Discriminability of bursts of reproducible noise Susan Fallon Coble b) and Donald E. Robinson DepartmentofPsychology, Indiana University, Bloomington, Indiana 47405
(Received15April 1991;acceptedfor publication20 July 1992)
The ability of humanlistenersto discriminatepairsof burstsof reproduciblenoisewas examined.A same-differentpsychophysical methodwasused.Burstsin a pair wereidentical on sametrials. On differenttrials, burstswereidenticalexceptfor r ms of independentnoise locatedat eitherthe beginning,middle,or end of the pairsof bursts.As the temporalposition of the r ms of independentnoisewasmovedfrom the beginningto the end of the burstsof noise,discriminabilityincreased.For eachtemporalpositionof the independentnoise, discriminabilitywasa functionof the ratio of the durationof the independentnoise(r) to the total burst duration.
PACS numbers: 43.66.Fe, 43.66.Mk
INTRODUCTION
decreasedin the beginningcondition,and wasworst in the middle condition.
The presentexperimentcontributesto the understandThe presentexperimentextendsthe studyof the discriing of how complexsoundsheard in the "real world" are minability of bursts of reproduciblenoise performed by processed by the auditorysystem.The experimentpresented Hanna (1984) by examiningthe effectsof temporalposition here examinesthe ability of human listenersto discriminate of the altered portion of the noiseburstsover a range of betweenpairsof one particulartype of complexnonspeech durations. sound.The soundexaminediswhitenoise.Noiseisof particular interestbecauseit is presentin the everydayenvironI. METHOD mentandisoneof the few complexsoundsthat hasbeenused Data werecollected2 h a day, 4 to 5 daysa week,for 9 to ratherextensivelyin the pasthalf-centuryof psychoacoustic research. 14 weeks.During a typical 2-h period, five blocksof trials were presentedin each of three sessions.Each sessionconSeveralexperiments(Watson et al., 1975, 1976;Hanna, sistedof onepracticeblock (20 trials) followedby four regu1984) useda same-differentpsychophysical paradigmto exlar blocks (100 trials each). A 2-min rest periodfollowed amine the discriminabilityof complexsounds.In theseexeach block within a session and there was a 5- to 7-min break periments,pairs of identicalwaveformswere presentedon between sessions. same trials. Waveforms presentedduring different trials A same-differentpsychophysical procedurewas used. were identicalexceptfor a selectedtemporalportion of the Eachblockof regulartrialshad exactly50% sametrials and secondwaveformin a pair that was altered relative to the 50% differenttrials. The order of presentationof sameand firstwaveform.Resultsshowedthat discriminabilityof complexwaveformsvariedasa functionof the temporalposition differenttrials within eachblock was pseudorandom.Each trial consistedoff a 250-mswarning light followedby a 250of the alteredportionof the pairsof waveformspresentedon different trials. mspause,the firstobservationinterval,a 300-mspause,and the second observation interval. Observation intervals were The effectsof the temporalpositionexhibitedin the tomarkedthroughoutby lights.The secondobservationinternal-patternexperimentsperformedby Watsonand his colinterval.At the endof the leaguesand thosereportedin Hanna's studyof the discri- val wasfollowedby a 2-sresponse minabilityof burstsof reproduciblenoisewerenot identical. responseinterval, one of two 200-ms feedbacklights was illuminated to indicate whether a same or different trial had For both typesof complexwaveforms,409.6-msburstsof noiseand 400-mstonal patterns,discriminabilitywas best beenpresented.Subjectswereinformedof their averageperwhenthe alteredportionwaslocatedat the endof the wave- formanceon everyblock of trials at the end of everyexperimental session. form. However,in the tonal pattern studies(Watson et al., Burstsof noisewere generatedon a Digital Equipment 1975), discriminabilitydecreasedmonotonicallyasthe temCorp. PDP-11/34A computerby a 33-bit softwareshiftregporal positionof targettone wasmovedfrom the end to the ister (Gilkey et al., 1988). The sequence of binarynumbers beginningof the tonal pattern,while the discriminabilityof bursts of noise (Hanna, 1984) was best when the altered generatedby the shiftregisterwasconvertedto voltagesby a portion was locatedat the end of the pair of noisebursts, 14-bitdigital-to-analogconverter(DAC) at a rate of 20 000 samplesper second.The output of the DAC was passed a)Portions ofthispaperwerepresented at the110thMeetingoftheAcousti- through an antialiasingfilter with a low-passcutoff of 6.3 cal Societyin Nashville, TN [J. Acoust. Soc. Am. Suppl. 1 78, S46 (1985) ].
kHz and a 80-dB/oct
rolloff. The bursts of noise were addi-
b)Presentlyat Psychoacoustics Laboratory,Departmentof Psychology, tionally filtered to a bandwidth of 100-3000 Hz using an SKL 302 filter with an 18-dB/oct rolloff. Within the 100- to University of Florida, Gainesville,FL 32611. 2630
J. Acoust.Soc. Am. 92 (5), November 1992
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3000-Hz passband,the long-termpower spectrumof the noisewaswhite, and the distributionof instantaneous pressureswas approximatelyGaussian.Burstswere presented diotically throughTDH-49 headphoneswith 001A circumaural cushionsto subjectsseatedin a SuttleCorp. doublewalledsound-attenuating chamber.A newburstof noisewas generatedat the beginningof eachtrial and, with the exception of sametrials, no noiseburst was repeatedduring the experiment. Schematicsof burstspresentedon sameand different trials are shownin Fig. 1. On eachtrial, subjectsheardtwo burstsof noiseof equal duration. Burstsin eachpair were
perimentalsessionthe levelof the calibrationnoisewasadjustedto equalthat of a thermal noisewith a levelof 50 dB SPL/Hz. Basedon this measurement,the spectrumlevel of
mance. Three durations (T) of the bursts were examined in
II. RESULTS
the bursts of noise was assumed to be 50 dB.
Three undergraduatestudents(one female,two male) with audiometricallynormal hearing were paid an hourly wageto work as subjectsin eachexperiment.The agesof subjectsrangedfrom 18-25 years.In eachexperiment,the leastdifficultexperimentalconditionwasusedto selectand train subjects.In this condition,pairs of identicalburstsof noisewerepresentedduringsametrials and duringdifferent trialscompletelyindependentburstswerepresented.During identical on same trials. On each different trial, bursts were the selectionprocess,prospectivesubjectslistenedto apidenticalexceptfor •- ms of independentnoiselocatedat proximately2400trials.Typically,only studentsableto diseither the beginning,middle, or end of the burst pairs.The criminatepairsof burstsat a P(C) greaterthan or equalto duration of •-was varied from T, the total duration of the 0.85 were hired as subjects.(Over the courseof 13 experiburstsin a pair, to nearzero.As the durationof •-decreased, ments, approximately65% of the 80 candidatesmet this differenttrials becameincreasinglysimilar to sametrials. criterion.) Each subjectrespondedto at least5000trialsbefore data werecollected.Data collectionbeganwhenall subFigure 1 alsoshowsthe threemethodsusedto generatedifjectsattaineda stablelevelof performance.At least400 trials ferenttrials.To generatethe secondburstin a pair, •- ms of noisewereremovedfrom eitherthebeginning,middle(symwerepresentedto eachsubjectin everyexperimentalcondimetric aboutthe midpointof the noiseburst), or end of the tion. To determineif performancevaried from the level burstof noisepresentedduringthe firstobservationinterval achievedat the end of the training period,the leastdifficult and replacedwith new noiseof equalduration. experimentalconditionwas repeatedperiodicallythroughIn eachexperimentalcondition,•-was initially equalto out eachexperiment.Performancein thisbaselinecondition T and was decreasedin subsequent experimentalsessions remainedstablethroughoutall experiments. until discriminabilityreachedthe level of chanceperforthe beginning,middle, and end conditions:25, 50, and 150 ms.Within an experimentalsession(which consisted of one practiceblock and four regularblocks), the durationsof •and T, andthetypeof condition(beginning,middle,or end) remained constant.
A singleburst of noisewas usedfor calibration.The calibrationnoisewasproducedrepeatedlywith no temporal gapbetweenonsetand offset.At the beginningof eachexSame
Tr'ials
Differ'ent
The data in the followingfiguresare averagedacross subjects.Each data point represents400 trials per subject. The trends exhibited in the data from each of the three sub-
jectsare accuratelydepictedby the averageddata. Figure 2 showsP(C) as a function of •-. The data for each of the three experimentalconditionsand three durations of the burstsare plotted separately.For each of the durations(T) examined,discriminabilitywasbestwhen the •-msof independent noisewerelocatedat the endof the pairs of bursts, decreasedin the middle, and was worst in the be-
Tnials
The portions of the noise bursts marked with a t are identical.
ginningcondition. Bursts are most discriminablewhen the independent noiseis locatedat the end of a pair. In this condition,the valuesof •- neededfor P(C) equal0.707 are approximately 0.2, 1.3, and 8.5 ms when T equaled 25, 50, and 150 ms, respectively.In many conditions,discriminabilityremained well abovechancefor very small valuesof •-. As shownin Fig. 2, the smallestvaluesof •-examinedwere:5 and 10msin the T = 150-mscondition;5.0, !.0, and 0.5 ms in the T = 50ms condition; and 0.5 and 0.1 ms in the T = 25-ms condi-
The portions of the no,se bursts marked with a 'r are d•fferent.
tion.
All Conditions
Int.
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FIG. 1. A schematicof burstspresentedon sameand differenttrials is shown.Two burstsof digitalnoisewerepresented on eachtrial. On each sametrial, the burstsin a pair wereidentical;the durationof the identical noise(ts) equalsthe burstduration(T). Differenttrialswerecomprisedof pairsof nonidentical burstof noise.On eachdifferenttrial, burstspresented in the beginning, middle,andendconditions wereidenticalexceptfor •' ms of independent noiselocatedat eitherthebeginning,the middle,or the end of eachburstpair. In the beginningandmiddleconditions,the durationof the portionof identicalnoise,to and te, respectively, equalsT-- •-. In the middlecondition,therearetwo portionsof identicalnoise(tin). The duration of eachportionequals( T-- •-)/2. 2631
J. Acaust. Sac. Am., Vol. 92, No. 5, November 1992
The datashownin Fig. 2 are replottedin Fig. 3 by dividing •-, the independentvariableplottedin Fig. 2, by T. This transformationpermitscomparisonsacrossdurations.The dataare presentedfor the beginning,middle,and endconditions.Figure 3 demonstratesthat discriminabilityis a functionof ratioof •-to Tand thetemporalpositionof the •-msof independentnoise. The standarderrorsshownin Figs. 2 and 3 were calculatedbasedon the significance testfor oneparameterisosensitivity functionsdevelopedby Gourevitch and Galanter S. Fallon Cable and D. E. Robinson:Noise discrimination
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FIG. 2. Performance, measured asP(C), isplottedasa functionof thedurationoftheindependent samples ofnoise,•-.Eachcondition(beginning, middle, andend)isseparately depicted forthe150-,50-,and25-msburstdurations. Thedataareaveraged oversubjects. Eachdatapointrepresents 400trialsper subject.
(1967). As discussedin Sec. I, the baseline conditions were
repeatedperiodicallyto insureperformancein theseconditions remained stable throughout the experiment. Three measurementsof performancein eachof the baselineconditionsare plotted at Tequal •-in Fig. 2. The dispersionof the three data pointsplottedat eachbaselineconditionis a measure of the variability in performancein theseconditions from week to week over the course of approximately a month, while the error bars measurethe dispersionof the data during a 30-min presentationof four 100 trial blocks.
III. DISCUSSION
Discriminability of burstsof widebandnoiseis affected by the temporal positionof the •- ms of independentnoise presentedon differenttrials and the ratio •'/T. Discriminability increases asthe temporalpositionof the •-msof independentnoiseismovedfrom thebeginningto theendof the pairs of noisebursts.At eachof the threetemporalpositionsof the independentnoise, discriminabilityvaries as a function of the ratio of the duration of the independentnoise(•-) to the burst duration (T).
The effect of the ratio •'/T on discriminabilityis describedby Weber's law. The duration of the independent noiseneededto determinethat a pair of burstsof noiseisjust 2632
J. Acoust. Soc. Am., Vol. 92, No. 5, November 1992
detectablydifferentis a constantproportionof the burstduration. It is importantto notethat the effectsof the temporal position of the independentnoise are not predicted by Weber'slaw. For eachof the threetemporalpositionsof the independentnoise,the ratio of the durationof the independent noise to the burst duration
is constant.
The constant of
proportionality decreasesas the independentnoise was movedfrom thebeginningto the endof the burstpairs.For a P(C) equal to 0.707, the constantof proportionalityis approximately 0.03 in the end condition, 0.14 in the middle condition,and 0.41 in the beginningcondition. The relationshipbetweendiscriminabilityand the ratio of the •- ms of independentnoiseto the durationof the noise burstsis similarto the resultsof studiesexaminingthediscriminability of pairsof ten-tonepatterns(Watson and Kidd, 1987;Watson et al., 1990). The tonal-patternstudiesperformedby Watsonand hiscolleagues demonstratethat componentresolutiondependsmainly on ratio of the durationof the targettoneto the durationof the patternwithin whichit occurs.Theseresultsare shownfor patternsrangingin duration from 62.5 ms to 2 s. Over this range,the numberand durationof the toneswithin the pattern are varied.Results indicatethat theabilityto detecta changein thefrequencyof the target tone is determinedby the fraction of the pattern that is occupiedby the target tone. S. Fallon Coble and D. E. Robinson: Noise discrimination
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FIG. 3.Thedatashownin Fig.2 arereplotted. Performance, measured asP(C), isplottedasa functionoftheratiooftheduration(•-) ofindependent samples of noisepresented ondifferenttrialsto theduration(T) of thebursts.Thedatafromthethreedurationsof thebursts(25, 50,and 150ms) areshownfor the beginning,middle,and end conditions.
The effectof the temporalpositionof the •- ms of independentnoiseon discriminabilityshownin the presentexperimentisalsoin generalagreement with theresultsof other experimentsexaminingthe discriminabilityof pairs of complexwaveforms.As previouslydiscussed, Watsonand hiscolleagues (Watsonet al., 1975) andHanna (1984) also useda same-differentparadigmto examinethe discriminability of complexwaveformsof approximately400-msduration. In eachof theseexperiments,the waveformspresented duringdifferenttrialsareidenticalexceptfor a selectedtemporalportionof thesecondwaveformin a pair that is altered relative to the first waveform.The presentexperiment,as well as the tonal pattern research(Watson et al., 1975), showsthat discriminabilityincreasesas the temporal position of the alteredportionismovedfrom thebeginningto the end of the waveform.
Hanna examinedthe discriminabilityof a singlepair of burstsof noiseunder conditionsof minimal uncertainty; at
the beginningof each block of trials, two 409.6-msnoise burstsare generated.The noiseburstspresentedduringdifferent trials are identicalexceptfor 25.6 ms of the second waveformin a pair that is alteredrelativeto the first waveform. Usingtheterminologyintroducedto describethe present experiment,the ratio of •'/T equaled0.0625 in Hanna's 2633
J. Acoust. Soc. Am., Vol. 92, No. 5, November 1992
experiment.The closestcomparabledata from the present experimentare at the longestburstdurationexamined( 150 ms) and the smallestvaluesof •- ( 5 and 10 ms). As shown in
Fig. 2, performancein theseconditionsis best in the end conditionand is nearly identical and approachingchance performancein the middle and beginningconditions.These results are similar but not identical to the results from Han-
na'sexperiment.Hanna foundthat discriminabilityisbestin the end condition (d'= 1.75), decreasesin the beginning condition (d' = 0.75), and is worst in the middle condition
(d' = 0.25).• Althoughtheresults ofHanna'sexperiments showa clearorderingof the threeconditions,asin the present experimentperformancein the discriminationtaskis superiorin the end conditionand decreases to similarbut not identicallevelsin the beginningand middle conditions. Hanna (1984) also examined the discriminability of burstsof reproduciblenoiseas a function of overall burst duration.The stimuliin Hanna'sexperimentareidenticalto thoseusedin the baselineconditionsin the presentexperiment. (In the baselineconditionsthe durationof T is equal to the duration of •-.) Hanna showsthat the effectof duration
on discriminabilityis relatively small. Discriminability is best at a duration
of 25.6 ms and decreases at the shortest
(0.1 ms) andlongest(409.6 ms) durationsexamined.There S. Fallon Coble and D. E. Robinson: Noise discrimination
2633
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memory. The first, referred to as the sensory-tracemode, assumesthat a preciserepresentationor trace of the first oneofthetwocomparable points. 2 Hannareportsa decrease stimulusis maintainedby rehearsaland comparedwith the secondstimulus. In the secondmode of memory, the conin performancein the discriminationtask asthe burst duratext-codingmode, the stimuli are comparedto the general tion increases from 25.6 to 102.4ms. In the presentexperiment, discriminabilityis independentof the overall burst contextof the soundsin the experiment,and, basedon this duration (T) overa rangeof durationsfrom 25.0-150.0 ms. comparison,transformedinto an abstractand morepermaThe difference between Hanna's data at a burst duration of nent representationof the stimulus. 102.4 ms (d'=2.75) and the 150.0-ms condition Becausethe presentexperimentwasnot designed to test the model of memorydescribedby Durlach and Braida, it is (d' = 3.25) in the presentexperimentisrelativelysmalland is assumedto be within the range of error attributable to unclear whether listenersperforming the same-different methodologicaldifferencesor practiceeffects. noisediscriminationtask generatepreciseor abstractrepreThis assumptionis not unreasonablebased on the sentationsof the stimuli. To determinewhetherprecise,abamountof variability shownin weeklymeasurements of perstract,or both typesof representations are usedin the noise formance in the baselineconditionsin the presentexperidiscrimination task, further studies must be performed. ment. As shownin Fig. 2 at the pointsT equalr, the disper- Thesestudiesincludeexaminingthe effectsof intervening sion of three measurementsof performancein each of the maskerson discriminability,varying the duration of the interstimulusinterval, and varying the sizeof the stimulusset baselineconditionsis greaterthan the error associated with any singledatapoint.The dispersionof the threedata points and the numberof timeseachstimulusis presented. Based on Durlach and Braida's model, it is assumedthat plottedat eachbaselineconditionis a measureof the fluctuationin performancein the discriminationtaskfrom weekto the listenersgeneratesomeform or formsof internalrepreweek,whilethe errorbarsmeasurethe dispersionof the data sentationof the stimuli that lead to the productionof optimal decisionstatistics.The representationof the burst of during a single30-min presentationof four blocksof 100 pairsof noisebursts. noisepresentedin the firstobservationintervalisthenstored Further noise discrimination experimentsare being and comparedwith the representationof the secondnoise conductedto examinethe effectsof the temporalpositionof burst. In order to accountfor the temporalpositionof the the r ms of independentnoisepresentedon differenttrials as independentnoiseon discriminability,it is assumedthat the a function of the overall burst duration (T). Studies of the representations of the stimuli are weighted. discriminabilityof pairsoften 40-mstonalpatterns(Watson For example,for a fixedlevel of performance,the duraet al., 1975) supportthe hypothesisthat, asthe overallduration of the independentnoise neededto discriminatebeis a slight discrepancybetweenthe resultsof the data from the presentexperimentand the resultsof Hanna's study at
tion of the bursts of noise (T) is increased to 400 ms, discri-
tween 150-ms bursts of noise was 114, 78, and 53 ms in the
minability of any givenoverallduration (73 will increaseas a fixeddurationof independentnoise(r) is movedfrom the beginningto the end of the burstsof noisepresentedon different trials. However, Hanna's results(1984) suggestthat the orderingof the effectsof the temporalpositionof the r ms of independentnoisepresentedon differenttrials on discriminability will vary as the overall duration is increased. Basedon Hanna'sdata, it would be expectedthat the burst pairs will be most discriminablein the end condition and that the differencebetweenperformancein the middle and beginningconditionswill diminish. A conceptualframework that accountsfor the conclusionsof the presentexperimentis outlinedin the remaining portion of the discussionsection.The similaritiesbetween the resultsof the presentexperimentand thosereportedin the studiesof tonal-patterndiscriminationreportedby Watson and his colleagues(Watson et al., 1975; Watson and Kidd, 1987;Watsonet al., 1990) suggestthat the qualitative modeldevelopedto describenoisediscriminationcouldalso accountfor the discriminabilityof tonal patterns. The qualitativemodelof noisediscriminationprocessis basedon the generalframework of the model of memory proposedby Durlach and Braida (1969). Durlach and Braida'smodelis composedof threebasicprocesses: the generation of internal representations of each stimulus,the comparison of the representations,and the calculation of a
beginning,middle, and end conditions,respectively.If the weighting function applied to a temporal window determinedby the burst durationincreasesmonotonicallyin approximatelya linear manner, the duration of the independent noiseneededto discriminatebetweenpairsdecreases as the r ms of independentnoiseare movedfrom the beginning to the end of the burstpairs. It is alsonecessary to incorporatea mechanisminto the
decision statistic.
Durlach
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and Braida described two distinct modes of
J. Acoust.Soc. Am., Vol. 92, No. 5, November1992
model that accounts for the effect of the ratio r/T
on discri-
minability.It ispossiblethat a singleweightingfunctionnormalizedby burstdurationcouldaccountfor both the effects of the temporalpositionof the r ms of independentnoiseon discriminabilityand the Weber'sLaw relationshipbetween discriminabilityand the proportion of independentnoise presentedon differenttrials. It is alsopossiblethat independent mechanisms
are needed to account for the effects of
temporalpositionand proportionaldurationon the discriminability of noisebursts. It is unclear where, within Durlach and Braida's model,
it wouldbe mostappropriateto placeoneor moretemporal weightingfunctions.It isconceivable that eachburstof noise is weightedprior to the comparisonprocessor that the weightingstageof themodeloccursafterthe representations of the two waveformsare compared. The use of a qualitativemodel basedon Durlach and Braida's (1969) model of intensity perceptionprovidesa generalframeworkfor organizingand conceptualizing the data. Further modelingis currently beingperformedto inS. Fallon Coble and D. E. Robinson:Noisediscrimination
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corporatethe data from the presentexperimentand unpublished data from related noisediscriminationexperiments into a quantitativemodel. ACKNOWLEDGMENTS
This researchwasa portionof the doctoralthesisof the first authorand wassupportedby grantsfrom the Air Force Office of ScientificResearchto Indiana University and to the Indiana University Institute for the Study of Human Capabilities.Dr. Bruce G. Berg, Dr. David M. Green, and an anonymousreviewerprovidedhelpful commentson an earlierversionof this manuscript. The valuesof d' reportedin Sec.III for thenoisediscrimination experiment performedby Hanna in 1984are approximations determinedby visual inspection.
It isimportant to notethatHannameasured performance in thenoisediscriminationtask in units of d' and plottedthe data on semilogarithmic coordinates.If the differencesin data presentationbetweenthe present experimentand Hanna's study (1984) are not taken into account,the slightdiscrepancy in the resultsis exaggerated.
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Durlach, N. I., and Braida, L. D. (1969). "Intensity Perception.I. Preliminary theoryof intensityresolution,"J. Acoust.Soc.Am. 46, 372-383. Fallon, S. M., and Robinson,D. E. (1985). "The effectsof changesin correlation on the discriminabilityof noisesamples,"J. Acoust. Soc. Am. Suppl. 1 78, S46. Gilkey, R. H., Robinson,D. E., and Frank, A. S. (1988). "A softwarepseudorandomnoisegenerator,"J. Acoust. Soc.Am. 83, 829-831. Gourevitch,V., and Galanter, E. (1967). "A significance test for one parameterisosensitivity functions,"Psychometrika32, 67-72. Hanna, T. E. (1984). "Discriminationof reproduciblenoiseasa functionof bandwidthand duration,"Percept.Psychophys. 36, 409-416. Watson, C. S., Foyle, D.C., and Kidd, G. R. (1990). "Limits of auditory patterndiscriminationfor patternswith variousdurations,"J. Acoust. Soc. Am. 88, 2631-2638.
Watson C. S., Kelly W. J., and Wroton, H. W. (1976). "Factors in the discriminationof tonal patterns.II. Selectiveattentionand learningunder variouslevelsof stimulusuncertainty,"J. Acoust. Soc.Am. 60, 11751186.
Watson,C. S., and Kidd, G. R. (1987). "Proportionaltarget-toneduration as a factor in discriminabilityof tonal patterns," J. Acoust. Soc. Am. Suppl. 1 82, S40. Watson, C. S., Wroton, H. W., Kelly, W. J., and Benbassat,C. A. (1975). "Factorsin the discriminationof auditorypatterns.I. Componentfrequency,temporalposition,and silentintervals,"J. Acoust.Soc.Am. 57, 1175-1185.
S. FallonCobleandD. E. Robinson: Noisediscrimination
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(Received15April 1991;acceptedfor publication20 July 1992)
The ability of humanlistenersto discriminatepairsof burstsof reproduciblenoisewas examined.A same-differentpsychophysical methodwasused.Burstsin a pair wereidentical on sametrials. On differenttrials, burstswereidenticalexceptfor r ms of independentnoise locatedat eitherthe beginning,middle,or end of the pairsof bursts.As the temporalposition of the r ms of independentnoisewasmovedfrom the beginningto the end of the burstsof noise,discriminabilityincreased.For eachtemporalpositionof the independentnoise, discriminabilitywasa functionof the ratio of the durationof the independentnoise(r) to the total burst duration.
PACS numbers: 43.66.Fe, 43.66.Mk
INTRODUCTION
decreasedin the beginningcondition,and wasworst in the middle condition.
The presentexperimentcontributesto the understandThe presentexperimentextendsthe studyof the discriing of how complexsoundsheard in the "real world" are minability of bursts of reproduciblenoise performed by processed by the auditorysystem.The experimentpresented Hanna (1984) by examiningthe effectsof temporalposition here examinesthe ability of human listenersto discriminate of the altered portion of the noiseburstsover a range of betweenpairsof one particulartype of complexnonspeech durations. sound.The soundexaminediswhitenoise.Noiseisof particular interestbecauseit is presentin the everydayenvironI. METHOD mentandisoneof the few complexsoundsthat hasbeenused Data werecollected2 h a day, 4 to 5 daysa week,for 9 to ratherextensivelyin the pasthalf-centuryof psychoacoustic research. 14 weeks.During a typical 2-h period, five blocksof trials were presentedin each of three sessions.Each sessionconSeveralexperiments(Watson et al., 1975, 1976;Hanna, sistedof onepracticeblock (20 trials) followedby four regu1984) useda same-differentpsychophysical paradigmto exlar blocks (100 trials each). A 2-min rest periodfollowed amine the discriminabilityof complexsounds.In theseexeach block within a session and there was a 5- to 7-min break periments,pairs of identicalwaveformswere presentedon between sessions. same trials. Waveforms presentedduring different trials A same-differentpsychophysical procedurewas used. were identicalexceptfor a selectedtemporalportion of the Eachblockof regulartrialshad exactly50% sametrials and secondwaveformin a pair that was altered relative to the 50% differenttrials. The order of presentationof sameand firstwaveform.Resultsshowedthat discriminabilityof complexwaveformsvariedasa functionof the temporalposition differenttrials within eachblock was pseudorandom.Each trial consistedoff a 250-mswarning light followedby a 250of the alteredportionof the pairsof waveformspresentedon different trials. mspause,the firstobservationinterval,a 300-mspause,and the second observation interval. Observation intervals were The effectsof the temporalpositionexhibitedin the tomarkedthroughoutby lights.The secondobservationinternal-patternexperimentsperformedby Watsonand his colinterval.At the endof the leaguesand thosereportedin Hanna's studyof the discri- val wasfollowedby a 2-sresponse minabilityof burstsof reproduciblenoisewerenot identical. responseinterval, one of two 200-ms feedbacklights was illuminated to indicate whether a same or different trial had For both typesof complexwaveforms,409.6-msburstsof noiseand 400-mstonal patterns,discriminabilitywas best beenpresented.Subjectswereinformedof their averageperwhenthe alteredportionwaslocatedat the endof the wave- formanceon everyblock of trials at the end of everyexperimental session. form. However,in the tonal pattern studies(Watson et al., Burstsof noisewere generatedon a Digital Equipment 1975), discriminabilitydecreasedmonotonicallyasthe temCorp. PDP-11/34A computerby a 33-bit softwareshiftregporal positionof targettone wasmovedfrom the end to the ister (Gilkey et al., 1988). The sequence of binarynumbers beginningof the tonal pattern,while the discriminabilityof bursts of noise (Hanna, 1984) was best when the altered generatedby the shiftregisterwasconvertedto voltagesby a portion was locatedat the end of the pair of noisebursts, 14-bitdigital-to-analogconverter(DAC) at a rate of 20 000 samplesper second.The output of the DAC was passed a)Portions ofthispaperwerepresented at the110thMeetingoftheAcousti- through an antialiasingfilter with a low-passcutoff of 6.3 cal Societyin Nashville, TN [J. Acoust. Soc. Am. Suppl. 1 78, S46 (1985) ].
kHz and a 80-dB/oct
rolloff. The bursts of noise were addi-
b)Presentlyat Psychoacoustics Laboratory,Departmentof Psychology, tionally filtered to a bandwidth of 100-3000 Hz using an SKL 302 filter with an 18-dB/oct rolloff. Within the 100- to University of Florida, Gainesville,FL 32611. 2630
J. Acoust.Soc. Am. 92 (5), November 1992
0001-4966/92/112630-06500.80
¸ 1992 AcousticalSociety of America
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3000-Hz passband,the long-termpower spectrumof the noisewaswhite, and the distributionof instantaneous pressureswas approximatelyGaussian.Burstswere presented diotically throughTDH-49 headphoneswith 001A circumaural cushionsto subjectsseatedin a SuttleCorp. doublewalledsound-attenuating chamber.A newburstof noisewas generatedat the beginningof eachtrial and, with the exception of sametrials, no noiseburst was repeatedduring the experiment. Schematicsof burstspresentedon sameand different trials are shownin Fig. 1. On eachtrial, subjectsheardtwo burstsof noiseof equal duration. Burstsin eachpair were
perimentalsessionthe levelof the calibrationnoisewasadjustedto equalthat of a thermal noisewith a levelof 50 dB SPL/Hz. Basedon this measurement,the spectrumlevel of
mance. Three durations (T) of the bursts were examined in
II. RESULTS
the bursts of noise was assumed to be 50 dB.
Three undergraduatestudents(one female,two male) with audiometricallynormal hearing were paid an hourly wageto work as subjectsin eachexperiment.The agesof subjectsrangedfrom 18-25 years.In eachexperiment,the leastdifficultexperimentalconditionwasusedto selectand train subjects.In this condition,pairs of identicalburstsof noisewerepresentedduringsametrials and duringdifferent trialscompletelyindependentburstswerepresented.During identical on same trials. On each different trial, bursts were the selectionprocess,prospectivesubjectslistenedto apidenticalexceptfor •- ms of independentnoiselocatedat proximately2400trials.Typically,only studentsableto diseither the beginning,middle, or end of the burst pairs.The criminatepairsof burstsat a P(C) greaterthan or equalto duration of •-was varied from T, the total duration of the 0.85 were hired as subjects.(Over the courseof 13 experiburstsin a pair, to nearzero.As the durationof •-decreased, ments, approximately65% of the 80 candidatesmet this differenttrials becameincreasinglysimilar to sametrials. criterion.) Each subjectrespondedto at least5000trialsbefore data werecollected.Data collectionbeganwhenall subFigure 1 alsoshowsthe threemethodsusedto generatedifjectsattaineda stablelevelof performance.At least400 trials ferenttrials.To generatethe secondburstin a pair, •- ms of noisewereremovedfrom eitherthebeginning,middle(symwerepresentedto eachsubjectin everyexperimentalcondimetric aboutthe midpointof the noiseburst), or end of the tion. To determineif performancevaried from the level burstof noisepresentedduringthe firstobservationinterval achievedat the end of the training period,the leastdifficult and replacedwith new noiseof equalduration. experimentalconditionwas repeatedperiodicallythroughIn eachexperimentalcondition,•-was initially equalto out eachexperiment.Performancein thisbaselinecondition T and was decreasedin subsequent experimentalsessions remainedstablethroughoutall experiments. until discriminabilityreachedthe level of chanceperforthe beginning,middle, and end conditions:25, 50, and 150 ms.Within an experimentalsession(which consisted of one practiceblock and four regularblocks), the durationsof •and T, andthetypeof condition(beginning,middle,or end) remained constant.
A singleburst of noisewas usedfor calibration.The calibrationnoisewasproducedrepeatedlywith no temporal gapbetweenonsetand offset.At the beginningof eachexSame
Tr'ials
Differ'ent
The data in the followingfiguresare averagedacross subjects.Each data point represents400 trials per subject. The trends exhibited in the data from each of the three sub-
jectsare accuratelydepictedby the averageddata. Figure 2 showsP(C) as a function of •-. The data for each of the three experimentalconditionsand three durations of the burstsare plotted separately.For each of the durations(T) examined,discriminabilitywasbestwhen the •-msof independent noisewerelocatedat the endof the pairs of bursts, decreasedin the middle, and was worst in the be-
Tnials
The portions of the noise bursts marked with a t are identical.
ginningcondition. Bursts are most discriminablewhen the independent noiseis locatedat the end of a pair. In this condition,the valuesof •- neededfor P(C) equal0.707 are approximately 0.2, 1.3, and 8.5 ms when T equaled 25, 50, and 150 ms, respectively.In many conditions,discriminabilityremained well abovechancefor very small valuesof •-. As shownin Fig. 2, the smallestvaluesof •-examinedwere:5 and 10msin the T = 150-mscondition;5.0, !.0, and 0.5 ms in the T = 50ms condition; and 0.5 and 0.1 ms in the T = 25-ms condi-
The portions of the no,se bursts marked with a 'r are d•fferent.
tion.
All Conditions
Int.
I
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FIG. 1. A schematicof burstspresentedon sameand differenttrials is shown.Two burstsof digitalnoisewerepresented on eachtrial. On each sametrial, the burstsin a pair wereidentical;the durationof the identical noise(ts) equalsthe burstduration(T). Differenttrialswerecomprisedof pairsof nonidentical burstof noise.On eachdifferenttrial, burstspresented in the beginning, middle,andendconditions wereidenticalexceptfor •' ms of independent noiselocatedat eitherthebeginning,the middle,or the end of eachburstpair. In the beginningandmiddleconditions,the durationof the portionof identicalnoise,to and te, respectively, equalsT-- •-. In the middlecondition,therearetwo portionsof identicalnoise(tin). The duration of eachportionequals( T-- •-)/2. 2631
J. Acaust. Sac. Am., Vol. 92, No. 5, November 1992
The datashownin Fig. 2 are replottedin Fig. 3 by dividing •-, the independentvariableplottedin Fig. 2, by T. This transformationpermitscomparisonsacrossdurations.The dataare presentedfor the beginning,middle,and endconditions.Figure 3 demonstratesthat discriminabilityis a functionof ratioof •-to Tand thetemporalpositionof the •-msof independentnoise. The standarderrorsshownin Figs. 2 and 3 were calculatedbasedon the significance testfor oneparameterisosensitivity functionsdevelopedby Gourevitch and Galanter S. Fallon Cable and D. E. Robinson:Noise discrimination
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FIG. 2. Performance, measured asP(C), isplottedasa functionof thedurationoftheindependent samples ofnoise,•-.Eachcondition(beginning, middle, andend)isseparately depicted forthe150-,50-,and25-msburstdurations. Thedataareaveraged oversubjects. Eachdatapointrepresents 400trialsper subject.
(1967). As discussedin Sec. I, the baseline conditions were
repeatedperiodicallyto insureperformancein theseconditions remained stable throughout the experiment. Three measurementsof performancein eachof the baselineconditionsare plotted at Tequal •-in Fig. 2. The dispersionof the three data pointsplottedat eachbaselineconditionis a measure of the variability in performancein theseconditions from week to week over the course of approximately a month, while the error bars measurethe dispersionof the data during a 30-min presentationof four 100 trial blocks.
III. DISCUSSION
Discriminability of burstsof widebandnoiseis affected by the temporal positionof the •- ms of independentnoise presentedon differenttrials and the ratio •'/T. Discriminability increases asthe temporalpositionof the •-msof independentnoiseismovedfrom thebeginningto theendof the pairs of noisebursts.At eachof the threetemporalpositionsof the independentnoise, discriminabilityvaries as a function of the ratio of the duration of the independentnoise(•-) to the burst duration (T).
The effect of the ratio •'/T on discriminabilityis describedby Weber's law. The duration of the independent noiseneededto determinethat a pair of burstsof noiseisjust 2632
J. Acoust. Soc. Am., Vol. 92, No. 5, November 1992
detectablydifferentis a constantproportionof the burstduration. It is importantto notethat the effectsof the temporal position of the independentnoise are not predicted by Weber'slaw. For eachof the threetemporalpositionsof the independentnoise,the ratio of the durationof the independent noise to the burst duration
is constant.
The constant of
proportionality decreasesas the independentnoise was movedfrom thebeginningto the endof the burstpairs.For a P(C) equal to 0.707, the constantof proportionalityis approximately 0.03 in the end condition, 0.14 in the middle condition,and 0.41 in the beginningcondition. The relationshipbetweendiscriminabilityand the ratio of the •- ms of independentnoiseto the durationof the noise burstsis similarto the resultsof studiesexaminingthediscriminability of pairsof ten-tonepatterns(Watson and Kidd, 1987;Watson et al., 1990). The tonal-patternstudiesperformedby Watsonand hiscolleagues demonstratethat componentresolutiondependsmainly on ratio of the durationof the targettoneto the durationof the patternwithin whichit occurs.Theseresultsare shownfor patternsrangingin duration from 62.5 ms to 2 s. Over this range,the numberand durationof the toneswithin the pattern are varied.Results indicatethat theabilityto detecta changein thefrequencyof the target tone is determinedby the fraction of the pattern that is occupiedby the target tone. S. Fallon Coble and D. E. Robinson: Noise discrimination
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I.OO -
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FIG. 3.Thedatashownin Fig.2 arereplotted. Performance, measured asP(C), isplottedasa functionoftheratiooftheduration(•-) ofindependent samples of noisepresented ondifferenttrialsto theduration(T) of thebursts.Thedatafromthethreedurationsof thebursts(25, 50,and 150ms) areshownfor the beginning,middle,and end conditions.
The effectof the temporalpositionof the •- ms of independentnoiseon discriminabilityshownin the presentexperimentisalsoin generalagreement with theresultsof other experimentsexaminingthe discriminabilityof pairs of complexwaveforms.As previouslydiscussed, Watsonand hiscolleagues (Watsonet al., 1975) andHanna (1984) also useda same-differentparadigmto examinethe discriminability of complexwaveformsof approximately400-msduration. In eachof theseexperiments,the waveformspresented duringdifferenttrialsareidenticalexceptfor a selectedtemporalportionof thesecondwaveformin a pair that is altered relative to the first waveform.The presentexperiment,as well as the tonal pattern research(Watson et al., 1975), showsthat discriminabilityincreasesas the temporal position of the alteredportionismovedfrom thebeginningto the end of the waveform.
Hanna examinedthe discriminabilityof a singlepair of burstsof noiseunder conditionsof minimal uncertainty; at
the beginningof each block of trials, two 409.6-msnoise burstsare generated.The noiseburstspresentedduringdifferent trials are identicalexceptfor 25.6 ms of the second waveformin a pair that is alteredrelativeto the first waveform. Usingtheterminologyintroducedto describethe present experiment,the ratio of •'/T equaled0.0625 in Hanna's 2633
J. Acoust. Soc. Am., Vol. 92, No. 5, November 1992
experiment.The closestcomparabledata from the present experimentare at the longestburstdurationexamined( 150 ms) and the smallestvaluesof •- ( 5 and 10 ms). As shown in
Fig. 2, performancein theseconditionsis best in the end conditionand is nearly identical and approachingchance performancein the middle and beginningconditions.These results are similar but not identical to the results from Han-
na'sexperiment.Hanna foundthat discriminabilityisbestin the end condition (d'= 1.75), decreasesin the beginning condition (d' = 0.75), and is worst in the middle condition
(d' = 0.25).• Althoughtheresults ofHanna'sexperiments showa clearorderingof the threeconditions,asin the present experimentperformancein the discriminationtaskis superiorin the end conditionand decreases to similarbut not identicallevelsin the beginningand middle conditions. Hanna (1984) also examined the discriminability of burstsof reproduciblenoiseas a function of overall burst duration.The stimuliin Hanna'sexperimentareidenticalto thoseusedin the baselineconditionsin the presentexperiment. (In the baselineconditionsthe durationof T is equal to the duration of •-.) Hanna showsthat the effectof duration
on discriminabilityis relatively small. Discriminability is best at a duration
of 25.6 ms and decreases at the shortest
(0.1 ms) andlongest(409.6 ms) durationsexamined.There S. Fallon Coble and D. E. Robinson: Noise discrimination
2633
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memory. The first, referred to as the sensory-tracemode, assumesthat a preciserepresentationor trace of the first oneofthetwocomparable points. 2 Hannareportsa decrease stimulusis maintainedby rehearsaland comparedwith the secondstimulus. In the secondmode of memory, the conin performancein the discriminationtask asthe burst duratext-codingmode, the stimuli are comparedto the general tion increases from 25.6 to 102.4ms. In the presentexperiment, discriminabilityis independentof the overall burst contextof the soundsin the experiment,and, basedon this duration (T) overa rangeof durationsfrom 25.0-150.0 ms. comparison,transformedinto an abstractand morepermaThe difference between Hanna's data at a burst duration of nent representationof the stimulus. 102.4 ms (d'=2.75) and the 150.0-ms condition Becausethe presentexperimentwasnot designed to test the model of memorydescribedby Durlach and Braida, it is (d' = 3.25) in the presentexperimentisrelativelysmalland is assumedto be within the range of error attributable to unclear whether listenersperforming the same-different methodologicaldifferencesor practiceeffects. noisediscriminationtask generatepreciseor abstractrepreThis assumptionis not unreasonablebased on the sentationsof the stimuli. To determinewhetherprecise,abamountof variability shownin weeklymeasurements of perstract,or both typesof representations are usedin the noise formance in the baselineconditionsin the presentexperidiscrimination task, further studies must be performed. ment. As shownin Fig. 2 at the pointsT equalr, the disper- Thesestudiesincludeexaminingthe effectsof intervening sion of three measurementsof performancein each of the maskerson discriminability,varying the duration of the interstimulusinterval, and varying the sizeof the stimulusset baselineconditionsis greaterthan the error associated with any singledatapoint.The dispersionof the threedata points and the numberof timeseachstimulusis presented. Based on Durlach and Braida's model, it is assumedthat plottedat eachbaselineconditionis a measureof the fluctuationin performancein the discriminationtaskfrom weekto the listenersgeneratesomeform or formsof internalrepreweek,whilethe errorbarsmeasurethe dispersionof the data sentationof the stimuli that lead to the productionof optimal decisionstatistics.The representationof the burst of during a single30-min presentationof four blocksof 100 pairsof noisebursts. noisepresentedin the firstobservationintervalisthenstored Further noise discrimination experimentsare being and comparedwith the representationof the secondnoise conductedto examinethe effectsof the temporalpositionof burst. In order to accountfor the temporalpositionof the the r ms of independentnoisepresentedon differenttrials as independentnoiseon discriminability,it is assumedthat the a function of the overall burst duration (T). Studies of the representations of the stimuli are weighted. discriminabilityof pairsoften 40-mstonalpatterns(Watson For example,for a fixedlevel of performance,the duraet al., 1975) supportthe hypothesisthat, asthe overallduration of the independentnoise neededto discriminatebeis a slight discrepancybetweenthe resultsof the data from the presentexperimentand the resultsof Hanna's study at
tion of the bursts of noise (T) is increased to 400 ms, discri-
tween 150-ms bursts of noise was 114, 78, and 53 ms in the
minability of any givenoverallduration (73 will increaseas a fixeddurationof independentnoise(r) is movedfrom the beginningto the end of the burstsof noisepresentedon different trials. However, Hanna's results(1984) suggestthat the orderingof the effectsof the temporalpositionof the r ms of independentnoisepresentedon differenttrials on discriminability will vary as the overall duration is increased. Basedon Hanna'sdata, it would be expectedthat the burst pairs will be most discriminablein the end condition and that the differencebetweenperformancein the middle and beginningconditionswill diminish. A conceptualframework that accountsfor the conclusionsof the presentexperimentis outlinedin the remaining portion of the discussionsection.The similaritiesbetween the resultsof the presentexperimentand thosereportedin the studiesof tonal-patterndiscriminationreportedby Watson and his colleagues(Watson et al., 1975; Watson and Kidd, 1987;Watsonet al., 1990) suggestthat the qualitative modeldevelopedto describenoisediscriminationcouldalso accountfor the discriminabilityof tonal patterns. The qualitativemodelof noisediscriminationprocessis basedon the generalframework of the model of memory proposedby Durlach and Braida (1969). Durlach and Braida'smodelis composedof threebasicprocesses: the generation of internal representations of each stimulus,the comparison of the representations,and the calculation of a
beginning,middle, and end conditions,respectively.If the weighting function applied to a temporal window determinedby the burst durationincreasesmonotonicallyin approximatelya linear manner, the duration of the independent noiseneededto discriminatebetweenpairsdecreases as the r ms of independentnoiseare movedfrom the beginning to the end of the burstpairs. It is alsonecessary to incorporatea mechanisminto the
decision statistic.
Durlach
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and Braida described two distinct modes of
J. Acoust.Soc. Am., Vol. 92, No. 5, November1992
model that accounts for the effect of the ratio r/T
on discri-
minability.It ispossiblethat a singleweightingfunctionnormalizedby burstdurationcouldaccountfor both the effects of the temporalpositionof the r ms of independentnoiseon discriminabilityand the Weber'sLaw relationshipbetween discriminabilityand the proportion of independentnoise presentedon differenttrials. It is alsopossiblethat independent mechanisms
are needed to account for the effects of
temporalpositionand proportionaldurationon the discriminability of noisebursts. It is unclear where, within Durlach and Braida's model,
it wouldbe mostappropriateto placeoneor moretemporal weightingfunctions.It isconceivable that eachburstof noise is weightedprior to the comparisonprocessor that the weightingstageof themodeloccursafterthe representations of the two waveformsare compared. The use of a qualitativemodel basedon Durlach and Braida's (1969) model of intensity perceptionprovidesa generalframeworkfor organizingand conceptualizing the data. Further modelingis currently beingperformedto inS. Fallon Coble and D. E. Robinson:Noisediscrimination
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corporatethe data from the presentexperimentand unpublished data from related noisediscriminationexperiments into a quantitativemodel. ACKNOWLEDGMENTS
This researchwasa portionof the doctoralthesisof the first authorand wassupportedby grantsfrom the Air Force Office of ScientificResearchto Indiana University and to the Indiana University Institute for the Study of Human Capabilities.Dr. Bruce G. Berg, Dr. David M. Green, and an anonymousreviewerprovidedhelpful commentson an earlierversionof this manuscript. The valuesof d' reportedin Sec.III for thenoisediscrimination experiment performedby Hanna in 1984are approximations determinedby visual inspection.
It isimportant to notethatHannameasured performance in thenoisediscriminationtask in units of d' and plottedthe data on semilogarithmic coordinates.If the differencesin data presentationbetweenthe present experimentand Hanna's study (1984) are not taken into account,the slightdiscrepancy in the resultsis exaggerated.
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Durlach, N. I., and Braida, L. D. (1969). "Intensity Perception.I. Preliminary theoryof intensityresolution,"J. Acoust.Soc.Am. 46, 372-383. Fallon, S. M., and Robinson,D. E. (1985). "The effectsof changesin correlation on the discriminabilityof noisesamples,"J. Acoust. Soc. Am. Suppl. 1 78, S46. Gilkey, R. H., Robinson,D. E., and Frank, A. S. (1988). "A softwarepseudorandomnoisegenerator,"J. Acoust. Soc.Am. 83, 829-831. Gourevitch,V., and Galanter, E. (1967). "A significance test for one parameterisosensitivity functions,"Psychometrika32, 67-72. Hanna, T. E. (1984). "Discriminationof reproduciblenoiseasa functionof bandwidthand duration,"Percept.Psychophys. 36, 409-416. Watson, C. S., Foyle, D.C., and Kidd, G. R. (1990). "Limits of auditory patterndiscriminationfor patternswith variousdurations,"J. Acoust. Soc. Am. 88, 2631-2638.
Watson C. S., Kelly W. J., and Wroton, H. W. (1976). "Factors in the discriminationof tonal patterns.II. Selectiveattentionand learningunder variouslevelsof stimulusuncertainty,"J. Acoust. Soc.Am. 60, 11751186.
Watson,C. S., and Kidd, G. R. (1987). "Proportionaltarget-toneduration as a factor in discriminabilityof tonal patterns," J. Acoust. Soc. Am. Suppl. 1 82, S40. Watson, C. S., Wroton, H. W., Kelly, W. J., and Benbassat,C. A. (1975). "Factorsin the discriminationof auditorypatterns.I. Componentfrequency,temporalposition,and silentintervals,"J. Acoust.Soc.Am. 57, 1175-1185.
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