn th disguisd gallop and th control condition was prformd. As hypothsizd, thr was no significant diffrnc [F(l,14) = 0.636, p =.439]. A Bonfrroni-adjustd t tst of all pairs of mans at th.05 lvl showd that th monotic (lft-ar silnc) condition inducd rliably mor sgrgation than did all th othr conditions. Among th binaural conditions, th nois-plus-gallop condition inducd significantly mor sgrgation than did th disguisd gallop and th control conditions. 7 MAX 6 S g5 r a t is 0 n 2 MIN Frquncy sparation (smitons) -~B~ Disguisd Monotic INDU CT1ON GALLOP -- Diotic * Nois~gaiiop -~~-- Control Figur 7. Rsults of Exprimnt 3: Inducd sgrgation as a function of tst squnc frquncy sparation, shown for ach lvl of th induction condition factor.

10 188 ROGERS AND BREGMAN Discussion Th data provid vidnc for a cntral procss in th straming of two-frquncy squncs: th xprimntal conditions wr not qual in ffct, vn though all had idntical right-ar stimuli. In particular, th disguisd gallop did not induc any straming on th tst squnc gallop. This implis that adaptation of jump dtctors, if it xists at all, must involv th tons within a prcivd stram, rathr than just th tons at a singlar. Th main ffct of frquncy sparation and th larg diffrnc btwn th monotic and th control conditions confirmd that th data ar valid. Also of intrst wr th rsults from th diotic gallop condition, which had smd likly to induc sgrgation vry strongly. Howvr, it did not; th rating, avragd ovr all subjcts and frquncy-sparation conditions, was 2.4. On possibl xplanation, which is consistnt with th rsults of Exprimnt 2, is that th induction squnc had sgrgatd but that a suddn chang in stram proprtis was rsponsibl for rstting th straming mchanism. Th transition from th induction to th tst squnc includd a chang in both th intnsity and spatial location of th gallop, which could b intrprtd as vidnc that th tst squnc tons blongd to a compltly diffrnt stram. This maks sns, bcaus thr is no natural objct that can mov through 900 instantanously; th auditory systm would b corrct in trating th nw tons as part of a nw objct. GENERAL DISCUSSION Thr is a ndto rvaluat th thr thoris in light of th nw data. Jons s (1976) thory of a prdictiv rulbasd procss in sgrgation is not supportd by ths data. In Exprimnt 1, th rgular and irrgular monoton induction squncs, with othrwis similar proprtis, had th sam ffct. According to hr thory, any incrasd knowldg of th squncs, gaind during th xprimnt, ought to hav hlpd th subjcts do what thy wr trying to do, namly to hold th tst squnc togthr. Th cumulativ ffct obsrvd in our xprimnt, thrfor, cannot hav bn a gnral incrasd knowldg of th stimulus. It must b viwd as somthing that spcifically coms to favor a two-stram ovr a on-stram intrprtation. Jons, Kidd, and Wtzl (1981) provd that som aspct of rhythm must b a factor in straming. Thir rsults can now b xplaind in trms of grouping by similarity (in proprtis such as vnt dnsity or total ton duration) rathr than prdictiv attntion. Th thory of priphral frquncy-jump dtctors, as proposd by Anstis and Saida (1985) and Van Noordn (1975), would nd modification bfor it could b rtaind. First, frquncy-jump dtctors ar not ncssary to xplain straming, as is shown by th induction of straming by squncs of only on frquncy. Thus this thory is incomplt, spcially sinc it also fails to dal with comptition among grouping procsss (Brgman, 1990). Scond, vn if frquncy-jump dtctors do contribut to straming, thy ar not priphral to th suprior olivary complx, whr information from th two ars first convrgs; or, at th vry last, thir output is rintrprtd by cntral procsss. On might modify th thory to say that th adaptation to altrnation of frquncis occurs somwhr subsqunt to binaural intgration but still in tim to contribut to scn analysis. Anothr way in which th thory would hav to chang is rlatd to our finding that th adaptation rcovrs quickly undr som conditions, such as th instantanous chang in both intnsity and spatial location of th diotic gallop condition in Exprimnt 3. Prhaps th modifid thory might propos that whn proprtis chang suddnly, th nw inputs ar routd to a diffrnt population of nurons, with a frsh supply of frquncy-jump dtctors. This might provid a mchanism similar to that discovrd by Haftr and Bull (1990), which shows adaptation to unchanging information and thn rapid rcovry whn a chang dos occur. That is, th rsults obtaind by Anstis and Saida (1985) may hav bn causd by adaptation to th lack of chang (constant rptition) in th altrnating-ton squncs, rathr than by adaptation to th frquncy jumping. Brgman s scn-analysis thory is largly supportd by th data from th prsnt xprimnts. Thy confirmd that th whol auditory contxt is usd (not just local information at th priphry), and thy showd that squntial grouping by similarity (of incoming sounds to xisting strams) occurs. Proprtis involvd in th calculation of similarity may includ vnt dnsity, total ton duration, spatial location and sound lvl. Howvr, th cumulativ ffcts of straming ar not dominatd by a procss that rsponds to cumulativ ton duration in a frquncy rgion. Furthrmor, contrary to prvious rsults (Brgman, 1978b), cumulativ straming can b rvrsd rapidly that is, without a tim priod of svral sconds. This occurrd in th transition btwn th induction and tst squncs of th diotic gallop condition of Exprimnt 3. Th procsss of hystrsis apparntly do not prvnt rapid changs in straming whn thr is good vidnc for gnuin changs in th nvironmntal sourcs of incoming sounds (.g., nw spatial location of nwly arriving tons, and disapparanc of tons in th spatial location of th stablishd stram). Thr ar othr implications of th nw data, apart from thir rsolving som issus about th thr thoris of sgrgation. Th mchanisms that comput th proprtis of xisting strams may hlp implmnt grouping by similarity by providing fdback (of proprtis drivd from prvious grouping) to th scn analysis procsss. It is possibl that a comparator mchanism is always at work on sounds tntativly assignd to a stram to vrify that any chang in stram proprtis is gradual nough to b consistnt with th nw sounds blonging to th ongoing stram. Th output from th comparator may signal to th scn analyzr whthr or not to continu making groupings in th sam way.