Lateral-Position-Based Models of Interaural Discrimination

LETTERS

TO THE EDITOR

Lateral-position-based models of interaural discrimination RichardM. Stem, Jr. Department of ElectricalEngineering andBiomedical Engineering Program,Carnegie-Me!Ion University, Pittsburgh. Pennsylvania 15213 H. Steven Colbum

Department of Biomedical Enginecring• BostonUniversity, Boston, Massachusetts 02215andResearch Laboratory of Electronic•Massachusetts Instituteof Technology, Cambridge, Massachusetts 02159

(Received 29 July 1983;accepted for publication 20 September 1984) Thisletterinvestigates thehypothesis thatlateralpositionistheonlycueavailablefor interaural discrimination experiments using500-Hzstimuli.Thediscussion of thishypothesis isin the contextof comparisons of theexperimental datato predictions of the "position-vatiable" model of binauralinteraction. The modelpredictsthemeanandvarianceof thesubjective lateral positionof stimuliusedin thediscrimination experiments, assuming thatdiscrimination performance isbasedon optimalprocessing of thissubjective position.To theextentthatthe lateraritypredictions of themodelareaccurate, datathatareinconsistent with itspredictions wouldalsobeproblematical for anymodelbasedonthesubjective lateralityof a singlebinaural image.Thepredictions (atleastqualitatively) describe muchof theobserved experimental data, including a numberof resultsthathavenotbeenaddressed byanyprevious theory.Nevertheless, theobserved performance is signiticanfiy betterthanthecorresponding predictions for three. typesof experiments in whichtheutilityof thepositioncuehasbeeneliminated by experimental design. Webelievethatourresultsindicatethatchanges in lateralpositionaretheprimarycuein mostinterauraldiscrimination experiments, butthatsecondary attributesof theperceptual images canbeusefulwhenperformance-based onposition alonewouldbepoor.

PACSnumbers: 43.66.Pn,•3.66.Ba,43.66.Rq[FLW]

!970},andinteraural timeandamplitude jn•ls(Domnitz and

INTRODUCTION

Colbum, 1977).Most of thesemodels(e.g.,Halter and Cartier, 1970;Yost, 1970)haveassumed thatthelateralposition

Subjectivelateralpositionis a prominentcuein many peychoacoustical experiments in whichinterauraltime dif- ß.ofthedominant subjective binaural image canbemodeled as ferences (ITDs) or interauralintensitydifferences (IlDs) are a linear combination the the ITD and lid of the stimulus, varied. For example, Klumpp and Eady (1956), and that the variabilityof this subjectivelateralpositionis HershkowitzandDurlach{1969),Yost{1974},andDomnitz independent of the stimulusITD and liD. Accordingto andColbum(1977}haveall specifically citedthe useof laavailabledata on subjectivelateral position(Sayers,1964; teralpositioncuesin theirexperiments measuring just noDomnitz and Colburn, 1977;Yost, 1981},the formerasticeabledifferences (jnds)oflTD or IID. Theoreticalstudies sumptiononlyapplies(for 500-Hzstimuli)to lids of apcomparingvarious interaural discriminationresults to proximately0 dB and ITDs of approximately 0 or + 1000 changes in subjective positionincludework by Hafter and ps. For theseITDs and lIDS, all modelsprovidethe same Carrier(1970),Halter{1971),Yost(1970),andDomnltzand description of lateralposition,andpredictions developed in Colbum(1977). the presentpaperapplygenerallyto all models.For ITDs In thisletterwe investigate the hypothesis that lateral and lids outsidethisrangeof values,the lineardescription positionistheonlycueavailable(otherthanpitchandloudof positionis inappropriateand predictionsof the simple nesscues)for binauralexperiments in whichsubjectsdislaterali•tion modelshavegenerallynotbeenappliedto the available data. criminate500-Hz stimuli presentedwith slighfiydifferent ITDs and/or IIDs. The discussion of thishypothesis isin the To the extentthat the lateralitypredictions of the posicontextof the "position-variable" model (Sternand Coltion-variable model are accurate, discrimination data that bum, 1978)which predictsthe subjectivelateral position, cannotbedescribed by thismodelwouldalsobeproblematianditsvariability,of thesingledominantimageof thestimucal for any modelbasedsolelyon the subjectivelateralposili usedin theseinterauraldiscriminationexperiments. The tion of the dominantbinauralimage.Hence,we considera modelassumes that discriminationperformanceis basedon comprehensive review of the successes and failuresof the the optimalprocessing of thissubjective position.This apmodel in predictingresultsfrom interauraldiscrimina• proachhas previouslybeenusedin applyingthe theories experimentsto be worthwhile,even thoughmany of the collectivelyreferredto as "lateralizationmodels"to tonecomparisons aresimilarto resultsobtainedin previous studies. on-tonemaskingresults(Hafter and Carder, 1970;Yost, 753

J. •

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ownloaded 26 May 2010 to 71.182.225.48. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.js

I. SUMMARY OF COMPARISONS

OF PREDICTIONS AND

DATA

The position-variable modelcharacterizes the subjec-

tivelater• position of a •}O-Hzstimulus by therandom variableP, whichis assumed to bederivedfrom the stochas-

ticaudi•tory-nerve response tothesound (Stem andColburn, 1978).P is specifically assumed to be normallydhtribut•!, with a meanandvariancethatdependontheITD andIID of the sthnulns.Predictionsfor the resultsof varioustypesof /nterauraldiscriminationexperiments wereobtainedby calculatingthe valueof the independent stimulusvariablethat would produce threshold discriminationperformance. Thesepredictionsare describedin detail in Sternand Col-

burn(1985}. • Freeparameters of themodelwerefittedona subject-by-subject badsin someof theseexperiments sothat thepredicted absolute sensitivity of themodelandthemod-

On the otherhand,severalaspects of the theoretical predictions areclearlyincompatible withtheobserved data: (1)Themodelpredicts thatinterauraltimejndsbecome extremelylargefor stimulipresented with certainvaluesof ITD. Althoughtheselargejndsareobserved for a smallminorityofsubjects, theperformance ofmøStsubjects issignificantlybetterthanthe predictions for theseITDs. (2) Observedperformance in NoSrrdetectionexperimentswithcoherenttargetsandmaskers issubstantially better than what is predictedby the modelfor somevaluesof target-to-masker phaseanglesbetween90' and 150. (All otherlateralization modelsfail in a similarfn,ghion.} (3) Subjectsin time-intensitytradeabilitystudiescan achievegooddiscrimination performance with twobinaural tonesforwhichthedominant subjective positions areidentical. Sincepredictionsof the position-variable model are basedonthesubjective hteralposition of a singleimage,it predictschanceperformancewith thesestimuli. We notethat in aHcaseswhenexperimental resultsare in obvious conflictwiththeoretical predictions, theobserved performance isbetterthanpredicted. All of theseresultsare problematical not only for the position-variable model,but alsofor anyothermodelthatattempts to relatediscrimlnationperformance to thesubjective lateralposition ofa single dominantbinauralimage.

el'seffectivethne-intens/ty tradingratio wouldbestdescribe theexperimental data.The experiments to whichthemodel was comparedincludethe interauraltime-jnd measurementsof HershkowitzandDurlach(1969},Domnitz0973), Yost {1974),Domn/tz andColburn(1977),and Moss(1979); theinterauralamplitudejnd measurements of Rowlandand Tobias{1967),HershkowitzandDurlach(1969),and Domnitz and Colburn(1977);coherenttone-on-tone and noiseon-noisemaskingstudies,with detailedcomparisons to the resultsof Yost et aL (1974)and letfressand McFadden IL DISCUSSION (1971);and the time-intensitytradeabilityexperimentsof Halter andCarder (1972},CxilliomandSorkin(1972),Smith In general,the comparisonsof predictionsand data {1976),and Ruotoloet aL (1979). supportthehypotheses that performance in mostinteraural In comparingthe predictionsof the position-variable discriminationexperiments is basedon changesof lateral model to the results of these interaural

discrimination

ex-

perimentswe foundthe followingaspectsof the data to be correctlydescribed by the model: {l) For pureinterauraltimejnds,the modelcorrectly predicts thegeneralincrease of t/mejndsasthemagn/tude of the lid increases, andthe asynunetry with respectto time delayof timejndsobtainedfor stimulipresented with interauralamplitudedifferences. The modelalsocorrectlyprediets the reversal in d/rection of the lateral/zation cue that

hasbeenreportedby subjects in time--jndexperiments when the ITD is at or near -t- 1000/zs. (2) For pureinterauralamplitudejnds,the modelcorrectlypredictsa lack of cuereversals,a weakerdependence of amplitudejnds (comparedto timejnds)on ITD, and an overallincrease of amplitudejndswith increasing IIDs. The quantitative agreement between thepreclictions of themodel and the actualtime and amplitudejnds variesconsiderably from subjectto subject. (3)The modelcorrectlypredicts,asdoall lateralization models, the general dependenceof NoS•r tone-on-toue maskingresultsfortarget-to-masker phaseangles between0' and 90'. In addition,the position-variable modelcorrectly decribes m•ny of the trendsof theN•rSocoherent-masking

resultsof Yostetal. (1974),although• thequantitative agreement of thepredictions anddatavariesfromsubject to subject. The modelalsoprovides a gooddescription of all of the resultsof Jefftess andMcFadden's(197l) lateralization and a-sweepexperiments usingcoherentnarrowbandtargetsand maskers,as.doaHlateralizationmodels. 754

J. Acoust. Soc. Am., Vol. 77, No. 2, February 1985

position, but additionalinformation maybeusedin experimentsfor whichperformance basedonpositionalonewould bepoor.We postulate thatthesubject-to-subject variabiltity in thetime-jnddataat largereference delays, andthegood performance ofsometime-jndsubjects relativetothe,predictionsof thepositionvariablemodel,mayindicatethat these subjects havelearned-to makeuseof otherspatialattributes of theperceptual imageof thesounds whenchanges in their primarysubjective lateralpositionare small.The potential utility of additionalpereceptualcuesin interauraldiscriminationexperiments hn.•previously beensuggested byJefftess and McFadden (1971)in discussions of their detectionexperimentswith coherentmaskers,andby Hafier andCarrier

{1972}for time'intensity tradeability studies, andit is supportedby descriptions of perceptualcuesusedby subjects in Moss'{1979}time-jndexperiments. While a numberof investigators have suggested that performancein time-intensitystudiescan be improvedby the use of secondarycomponents of the stimulusimage, thereis a lack of agreementconcerningthe exactnatureof the additional perceptual information. Specifically, WhitworthandJefftess (1961}andHafterandJefftess (1968}

havedescribed a "timeimage"thatisalmostcompletely independent of the lid of the stimulus.A differenttypeof secondary image,described by somesubjects in theexperimentsof Ruotoloet al. {1979},appearsat the ear receiving the signalthat is of greaterintensitybut laggingin time,A third typeof secondaryimagecanbe perceivedin eitherear whencertainsubjects arepresented with an equal-intensity Letters to the Editor

754


puretoneofITD equaltohalfthestimulus period, asreport~ ACKNOWLEDGMENTS edbySayers (1964)andYost(1981).In additionto theposiThisworkwassupported by theNationalInstitutes of tionof a secondary spatialimage,theactualsubjective cue Health(•ants 5ROI NS01344,5ROI NS10916,and5ROI (besides lateralposition) usedin the discrimination experi- NS14908).Preparation of themanuscript waspartiallysup-

mentsconsidered couldbea difference in imagediffuseness, portedby NIH Traininggrant 5T32 GMO7477. We also shape,or someotherdifference in qualitybetweenthe two thank.C. Trahiotisfor helpfulcommentson many earlier stimuli to be discriminated. It is not at all obvious that the draftsof thismanuscript. sametypeof additionalinformationis usedfor eachof the threeclasses of resultsthatarenotpredicted bytheposition- •SeeAlP document no,PAPSJASMA-77-753-29 forall 29 pages of the variablemodel.Thesesubtleties in the subjective imageof longerpaper"Subjective LateralPositionandlnteraurnlDiscr/mination." Order by PAPS numberandjournal refaence from AmericanInstituteof binauralsoundsmay be of concernto thosewho measure Physics, Physica AuxilliaryPublication Service, 335East45thStreet,New binauralperceptionin a clinicalsetting,in that untrained York, NY 10017.Thepriceis$1.50foreachmicrofiche {98pages}, or $5.00 subjects mayor maynot respondaccordingto thesesecon- fo• photocopies of up to 30 pageswith $0.1$ for eachpageover30 page• darystimulusattributesof the stimuliwhenchanges of laAimrail additional.Make checl• payableto th• AmericanInstituteof teridpositionare not availableasa cue.

Although it isrelatively easy tomoc•fy theposition-

variablemodelto predictthetypesof secondary imagesthat havebeendescribed to date,wearenot attemptingto rigorouslydevelopdecisionvariablesto describeany of theseimagesat present.We havefoundthe subjective perceptsto be

extremely complex evenfor a puretone,andwehavehad difficultyseparating thiscomplexity intosimplevariables. It appears thatthesystemisrelatively"plastic"andthatmany differentaspects of thestimulus canbeisolatedby a careful, trained observer and used as a decision variable.

In general,a modelbasedon lateralposition{andthe position-variable modelin particular}canaccountfor many of thesalientfeatures of thedata.Thereareotherperceptual attributesthat areusedin someexperiments, but theseother attributesappearto affectdiscrimination performanceonly whenthe meanlateralpositioncueis eliminatedby careful experimental design.Almostall of the otherresultsin interauraldiscrimination canbeaccounte•for by a modelbased on changes of the lateralpositionof a singleimageand its variability.

Domnitz,R. H. {1973)."The interauralt/mejnd aaa simultaneous function of interauraltime and interauralamplitude,"•. Acoust.Soc.Am. •3, 1549-1552.

Domnitz,R. H., andColbum,H. $. (1977)."Lateralpositionandinteraural 'dmcriminafion," •. Acousc Soc.Am. 61, 1586-1598.

C-illiom,$. D., andSorkin,R- D. (1•2). "Discr/m;nation ofinterauraltime and intensity,"J, Acoust.Soc.Am. 52, 1635--1644. I-lafi•, E. R. {1•71}."Quantitative evaluation ofa laternl;,•tionmodelof madring-level d/•rence%"J. Acoust.Soc.Am. •0, I 116-1122. Hafier,E. R., andCarrier,S.C. (1970)."M•king-levelditferencen obtained witha pulsedtonalrn•.•ker,"J. Acoust.Soc.Am. 47, 1041-1047. Halter, E. R-, andCarder,S.C. {1•/2). "Binauralinteractionin low-frequencysthnuli:The inabilityto tradet/meandintensitycomplctely," Acoust. Soc. Am. $1, lg52--lg62.

Halter, E. R., andJefere•e,L. A. {1•).

'q'wo-ima•ehteralizafionof tones

and clicks" J. Acoust. Soc. Am. 44, 563-569.

Hershkowitz, R. M., andDurtach,N. I. {1969}. "Interaural timeandamplitndejnds for a 500-Hz tone,"L Acoust.Soc.Am. 46•, 1464-1467. Jeffre• L. A., andMcFadden,D. (1•71}."Diferencesof interauralphase !179.

Klumpp,R• G., andEady,H. R. {1956}."Somemeasurements ofintemural time di•erence thresholds,"J. Acoust. Soc. Am. 28, 859-860.

IlL CONCLUSIONS

We havecomparedthe performanceof the positionvariable model of binaural interaction to the results of inter-

aural'd•scrimination experiments using500-I-Izstimuli.We havefoundthata singleposition variableappears to beadequateformanydiscrimir•ation experiments; however, anadditionaldecision variableis requiredto describe othertypes of experiments. Themodelgenerally provides goodpredictionsforexperiments in whichsubjects reportusingposition asthemajorcue,whilein manyof theexperimental results thatdifferfromthepredictions, greatintersubject variability is notedand the task is characterizedby subjectsas difficult

Mo• P. I. {197J)."Lateral positionand interauralt/me discrimination," M. S. thesh,Elec.Eng.Dept.,M.I.T., Cambridgc, Rowland,R- C., and Tobias,I. V. {1•7}. "Interauralintensitydifference iiman,"J. SpeechHear. Res.10, 745-756. Ruotolo,B. R-, Stern,R- M., Jr., and Colbum,H. S. (1•}. "Discr•mi,ation of symmetric,t/me-intensitytradedbinauralstimuli," J. Acoust. So•. Am. 66. 1733-1737.

Sayers, B.McA. (1964}."Acouadc-/mage lateral/zntion judgments withbinaural tone•" J. Acoust. Soc.Am. 36, 923-926.

Smith,L. E. {!•]6}. "Theeffects oft/meandintensity onthelaternli•ation of sounds," M. A. thesis,Psychol. Dept.,Universityof Ill/nois. Stem,R. M., Jr.,andColbum,H. S.{19•1}."Theoryofbinauralinteraction b•ed on auditory-nervedat• IV. A modelfor subjectivelateral position," J. Acoust Soc.Am. 64, 127-140. Stern,R. M., Jr., andColburn,H. S. {19•5). "Subjectivelateralpositionand intera•wal discrimination."

AIP document no. PAPS and JASMA-77-

to perform.We believethat lateralpositionis usedas the 753-29{seefootnote1). subjective cuein all experiments considered, andthat some Wh/tworth, R- H., andJefresn,L. A. {1961}.'•fime vsintensityin thelocalization of tones,"L Acoust. Soc.Am. 33, 925-929. •__ditional cuesareusedfor thoseexperiments in whichperbinauralmasking,"Ph.D. thesis,Psyformanceonthebasisof lateralpositionalonewouldbevery Your,W. A. {19•0}.'•Fone-on-tone chol.Dept.,Ind;nnnUniversity. poor.To theextentthatourmodelaccurately characterizes ¾o• W. A. {1974)."Interauralphasediscrimination,"J. Acoust.Soc.Am. lateral positionand its variability, these discrepancies •[, 1299-1303. ¾o• W. A. {19•1}."Lateralposition ofsinusolds presented withinteraural betweentheoreticalprech'ctions and interauraldi•riminationdatawouldahobeseenin theperformance of anymodel Your,W. A., Nielsen,D. W., Tan/s,D.C., andBergert,B. {1•/4}. "Tone-onin whichjudgments are assumed to be basedsolelyon the tonebinauralransiring withanantiphasic masker,"Percept.Psychophys. 15, 233-237. lateralpositionof a singleperceptual image. 755

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