Frequency discrimination in forward and backward masking ChristopherW. Turner Communication Sciences andDisordersandInstitutefor SensoryResearch,SyracuseUniversity, Syracuse,New York 13244
Fang-Gang Zeng HouseEar Institute,2100 }VestThirdStreet,LosAngeles,California90057 Evan M. Relkin
Institutefor Sensory ResearchandDepartmentofBioengineering, SyracuseUniversity, Syracuse, New York 13244
Amy R. Horwitz CommunicationSciences and Disorders,SyracuseUniversity,Syracuse,New York 13244
(Received11June1992;acceptedfor publication10August1992) Frequencydifferencelimensfor puretonesprecededby a forwardmaskeror followedby a backwardmaskerwereobtainedacrossa wide rangeof signallevels.Relkin and Doucet [Hear. Res. 55, 215-222 ( 1991) ] haveshownthat at a masker-signaldelayof 100ms, the thresholds of high-SR (spontaneous rate) auditory-nervefibersare recovered,while the low-SR fiber thresholdsare not. Therefore,forward-maskedfrequencydiscriminationpotentiallyoffersa methodto investigatethe role of low-SR fibersin the codingof frequency.It hasbeenshown that when an intenseforward maskeris presented100 msbeforea pure-tonesignal,intensity differencelimensare elevatedfor mid-levelsignals[ Zeng et al., Hear. Res. 55, 223-230 ( 1991) ]. However, Plack and Viemeister [J. Acoust. Soc.Am. 92, 3097-3101 (1992) ] have shownthat a similar elevationin the intensitydifferencelimen is obtainedunder conditionsof backwardmasking,whereselectiveadaptationof the auditoryneuronswould not be expected to occur.A conditionof backward-masked frequencydiscriminationwasthereforeincludedto investigate the role of interference resultingfrom addingadditionalstimulito a discrimination task.For signalsat 1000and 6000 Hz, therewasno effectof a forwardmaskeruponfrequency differencelimens.For the backward-masked conditions,an elevationof the frequency differencelimen wasobservedat all signallevels,demonstratingthat the effectsof forwardand backwardmaskersuponfrequencydiscriminationare dissimilarand suggesting that cognitive effectsare presentin backward-masked discriminationtasks.This differencebetweenintensity and frequencydiscriminationsupportsthe ideathat intensityand frequencyare encodedin differentmannersby the auditory system.The absenceof an effectof forward maskeron frequencydiscriminationsuggests that the rate response of low-SRfibersdoesnot contribute usefulinformationto a frequencydiscriminationtask. PACS numbers:43.66.Ba, 43.66.Dc, 43.66.Fe, 43.66.Hg
INTRODUCTION
Recent reportsfrom our laboratory (Zeng et al., 1991; Zeng and Turner, 1992) demonstratethat when pure-tone intensitydiscriminationis measuredfor signalspresented 100ms followingan intensenarrow-bandforwardmaskera large midlevel increasein the Weber fraction is observed. For the combination ofmasker level ( 90 dB SPL) and mask-
er-signal delay used in the studies,no shift in detection threshold was observedfor signalsat the test frequency. Thesesameresultswerereplicatedby Plack and Viemeister (1992a). The resultsof theseexperimentswere unusualin that an effect of a forward
masker is demonstrated
for su-
prathresholdtaskswhen the forward maskerproducesno elevation of the detection threshold. The choice of forward
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maskerlevel and masker-signaldelay in theseexperiments wasdesignedto take advantageof the recentfindingby Relkin and Doucet ( 1991), that at 100 msfollowingthe adapting stimulus,low-SR (low spontaneousrate) eighth-nerve fiberswere not completelyrecoveredfrom adaptation (in termsof their rate responsethreshold),while the high-SR fiberswere recovered.Zeng et al. (1991) and Zeng and Turner (1992) hypothesizedthat the high-SR fibers are thereforeresponsible for detectionof signalsnearthreshold, while low-SR fibersprovide an important contributionto suprathresholdintensitydiscriminationat higherlevels.Under certain conditionsof forward maskingtheselow-SR fibers are not completelyrecovered,thus leading to the observeddeteriorationin intensitydiscriminationperformance following a forward masker.
0001-4966/92/123102-07500.80
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The mechanismby which the auditory systemencodes the frequencyof a pure tone is uncertain.In termsof the auditory-nerveresponses,both rate-place (excitation pattern) schemes(e.g., Bekesy, 1960; Zwicker, 1970) and phaselockingschemes(e.g., GoldsteinandSrulovicz,1977; Wever, 1949) havebeenproposedas the relevantcode,althougheachhas theoreticallimitations.For modelsbased uponphaselocking,it hasbeenshownthat the information providedby timing at low stimulusfrequenciesis precise enoughto accountfor behavioraldata (Siebert,1970);however, it is alsoknown that accuratephaselockingis greatly diminishedor absentat frequenciesabove5000 Hz (Johnson, 1980). For modelsbasedupon rate-placecodes,one limiting factoris the low soundlevelsat whichthe firingrate of high-SR neuronssaturates,which would tend to reduce the sharpnesswith which the "excitationpattern" can be represented.Low-SR neuronsdo not saturateat theselow soundlevels,and thereforemight providean importantcontribution to placecodingat higher stimuluslevels.Shofner andSachs(1986) haveshownthat the populationoflow-SR fibersmaintainsa sharppeakin the neural-ratepopulation response to a 1500-Hz tonefor highsoundlevelswhile highSR fibersdo not. Kim et al. (1991) have similarly shown that a neuralresponseprofilebaseduponrate responsefrom a populationof high-SRfiberslosesits sharpness for a 1000Hz toneat highlevels,whilethe populationof low-SRfibers maintainsa sharp pattern. In contrast,Kim et al. showed that both the high-SR and low-SR neural responseprofiles maintaina sharppatternevenat high signallevelsfor a 5000Hz tone. In particular, at 5000 Hz, the high-SR neurons showa sharpapicaledgefor the patternacrossa widerange of signallevels. In the first experimentdescribedin this paper,we measuredfrequencydiscriminationof tone pairs, eachmember of the pair precededby a forward maskerpresented100 ms prior to the toneonset.Test frequencies were 1000and 6000 Hz, to investigatethe effectsof selectivelyadaptinglow-SR fibersin frequencyregionswheretemporalcodingmay and may not be important.Viewed in a straightforwardmanner, we were interestedin determiningif the effectsof an adapting tone,onethat doesnot alter a subject'ssensitivitythreshold, will havean effectuponthe suprathreshold task of frequency discrimination.If not, such a result implies that there are fundamentaldifferencesbetweenthe way that intensity discriminationand frequencydiscriminationtasks are performedby a listener.At a morespeculativelevel,since an adaptingstimuluspresented100msbeforethe testsignal hasbeenshownto lead to a differentialrecoveryof the rate responses of low- and high-SRsignals(Relkin and Doucet, 1991), this experimentwould seemto offeran opportunity to investigatethe role of the rate response oflow-SR fibersin
scribed above, was obtained when the test stimuli were fol-
lowed by a narrow-band-noisebackward masker. Such a
midlevel "hump"•lueto backward masking should notbe the result of selectiveadaptationof certain auditory-nerve fibers;instead,a more central,or evencognitive,mechanism for the elevation
ter (1992b), that demonstrated that a midlevel elevation of
intensitydifferencelimens,similarin shapeto the midlevel "hump" in forward-masked intensity discrimination de3103
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It is of
interest,then,to determinethedegreeto whichforward-and backward-masked "humps"in discriminationexperiments are related.
Massaro (1975) reported that a pure-tonebackward maskerpresentedup to 240 ms following a test stimulus produceda reductionin the percentcorrect obtainedin a 2AFC frequencydiscriminationtask.The maskerfrequency was similar to that of the test stimuli and both masker and
test signalswere presentedat the samelevel (81 dB SPL). This effectwas observedfor signal-maskerintervalsfrom 0 to 240 ms, with the greatestreductionsin performanceoccurringfor shortersignal-maskerintervals.Massaroattributed this effectto an interferencewith subjects'"auditory storage"memory. In contrast,Yost et al. (1976) found no effectof forward or backwardmaskersupon frequencydiscriminationwhenboththemaskerandsignalwerepresented at 70 dB SPL usingmasker-signalintervalsof 5 or 100 ms. In a secondexperiment,we further examinedthis effect in a frequency discriminationtask for backward-masked conditions.The Massaro(1975) dataprovidesevidencethat under some conditions a backward masker can have an effect
upon frequencydiscrimination;it was of interestto determine the form of the frequencydifferencelimen elevation obtainedfor the specificmaskingconditionsusedin our experiments. In the present experiments,we use the same maskingconditionsusedby Zeng et al. (1991), Zeng and Turner ( 1992 ), and Plack and Viemeister (1992a,b) in their
studiesof intensitydiscrimination.Of even greaterinterest however,is the questionof whether the similar "humps"
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frequencydiscrimination. A secondexperiment is also described,in which frequencydiscriminationof a tone pair, each followed by a
backwardmasker,was investigated.This experimentwas suggested followingthe recentreportby Plack and Viemeis-
of difference limens is indicated.
0 0
20
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Level (dB SPL) FIG. 1. Plot of the 1000-Hz frequencydifferencelimensas a function of signallevel for both the unmaskedand narrow-bandforward-maskedconditions. Data points representthe mean of five normal-hearinglisteners. Vertical
bars indicate the standard deviation across the five listeners.
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observedin intensity discrimination for both forward and backward masking, exist for both forward- and backwardmaskedconditionsin frequencydiscrimination.If both forward and backward maskersproduce the same effect for frequencydiscrimination,it is more likely that the elevation of the intensitydifferencelimen originallyobservedin for-
ward maskingby Zenget al. ( 1991) is not specificallyrelatedto auditory-nerveadaptation,but ratherisa moregeneral cognitiveeffectof addingadditional stimuli to a discrimination task (e.g., Leek and Watson, 1988). On the other hand, if both maskersdo not producethe sameeffectfor frequency discrimination,this suggests that the effectsof forward and backward maskingare not related. Lutfi (1992) has shown that the final stimulusin a sequencepresentedto a subject hasa uniquelydisruptiveeffectuponintensityor frequency discriminationfor a prior tone embeddedin the sequence. Adding backwardmaskersto a discriminationexperiment essentiallyadds a final interfering stimulus to the task, whereasin the forward-masked condition, the final stimulus
is one of the test signalsto be attendedto. I. EXPERIMENT
1: FORWARD-MASKED
FREQUENCY
DISCRIMINATION A. Methods 1. Stimuli
Test signalswere 25-ms (specifiedas the duration betweentheO-voltage points)puretonesat either1000or 6000 Hz. Riseandfall timeswere3-mslinearramps.Thesesignals weredigitallygeneratedby a Mac IIx computerin conjunction with a DigiDesign,16-bit,digital-to-analogconverter. The samplingrate was44.1 kHz and signalswerelow-pass filtered at 20 kHz. Frequency differencelimens were obtainedat 10-dBstepsfor signallevelsrangingfrom 20 to 90
2. Procedures
A two-intervaltwo-alternativeforced-choicelistening paradigm(2IFC) wasemployedfor the measurementof frequencydiscrimination.A silentinterval of 500 ms was insertedbetweenthe two listeningintervalsof eachtrial. The subjectrespondedby pressinga buttoncorresponding to the interval containingthe higher frequencytone. The correct answerwasindicatedvia a small light followingeachtrial. The stimuli for the followingtrial began2200 ms after the subject'sresponse. A two-down,one-upadaptivetracking procedurecontrolled the frequencydifferences presentedin an experimental run. This proceduretracksthe 71%-correctpoint on the psychometricfunction (Levitt, 1971). For the 1000-Hz frequencydiscriminationexperiment,the stepsizewas 1 Hz, for 6000 Hz, the stepsizewas 2 Hz. The procedurecontinued until 14 reversalswere obtained.The frequencydifference limen for each run was calculated
as the mean of the
final ten reversals.Subjectswere run until performancewas asymptoticand stable in each condition (minimum of six runs per condition). The final value for eachdata point for an individual subjectwas taken as the mean of the final six runs.A similar 2IFC procedurewas employedfor the measurementof detectionthresholdsfor the 25-mstonal signals underconditionsof forwardmaskingand in quiet.Under the conditionsof 100-msmasker-signalinterval and 90-dB narrow-band maskers, no threshold shift was observed (mean threshold shifts of 0.4 and 0.5 dB for 1000 and 6000 Hz,
respectively).The same result was obtainedfor the wideband maskerspresentedat 100 dB SPL (thresholdshiftsof - 1.8 and 1.6 dB for 1000and 6000 Hz, respectively).Fi;ve normal-hearingsubjectsparticipatedin the narrow-bandmaskedconditions,a singlenormal-hearingsubjectcompletedthe wideband-masked conditions.
dB SPL at 1000 Hz and from 20 to 80 dB SPL at 6000 Hz. The forward maskers were either a narrow band of noise
centeredat the test frequency(900-1100 Hz or 5600-6400 Hz) or in somecases,a widebandnoise(low passat 12 000 Hz). The noisemaskerswere 102msin duration,including 2-ms raised-cosine ramps.The maskerswere generatedby an analograndomnoisegenerator(MDF model8156); narrow-bandnoiseswerebandpass filteredwith rejectionslopes of 90 dB/oct (Kemo modelVBF 8). All signalswere presentedvia BeyerDT-48 A.0 headphones andsignallevelsare expressed asthe levelsdevelopedin an NBS-9A coupler. The narrow-bandmaskerswerepresentedat 90 dB SPL, while the wideband maskers were 100 dB SPL. In the for-
ward-maskedconditions,the maskerprecededthe test signal by 100 ms (masker offsetto signalonset). Under these conditions, there was no threshold shift for the detection of a
25-ms test tone at the signalfrequency.It shouldbe noted that the maskerand signalcharacteristics in the 1000-Hz narrow-band
masker condition were identical to those em-
B. Results
Figure 1 displaysthe frequencydifferencelimens at 1000 Hz in the narrow-band
masked condition as a function
of signallevel.Also plotted are the differencelimensfor the unmaskedcontrolcondition.The individualdatapointsrepresentmeanvaluesof the fivenormal-hearinglisteners.The verticalbarsindicateplusand minusonestandarddeviation acrossthe fivesubjectsin eachconditionat eachsignallevel. There is essentiallyno differencebetweenthe frequencydifference limens obtained
under the masked and unmasked
conditionsin the groupmean data;this sameresultwas observedin eachof the individualsubjects'data. In particular, no midlevelelevationin the frequencydifferencelimen was noted under forward masking, in contrast with the effect previously reported for intensity discrimination experiments.
Figure 2 displaysthe groupmean frequencydifference limens at 6000 Hz obtained from the same five normal-hear-
ployedby Zeng et al. ( 1991) with the exceptionthat in the Zeng et al. studyan incrementin intensitywasaddedto the signalin onelisteningintervalwhereasin the presentstudy, an incrementin signalfrequencywasaddedto oneinterval.
conditionare shown.The verticalbarsindicateplusand mi-
An unmasked, control condition was also run at each test
nus one standard deviation
frequencyand signallevel.
point is shownfor the forward-maskedfrequencydifference
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ing listenersin the narrow-bandmaskercondition.Both the unmasked control condition as well as the forward-masked across the five listeners. No data
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(dB SPL)
FIG. 2. Plot of the 6000-Hz frequencydifferencelimensas a functionof signallevelfor boththeunmasked andnarrow-band forward-masked conditions.Data pointsrepresentthe meanof five normal-hearinglisteners.
FIG. 3. Plot of the 1000-Hzfrequencydifferencelimensasfunctionof sig-
Vertical bars indicate the standard deviation across the five listeners.
indicate the standard deviation across the final six runs in each condition for
nal level for both the unmasked and wideband forward-masked conditions.
Data pointsaretheresultsfromonenormal-hearing listener.Verticalbars the listener.
limensat 20 dB SPL becauseonly three of the five subjects wereableto reliablyjudgethe pitchof the signalseparately from the narrow-bandmaskerwhenthe signalwaspresented
Again, no midlevelelevationis observed.In addition,the forward-maskedfrequencydifferencelimensat 6000 Hz are no longerelevatedwith respectto the controlcondition,as
at such a low sensationlevel. However, the mean value for
was observed in the narrow-band masker case at 6000 Hz,
the remainingthreesubjects undertheseconditionswas230 Hz, whichis similarto the other valueson thisfunction.The differencelimensasa functionof signallevelunderthe forward-maskedconditionwereparallelto, but slightlyhigher than, thoseof the control condition.As was the casefor the 1000-Hztones,no specificelevationof the differencelimen at midlevels under the forward-masked
condition
indicatingthat the pitch confusionwas successfully removed.
II. EXPERIMENT
2: BACKWARD-MASKED
FREQUENCY
DISCRIMINATION
A. Methods and procedures
is ob-
We obtainedfrequencydifferencelimensfor short1000served.Severallistenersreportedthat the forward masked Hz tones under conditions of no masker and under condifrequencydiscrimination taskat 6000Hz wasmoredifficult at all signallevelsthanat 1000Hz, dueto thetonelikequality of thehigh-frequency narrow-band masker,whichconfused the listenersas to which portionof the masker-signalcomplex to judgein termsof pitch. 5OO A wideband forward masker was then used in the re-
mainingforward-masking-experiments. One rationalewas to eliminatetheconfusing pitchcuesthat werepresentin the
6000 O• ---•--'
narrow-band maskers.In addition, becausethe resultsof the
narrow-bandmaskerexperimentswerenegative,in that no midlevelelevationof frequencydiscriminationwasnotedin the forward-maskedcondition,the possibilityexistedthat selectivelyadaptingthe rate responseof low-SR fibersin a narrowfrequencyregionsurroundingthe testtone wasinsufficientto disruptaccuratefrequencycoding.If the code for frequencyinvolveslookingat the entireneuralresponse profile,ratherthanjust the "peak"or the "edge"of the pattern, as suggested by the work of Emmerichet al. (1983), then adaptingonly a smallgroupof low-SRfiberswith the narrow-bandmaskersmay not havebeeneffective. Figure3 showsthe resultsof the widebandmaskerconditionfor thesinglesubjectat 1000Hz. Verticalbarsindicate the standarddeviationacrossruns for this subject.No midlevel elevationin the differencelimen is observed.Figure 4 shows the results from the wideband masker at 6000 Hz.
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Hz
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FIG. 4. Plot of the 6000-Hz frequencydifferencelimensasfunctionof signal level for both the unmasked and wideband forward-masked conditions.
Data pointsare the resultsfrom onenormal-hearing listener.Verticalbars indicate the standard deviation across the final six runs in each condition for the listener.
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80
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FIG. 5.Frequency difference limens at 1000Hz forboththeunmasked andnarrow-band backward-masked conditions. Individual subjects' dataareshown in (a)-(c). In (d), datapointsrepresentthe meansof the threenormal-hearing listeners.
tionsof backwardmaskingby a narrow-bandnoisecentered at 1000 Hz. The methods and stimuli for this experiment were identical
to that described for the 1000-Hz
encelimenswasconsistentand occurredfor all signallevels asshownin the groupmeandata [ (d) ].
narrow-
C. Discussion band-maskedfrequency discrimination experiment describedpreviously,with the exceptionthat the maskerwas The mostimportantfindingsof thisstudyarethe differpresented100 ms after eachmemberof the discrimination encesbetweenthe effectsof forward and backwardmasking pairin the2IFC task.Threenormal-hearing subjects partici- upon frequencyand intensitydiscrimination.Both a forpatedin theexperiment, two of thesesubjects werealsoparward- and backward-maskingintensenarrow-bandstimuticipantsin experiment1, the third wasan experienced lislus, separatedfrom the signalby 100 ms, havepreviously tenernewto frequencydiscriminationexperiments. In these beenshownto affectthe midlevel intensitycodingof a tone. subjects,the backwardmaskerproducedno thresholdshift In contrast,the presentexperimentsdemonstratethat for for a test tone at 1000 Hz (mean threshold shift of 0.7 dB). frequencydiscrimination,neithera wide nor narrow-band forwardmaskeraffectsthe frequencycodingof a tone (when B. Results pitch confusions are eliminated).However,the backward The four panelsof Fig. 5 displaythe resultsof theback- maskerdoesproducean elevationin the frequencydifference limen. ward-maskedfrequencydiscriminationexperiments.AlThe reasonfor the forward-maskingeffectin intensity thoughthe resultsof the three listenersshowedthe same trend, the magnitudeof the effectof the backwardmasker discriminationis not firmly established.The possibilityexdiffered among them. Therefore, (a)-(c) are provided iststhat the relevantsiteof action is not peripheralas suggestedby Zenget al. (1991), but rathera morecentralor showingthe resultsof the threeindividualsubjects, and (d) cognitive process, andthereforeunrelatedto thedifferential showsthe groupmeanresults.For subjectCT [(a)], the backwardmaskerproduceda large elevationin frequency adaptationof low- and high-SRfibers.In particular,this differencelimens, while for the other two listeners [ (b) and possibilitywassuggested by the findingof Plackand Viemeister(1992b) that a backwardmaskerproduceda similar (c) ], the elevation in differencelimens was smaller. Howmidlevel elevation of the differencelimen. Our finding that ever,for all threesubjects,the elevationin frequencydiffer-
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onlya backwardmaskerandnot a forwardmaskerproduces an elevationin frequencydiscriminationsuggeststhat the effectsof forward and backwardmaskingupon discrimination tasksare unrelated.However,an elevationin frequency differencelimensdue to backwardmaskingwasobservedin this studyand wasapproximatelyequalfor all signallevels. Plack and Viemeister (1992b) showed that the backward
maskerin their intensitydiscriminationtask producednot only a constantelevationof the differencelimen acrossall signal levels,but also an additional large elevation of the differencelimen seenonly for midlevelsignals.Thus the effect of a backwardmaskerupon discriminationtasksmay consistof two components; firsta generalelevationof differencelimensfor both frequencyand intensitytasks,and second, a midlevel elevation of the difference limen observed
only for intensity discrimination. The specificoriginsof theseeffectsremainspeculativeat the presenttime, althougha cognitiveeffectof either forward or backwardmaskersin discriminationexperimentsis certainly not ruled out. Recent data from our laboratory (Turner et aT., 1992) show that the effectof forward or backward maskersupon intensitydiscriminationdependsupon the methodusedfor obtainingthe differencelimens. It is alsonot entirely straightforwardhow to interpret the lack of effectfrom a forward maskeron frequencydiscrimination,evenif one acceptsthe Zeng et aT. ( 1991) hy-
pothesisthat the roles of differentiallyadaptedhigh- and low-SR fibersare involved in forward-maskedintensity discrimination.On onehand,it couldbe arguedthat the adaptation resulting from the 90-dB SPL forward masker employed here was not large enough to decrease the contributionof the low-SR fibersto the frequencydiscrimination task, but was sufficientto disrupt intensitycoding. However,sinceboth tasksare differencelimen experiments, in which the smallestperceptibledifferencebetweentwo stimuli is determined,any disruptionin codingof the rel-
criminationalso suggeststhat the two discriminationtasks differ in mechanism.
It is known that the adaptingstimuli usedin theseexperimentsselectivelydisruptsthe rate responseof low-SR fibersin chinchillas,elevatingneuralthresholds(Relkin and Doucet, 1991). The presentresultsthereforesuggesta role for phase-locking in the encodingof frequencyat 1000Hz, in viewof the reportsby Schofnerand Sachs(1986) and Kim et al. ( 1991) that high-SRauditory-nervefiberrate profilesfor low frequenciesare degradedin sharpnessat higher signal levels.Selectivelyadaptingthe low-SR fibersin this experiment producedno elevationof the differencelimen. Goldstein (1980) modeledfrequencydiscriminationusingrate responses from low- and high-SRfibersand concludedthat if rateresponses wereimportantfor behavioralperformance, low-SR fibersare necessary for performanceat high levels. At 6000 Hz, a role for phase-locking is not asstronglysupported. First, as mentionedpreviously,the accuracywith whichfiberscanphaselock to a 6000-Hz toneis rather poor to beginwith. Second,Kim et al.'s data suggestthat low-SR fibersmay not be necessaryin codinghigh frequenciesvia a rate-placecode.The "excitationpattern"for high-frequency stimuli transmittedby a populationof high-SR fibers remainssharpevenat high signallevels. The presentresultsthen offersupportto a dual-coding modelof frequencydiscrimination,in whichphaselockingis usedat lower frequenciesand rate-placecodesat higherfrequencies.A similar mechanismhasbeenproposedby others (e.g., Moore, 1973, 1989). However,the role of phaselocking at even6000 Hz cannotbe completely.ruledout, as the largefrequencydifferencelimensobtainedat 6000 Hz in this experimentand othersmay still be compatiblewith the degradedphaselockingat that frequency(Goldsteinand Srulovicz, 1977). ACKNOWLEDGMENTS
evant portion of the excitationpattern shouldresult in a decrementin performance.Such a decrement in perfor-
Nos. DC 00380 and DC 00377. The authors thank Anita
mancewas not observedin this study, whereassucha decrement has beenobservedfor intensitydiscrimination.
Young, SusanPlante and Lauren Forget for assistancein data collection.Dr. Jozef Zwislocki also providedhelpful
The absenceof an effectof the forward masker upon frequencydiscrimination under the same stimulus conditionsasthoseemployedin previousforward-maskedintensity discriminationexperimentsindicatesthat the two tasks are performedby the auditory systemin fundamentally different manners.The resultsof the presentfrequencydiscriminationstudy,alongwith previousintensitydiscrimination studies(Zeng et aT.,1991;ZengandTurner, 1992;Plack and Viemeister,1992), would seemto contradictthe general hypothesisoriginallyput forth by Zwicker (1956) that both intensityand frequencyare similarly encodedby the auditory nerverepresentationof the "excitationpattern" along the cochlea.Additional evidenceagainstsuch a common mechanismhasbeenpresentedby Moore (1992). The finding that a backward masker producesboth a "midlevel hump" in additionto a generalelevationin differencelimens for intensitydiscrimination (Plack and Viemeister, 1992b), whereasthe presentstudy'sfinding that only the parallel elevationis observedfor backward-masked frequencydis3107
J. Acoust. Soc. Am., Vol. 92, No. 6, December 1992
This researchwassupportedin part by NIDCD Grants
discussions.
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Goldstein,J. L., and Srulovicz,P. (1977). "Auditory-nervespikeintervals asan adequatebasisfor aural frequencydiscrimination,"in Psychophysicsand Physiology of Hearing, editedby E. F. Evansand J.P. Wilson ( Academic, London).
Goldstein,J. L (1980). "On the signalprocessing potentialof high threshold auditory nerve fibers," in Psychological, Physiologicaland BehaoiouralStudiesin Hearing, editedby G. van den Brink and F. A. Bilsen (Delft U.P., Delft, The Netherlands).
Johnson,D. H. (1980). "The relationshipbetweenspikerate and synchronyin responses of auditory-nervefibersto singletones,"J. Acoust.Soc. Am. 68, 1115-1122.
Kim, D. O., Parham,K., Sirianni,J. G., and Chang,S. O. (1991). "Spatial response profilesof posteroventral cochlearnucleusneuronsand auditory-nerve fibersin unanesthetizeddecerebratecats: Responseto pure tones," J. Acoust. Soc. Am. 89, 2804--2817.
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Leek,M. R., andWatson,C. S. (1988)."Auditoryperceptual learning of tonalpatterns," Percept.Psychophys. 43, 389-394. Levitt,H. (1971)."Transformed up-downmethods in psychoacoustics," J. Acoust. Soc. Am. 49, 467-477.
Lutfi,R. A. (1992). "Informational processing of complexsound.III: Interference,"J. Acoust.Soc.Am. 91, 3391-3401.
Massaro,D. W. (1975). "Backwardrecognition masking," J. Acoust.Soc. Am. 58, 1059-1066.
Moore,B.C. J. (1973). "Frequency difference limensfor short-duration tones,"J. Acoust.Soc.Am. 54, 610-619.
Moore,B.C. J. (1989).•4nIntroduction tothePsychology ofHearing(Academic, London).
Moore,B.C. J. (1992). "Detectionof combined frequency andamplitude modulation,"J. Acoust.Soc.Am. 91, 2332 (At).
Plack,C., andViemeister, N. F. (1992a) "The effects of notchednoiseon
intensity discrimination underforwardmasking," J. Acoust.Soc.Am. 92, 1902-1910.
Plack,C., andViemeister, N. F. (1992b)."Intensitydiscrimination under
backward masking," J.Acoust.Soc.Am.92, 3097-3101. Relkin,E. M., andDoucet(1991). "Recoveryfrompriorstimulation. I:
Relationship to spontaneous firingrateof primaryauditoryneurons," Hear. Res. 55, 215-222.
cy tonein the discharge rateof populations of auditorynervefibers," Hear. Res. 21, 91-95.
Siebert,W. M. (1970). "Frequency discrimination in theauditorysystem: placeor periodicity mechanisms?," Proc.IEEE 58,723-730. Turner, C. W., Horwitz, A. R., and Souza,P. E. (1992). "Forward- and backward-masked intensitydiscriminationmeasuredusingan adjust-
mentprocedure," J. Acoust.Soc.Am. 92, 2364 (A). Wever,E.G. (1949). TheoryoœHearing (Wiley, New York). Yost,W. A., Berg,K., andThomas,G. B. (1976). "Frequency recognition intemporal interference tasks: A comparison amongfourpsychophysical procedures," Percept.Psychophys. 20, 353-359.
Zeng,F. G., andTurner,C. W. (1992). "Intensitydiscrimination in forward masking,"J. Acoust.Soc.Am. 92, 782-787.
Zeng,F. G., Turner,C. W., andRelkin,E. M. (1991). "Recovery from priorstimulation II: Effectsuponintensitydiscrimination," Hear.Res. 55, 223-230.
Zwicker,E. (1956). "Die elementarenGrundlagenzur Bestimmender Informationskapazit//t desGeh/Srs," Acustica6, 365-381. Zwicker, E. (1970). "Masking and psychological excitationas conse-
quences oftheear'sfrequency analysis," inProceedings oœthe Symposium onFrequency •4nalysis andPeriodicity Detection in Hearing,editedbyR. PlompandG. Smoorenburg ($ijthoff,Leyden).
Shofner,W. P., andSachs,M. B. (1986). "Representation ofa low-frequen-
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Turnereta/.: Frequency discrimination inmasking
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Fang-Gang Zeng HouseEar Institute,2100 }VestThirdStreet,LosAngeles,California90057 Evan M. Relkin
Institutefor Sensory ResearchandDepartmentofBioengineering, SyracuseUniversity, Syracuse, New York 13244
Amy R. Horwitz CommunicationSciences and Disorders,SyracuseUniversity,Syracuse,New York 13244
(Received11June1992;acceptedfor publication10August1992) Frequencydifferencelimensfor puretonesprecededby a forwardmaskeror followedby a backwardmaskerwereobtainedacrossa wide rangeof signallevels.Relkin and Doucet [Hear. Res. 55, 215-222 ( 1991) ] haveshownthat at a masker-signaldelayof 100ms, the thresholds of high-SR (spontaneous rate) auditory-nervefibersare recovered,while the low-SR fiber thresholdsare not. Therefore,forward-maskedfrequencydiscriminationpotentiallyoffersa methodto investigatethe role of low-SR fibersin the codingof frequency.It hasbeenshown that when an intenseforward maskeris presented100 msbeforea pure-tonesignal,intensity differencelimensare elevatedfor mid-levelsignals[ Zeng et al., Hear. Res. 55, 223-230 ( 1991) ]. However, Plack and Viemeister [J. Acoust. Soc.Am. 92, 3097-3101 (1992) ] have shownthat a similar elevationin the intensitydifferencelimen is obtainedunder conditionsof backwardmasking,whereselectiveadaptationof the auditoryneuronswould not be expected to occur.A conditionof backward-masked frequencydiscriminationwasthereforeincludedto investigate the role of interference resultingfrom addingadditionalstimulito a discrimination task.For signalsat 1000and 6000 Hz, therewasno effectof a forwardmaskeruponfrequency differencelimens.For the backward-masked conditions,an elevationof the frequency differencelimen wasobservedat all signallevels,demonstratingthat the effectsof forwardand backwardmaskersuponfrequencydiscriminationare dissimilarand suggesting that cognitive effectsare presentin backward-masked discriminationtasks.This differencebetweenintensity and frequencydiscriminationsupportsthe ideathat intensityand frequencyare encodedin differentmannersby the auditory system.The absenceof an effectof forward maskeron frequencydiscriminationsuggests that the rate response of low-SRfibersdoesnot contribute usefulinformationto a frequencydiscriminationtask. PACS numbers:43.66.Ba, 43.66.Dc, 43.66.Fe, 43.66.Hg
INTRODUCTION
Recent reportsfrom our laboratory (Zeng et al., 1991; Zeng and Turner, 1992) demonstratethat when pure-tone intensitydiscriminationis measuredfor signalspresented 100ms followingan intensenarrow-bandforwardmaskera large midlevel increasein the Weber fraction is observed. For the combination ofmasker level ( 90 dB SPL) and mask-
er-signal delay used in the studies,no shift in detection threshold was observedfor signalsat the test frequency. Thesesameresultswerereplicatedby Plack and Viemeister (1992a). The resultsof theseexperimentswere unusualin that an effect of a forward
masker is demonstrated
for su-
prathresholdtaskswhen the forward maskerproducesno elevation of the detection threshold. The choice of forward
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J. Acoust.Soc. Am. 92 (6), December 1992
maskerlevel and masker-signaldelay in theseexperiments wasdesignedto take advantageof the recentfindingby Relkin and Doucet ( 1991), that at 100 msfollowingthe adapting stimulus,low-SR (low spontaneousrate) eighth-nerve fiberswere not completelyrecoveredfrom adaptation (in termsof their rate responsethreshold),while the high-SR fiberswere recovered.Zeng et al. (1991) and Zeng and Turner (1992) hypothesizedthat the high-SR fibers are thereforeresponsible for detectionof signalsnearthreshold, while low-SR fibersprovide an important contributionto suprathresholdintensitydiscriminationat higherlevels.Under certain conditionsof forward maskingtheselow-SR fibers are not completelyrecovered,thus leading to the observeddeteriorationin intensitydiscriminationperformance following a forward masker.
0001-4966/92/123102-07500.80
@ 1992 AcousticalSocietyof America
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The mechanismby which the auditory systemencodes the frequencyof a pure tone is uncertain.In termsof the auditory-nerveresponses,both rate-place (excitation pattern) schemes(e.g., Bekesy, 1960; Zwicker, 1970) and phaselockingschemes(e.g., GoldsteinandSrulovicz,1977; Wever, 1949) havebeenproposedas the relevantcode,althougheachhas theoreticallimitations.For modelsbased uponphaselocking,it hasbeenshownthat the information providedby timing at low stimulusfrequenciesis precise enoughto accountfor behavioraldata (Siebert,1970);however, it is alsoknown that accuratephaselockingis greatly diminishedor absentat frequenciesabove5000 Hz (Johnson, 1980). For modelsbasedupon rate-placecodes,one limiting factoris the low soundlevelsat whichthe firingrate of high-SR neuronssaturates,which would tend to reduce the sharpnesswith which the "excitationpattern" can be represented.Low-SR neuronsdo not saturateat theselow soundlevels,and thereforemight providean importantcontribution to placecodingat higher stimuluslevels.Shofner andSachs(1986) haveshownthat the populationoflow-SR fibersmaintainsa sharppeakin the neural-ratepopulation response to a 1500-Hz tonefor highsoundlevelswhile highSR fibersdo not. Kim et al. (1991) have similarly shown that a neuralresponseprofilebaseduponrate responsefrom a populationof high-SRfiberslosesits sharpness for a 1000Hz toneat highlevels,whilethe populationof low-SRfibers maintainsa sharp pattern. In contrast,Kim et al. showed that both the high-SR and low-SR neural responseprofiles maintaina sharppatternevenat high signallevelsfor a 5000Hz tone. In particular, at 5000 Hz, the high-SR neurons showa sharpapicaledgefor the patternacrossa widerange of signallevels. In the first experimentdescribedin this paper,we measuredfrequencydiscriminationof tone pairs, eachmember of the pair precededby a forward maskerpresented100 ms prior to the toneonset.Test frequencies were 1000and 6000 Hz, to investigatethe effectsof selectivelyadaptinglow-SR fibersin frequencyregionswheretemporalcodingmay and may not be important.Viewed in a straightforwardmanner, we were interestedin determiningif the effectsof an adapting tone,onethat doesnot alter a subject'ssensitivitythreshold, will havean effectuponthe suprathreshold task of frequency discrimination.If not, such a result implies that there are fundamentaldifferencesbetweenthe way that intensity discriminationand frequencydiscriminationtasks are performedby a listener.At a morespeculativelevel,since an adaptingstimuluspresented100msbeforethe testsignal hasbeenshownto lead to a differentialrecoveryof the rate responses of low- and high-SRsignals(Relkin and Doucet, 1991), this experimentwould seemto offeran opportunity to investigatethe role of the rate response oflow-SR fibersin
scribed above, was obtained when the test stimuli were fol-
lowed by a narrow-band-noisebackward masker. Such a
midlevel "hump"•lueto backward masking should notbe the result of selectiveadaptationof certain auditory-nerve fibers;instead,a more central,or evencognitive,mechanism for the elevation
ter (1992b), that demonstrated that a midlevel elevation of
intensitydifferencelimens,similarin shapeto the midlevel "hump" in forward-masked intensity discrimination de3103
J. Acoust. Soc. Am., Vol. 92, No. 6, December 1992
It is of
interest,then,to determinethedegreeto whichforward-and backward-masked "humps"in discriminationexperiments are related.
Massaro (1975) reported that a pure-tonebackward maskerpresentedup to 240 ms following a test stimulus produceda reductionin the percentcorrect obtainedin a 2AFC frequencydiscriminationtask.The maskerfrequency was similar to that of the test stimuli and both masker and
test signalswere presentedat the samelevel (81 dB SPL). This effectwas observedfor signal-maskerintervalsfrom 0 to 240 ms, with the greatestreductionsin performanceoccurringfor shortersignal-maskerintervals.Massaroattributed this effectto an interferencewith subjects'"auditory storage"memory. In contrast,Yost et al. (1976) found no effectof forward or backwardmaskersupon frequencydiscriminationwhenboththemaskerandsignalwerepresented at 70 dB SPL usingmasker-signalintervalsof 5 or 100 ms. In a secondexperiment,we further examinedthis effect in a frequency discriminationtask for backward-masked conditions.The Massaro(1975) dataprovidesevidencethat under some conditions a backward masker can have an effect
upon frequencydiscrimination;it was of interestto determine the form of the frequencydifferencelimen elevation obtainedfor the specificmaskingconditionsusedin our experiments. In the present experiments,we use the same maskingconditionsusedby Zeng et al. (1991), Zeng and Turner ( 1992 ), and Plack and Viemeister (1992a,b) in their
studiesof intensitydiscrimination.Of even greaterinterest however,is the questionof whether the similar "humps"
40
ß
i
ß
I
1000 •
30
'
i
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i
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Hz Unmasked
---1--
Masked
20
frequencydiscrimination. A secondexperiment is also described,in which frequencydiscriminationof a tone pair, each followed by a
backwardmasker,was investigated.This experimentwas suggested followingthe recentreportby Plack and Viemeis-
of difference limens is indicated.
0 0
20
40
6•
80
O0
Level (dB SPL) FIG. 1. Plot of the 1000-Hz frequencydifferencelimensas a function of signallevel for both the unmaskedand narrow-bandforward-maskedconditions. Data points representthe mean of five normal-hearinglisteners. Vertical
bars indicate the standard deviation across the five listeners.
Turner et al.' Frequency discriminationin masking
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observedin intensity discrimination for both forward and backward masking, exist for both forward- and backwardmaskedconditionsin frequencydiscrimination.If both forward and backward maskersproduce the same effect for frequencydiscrimination,it is more likely that the elevation of the intensitydifferencelimen originallyobservedin for-
ward maskingby Zenget al. ( 1991) is not specificallyrelatedto auditory-nerveadaptation,but ratherisa moregeneral cognitiveeffectof addingadditional stimuli to a discrimination task (e.g., Leek and Watson, 1988). On the other hand, if both maskersdo not producethe sameeffectfor frequency discrimination,this suggests that the effectsof forward and backward maskingare not related. Lutfi (1992) has shown that the final stimulusin a sequencepresentedto a subject hasa uniquelydisruptiveeffectuponintensityor frequency discriminationfor a prior tone embeddedin the sequence. Adding backwardmaskersto a discriminationexperiment essentiallyadds a final interfering stimulus to the task, whereasin the forward-masked condition, the final stimulus
is one of the test signalsto be attendedto. I. EXPERIMENT
1: FORWARD-MASKED
FREQUENCY
DISCRIMINATION A. Methods 1. Stimuli
Test signalswere 25-ms (specifiedas the duration betweentheO-voltage points)puretonesat either1000or 6000 Hz. Riseandfall timeswere3-mslinearramps.Thesesignals weredigitallygeneratedby a Mac IIx computerin conjunction with a DigiDesign,16-bit,digital-to-analogconverter. The samplingrate was44.1 kHz and signalswerelow-pass filtered at 20 kHz. Frequency differencelimens were obtainedat 10-dBstepsfor signallevelsrangingfrom 20 to 90
2. Procedures
A two-intervaltwo-alternativeforced-choicelistening paradigm(2IFC) wasemployedfor the measurementof frequencydiscrimination.A silentinterval of 500 ms was insertedbetweenthe two listeningintervalsof eachtrial. The subjectrespondedby pressinga buttoncorresponding to the interval containingthe higher frequencytone. The correct answerwasindicatedvia a small light followingeachtrial. The stimuli for the followingtrial began2200 ms after the subject'sresponse. A two-down,one-upadaptivetracking procedurecontrolled the frequencydifferences presentedin an experimental run. This proceduretracksthe 71%-correctpoint on the psychometricfunction (Levitt, 1971). For the 1000-Hz frequencydiscriminationexperiment,the stepsizewas 1 Hz, for 6000 Hz, the stepsizewas 2 Hz. The procedurecontinued until 14 reversalswere obtained.The frequencydifference limen for each run was calculated
as the mean of the
final ten reversals.Subjectswere run until performancewas asymptoticand stable in each condition (minimum of six runs per condition). The final value for eachdata point for an individual subjectwas taken as the mean of the final six runs.A similar 2IFC procedurewas employedfor the measurementof detectionthresholdsfor the 25-mstonal signals underconditionsof forwardmaskingand in quiet.Under the conditionsof 100-msmasker-signalinterval and 90-dB narrow-band maskers, no threshold shift was observed (mean threshold shifts of 0.4 and 0.5 dB for 1000 and 6000 Hz,
respectively).The same result was obtainedfor the wideband maskerspresentedat 100 dB SPL (thresholdshiftsof - 1.8 and 1.6 dB for 1000and 6000 Hz, respectively).Fi;ve normal-hearingsubjectsparticipatedin the narrow-bandmaskedconditions,a singlenormal-hearingsubjectcompletedthe wideband-masked conditions.
dB SPL at 1000 Hz and from 20 to 80 dB SPL at 6000 Hz. The forward maskers were either a narrow band of noise
centeredat the test frequency(900-1100 Hz or 5600-6400 Hz) or in somecases,a widebandnoise(low passat 12 000 Hz). The noisemaskerswere 102msin duration,including 2-ms raised-cosine ramps.The maskerswere generatedby an analograndomnoisegenerator(MDF model8156); narrow-bandnoiseswerebandpass filteredwith rejectionslopes of 90 dB/oct (Kemo modelVBF 8). All signalswere presentedvia BeyerDT-48 A.0 headphones andsignallevelsare expressed asthe levelsdevelopedin an NBS-9A coupler. The narrow-bandmaskerswerepresentedat 90 dB SPL, while the wideband maskers were 100 dB SPL. In the for-
ward-maskedconditions,the maskerprecededthe test signal by 100 ms (masker offsetto signalonset). Under these conditions, there was no threshold shift for the detection of a
25-ms test tone at the signalfrequency.It shouldbe noted that the maskerand signalcharacteristics in the 1000-Hz narrow-band
masker condition were identical to those em-
B. Results
Figure 1 displaysthe frequencydifferencelimens at 1000 Hz in the narrow-band
masked condition as a function
of signallevel.Also plotted are the differencelimensfor the unmaskedcontrolcondition.The individualdatapointsrepresentmeanvaluesof the fivenormal-hearinglisteners.The verticalbarsindicateplusand minusonestandarddeviation acrossthe fivesubjectsin eachconditionat eachsignallevel. There is essentiallyno differencebetweenthe frequencydifference limens obtained
under the masked and unmasked
conditionsin the groupmean data;this sameresultwas observedin eachof the individualsubjects'data. In particular, no midlevelelevationin the frequencydifferencelimen was noted under forward masking, in contrast with the effect previously reported for intensity discrimination experiments.
Figure 2 displaysthe groupmean frequencydifference limens at 6000 Hz obtained from the same five normal-hear-
ployedby Zeng et al. ( 1991) with the exceptionthat in the Zeng et al. studyan incrementin intensitywasaddedto the signalin onelisteningintervalwhereasin the presentstudy, an incrementin signalfrequencywasaddedto oneinterval.
conditionare shown.The verticalbarsindicateplusand mi-
An unmasked, control condition was also run at each test
nus one standard deviation
frequencyand signallevel.
point is shownfor the forward-maskedfrequencydifference
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J. Acoust.Soc. Am., Vol. 92, No. 6, December 1992
ing listenersin the narrow-bandmaskercondition.Both the unmasked control condition as well as the forward-masked across the five listeners. No data
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500
ß
i
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i
6000
ß
4O
i
Hz
1000
400
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Unmasked
---•--
Hz
3O
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Masked
Unmasked Wideband Masked
---•--
3OO 2O 200
10 lOO
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•
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Level
•
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•
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80
0
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40
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80
100
Level (dB SPL)
(dB SPL)
FIG. 2. Plot of the 6000-Hz frequencydifferencelimensas a functionof signallevelfor boththeunmasked andnarrow-band forward-masked conditions.Data pointsrepresentthe meanof five normal-hearinglisteners.
FIG. 3. Plot of the 1000-Hzfrequencydifferencelimensasfunctionof sig-
Vertical bars indicate the standard deviation across the five listeners.
indicate the standard deviation across the final six runs in each condition for
nal level for both the unmasked and wideband forward-masked conditions.
Data pointsaretheresultsfromonenormal-hearing listener.Verticalbars the listener.
limensat 20 dB SPL becauseonly three of the five subjects wereableto reliablyjudgethe pitchof the signalseparately from the narrow-bandmaskerwhenthe signalwaspresented
Again, no midlevelelevationis observed.In addition,the forward-maskedfrequencydifferencelimensat 6000 Hz are no longerelevatedwith respectto the controlcondition,as
at such a low sensationlevel. However, the mean value for
was observed in the narrow-band masker case at 6000 Hz,
the remainingthreesubjects undertheseconditionswas230 Hz, whichis similarto the other valueson thisfunction.The differencelimensasa functionof signallevelunderthe forward-maskedconditionwereparallelto, but slightlyhigher than, thoseof the control condition.As was the casefor the 1000-Hztones,no specificelevationof the differencelimen at midlevels under the forward-masked
condition
indicatingthat the pitch confusionwas successfully removed.
II. EXPERIMENT
2: BACKWARD-MASKED
FREQUENCY
DISCRIMINATION
A. Methods and procedures
is ob-
We obtainedfrequencydifferencelimensfor short1000served.Severallistenersreportedthat the forward masked Hz tones under conditions of no masker and under condifrequencydiscrimination taskat 6000Hz wasmoredifficult at all signallevelsthanat 1000Hz, dueto thetonelikequality of thehigh-frequency narrow-band masker,whichconfused the listenersas to which portionof the masker-signalcomplex to judgein termsof pitch. 5OO A wideband forward masker was then used in the re-
mainingforward-masking-experiments. One rationalewas to eliminatetheconfusing pitchcuesthat werepresentin the
6000 O• ---•--'
narrow-band maskers.In addition, becausethe resultsof the
narrow-bandmaskerexperimentswerenegative,in that no midlevelelevationof frequencydiscriminationwasnotedin the forward-maskedcondition,the possibilityexistedthat selectivelyadaptingthe rate responseof low-SR fibersin a narrowfrequencyregionsurroundingthe testtone wasinsufficientto disruptaccuratefrequencycoding.If the code for frequencyinvolveslookingat the entireneuralresponse profile,ratherthanjust the "peak"or the "edge"of the pattern, as suggested by the work of Emmerichet al. (1983), then adaptingonly a smallgroupof low-SRfiberswith the narrow-bandmaskersmay not havebeeneffective. Figure3 showsthe resultsof the widebandmaskerconditionfor thesinglesubjectat 1000Hz. Verticalbarsindicate the standarddeviationacrossruns for this subject.No midlevel elevationin the differencelimen is observed.Figure 4 shows the results from the wideband masker at 6000 Hz.
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J. Acoust.Soc.Am.,Vol.92, No.6, December1992
Hz
400
Unmasked Wideband Masked
3OO
1.1=
200
lOO
0
• 0
I 20
,
I 40
Level
,
I
•
60
I 80
, 100
(dB SPL)
FIG. 4. Plot of the 6000-Hz frequencydifferencelimensasfunctionof signal level for both the unmasked and wideband forward-masked conditions.
Data pointsare the resultsfrom onenormal-hearing listener.Verticalbars indicate the standard deviation across the final six runs in each condition for the listener.
Turnereta/.' Frequency discrimination inmasking
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80
.
[
.
(a)
[
.
,
.
,
.
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(c)
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.
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4O
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20
0
0
20
40
60
80
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100
Level (dB SPL)
20
40
60
80
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Level (dB SPL)
FIG. 5.Frequency difference limens at 1000Hz forboththeunmasked andnarrow-band backward-masked conditions. Individual subjects' dataareshown in (a)-(c). In (d), datapointsrepresentthe meansof the threenormal-hearing listeners.
tionsof backwardmaskingby a narrow-bandnoisecentered at 1000 Hz. The methods and stimuli for this experiment were identical
to that described for the 1000-Hz
encelimenswasconsistentand occurredfor all signallevels asshownin the groupmeandata [ (d) ].
narrow-
C. Discussion band-maskedfrequency discrimination experiment describedpreviously,with the exceptionthat the maskerwas The mostimportantfindingsof thisstudyarethe differpresented100 ms after eachmemberof the discrimination encesbetweenthe effectsof forward and backwardmasking pairin the2IFC task.Threenormal-hearing subjects partici- upon frequencyand intensitydiscrimination.Both a forpatedin theexperiment, two of thesesubjects werealsoparward- and backward-maskingintensenarrow-bandstimuticipantsin experiment1, the third wasan experienced lislus, separatedfrom the signalby 100 ms, havepreviously tenernewto frequencydiscriminationexperiments. In these beenshownto affectthe midlevel intensitycodingof a tone. subjects,the backwardmaskerproducedno thresholdshift In contrast,the presentexperimentsdemonstratethat for for a test tone at 1000 Hz (mean threshold shift of 0.7 dB). frequencydiscrimination,neithera wide nor narrow-band forwardmaskeraffectsthe frequencycodingof a tone (when B. Results pitch confusions are eliminated).However,the backward The four panelsof Fig. 5 displaythe resultsof theback- maskerdoesproducean elevationin the frequencydifference limen. ward-maskedfrequencydiscriminationexperiments.AlThe reasonfor the forward-maskingeffectin intensity thoughthe resultsof the three listenersshowedthe same trend, the magnitudeof the effectof the backwardmasker discriminationis not firmly established.The possibilityexdiffered among them. Therefore, (a)-(c) are provided iststhat the relevantsiteof action is not peripheralas suggestedby Zenget al. (1991), but rathera morecentralor showingthe resultsof the threeindividualsubjects, and (d) cognitive process, andthereforeunrelatedto thedifferential showsthe groupmeanresults.For subjectCT [(a)], the backwardmaskerproduceda large elevationin frequency adaptationof low- and high-SRfibers.In particular,this differencelimens, while for the other two listeners [ (b) and possibilitywassuggested by the findingof Plackand Viemeister(1992b) that a backwardmaskerproduceda similar (c) ], the elevation in differencelimens was smaller. Howmidlevel elevation of the differencelimen. Our finding that ever,for all threesubjects,the elevationin frequencydiffer-
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Turnereta/.' Frequency discrimination inmasking
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onlya backwardmaskerandnot a forwardmaskerproduces an elevationin frequencydiscriminationsuggeststhat the effectsof forward and backwardmaskingupon discrimination tasksare unrelated.However,an elevationin frequency differencelimensdue to backwardmaskingwasobservedin this studyand wasapproximatelyequalfor all signallevels. Plack and Viemeister (1992b) showed that the backward
maskerin their intensitydiscriminationtask producednot only a constantelevationof the differencelimen acrossall signal levels,but also an additional large elevation of the differencelimen seenonly for midlevelsignals.Thus the effect of a backwardmaskerupon discriminationtasksmay consistof two components; firsta generalelevationof differencelimensfor both frequencyand intensitytasks,and second, a midlevel elevation of the difference limen observed
only for intensity discrimination. The specificoriginsof theseeffectsremainspeculativeat the presenttime, althougha cognitiveeffectof either forward or backwardmaskersin discriminationexperimentsis certainly not ruled out. Recent data from our laboratory (Turner et aT., 1992) show that the effectof forward or backward maskersupon intensitydiscriminationdependsupon the methodusedfor obtainingthe differencelimens. It is alsonot entirely straightforwardhow to interpret the lack of effectfrom a forward maskeron frequencydiscrimination,evenif one acceptsthe Zeng et aT. ( 1991) hy-
pothesisthat the roles of differentiallyadaptedhigh- and low-SR fibersare involved in forward-maskedintensity discrimination.On onehand,it couldbe arguedthat the adaptation resulting from the 90-dB SPL forward masker employed here was not large enough to decrease the contributionof the low-SR fibersto the frequencydiscrimination task, but was sufficientto disrupt intensitycoding. However,sinceboth tasksare differencelimen experiments, in which the smallestperceptibledifferencebetweentwo stimuli is determined,any disruptionin codingof the rel-
criminationalso suggeststhat the two discriminationtasks differ in mechanism.
It is known that the adaptingstimuli usedin theseexperimentsselectivelydisruptsthe rate responseof low-SR fibersin chinchillas,elevatingneuralthresholds(Relkin and Doucet, 1991). The presentresultsthereforesuggesta role for phase-locking in the encodingof frequencyat 1000Hz, in viewof the reportsby Schofnerand Sachs(1986) and Kim et al. ( 1991) that high-SRauditory-nervefiberrate profilesfor low frequenciesare degradedin sharpnessat higher signal levels.Selectivelyadaptingthe low-SR fibersin this experiment producedno elevationof the differencelimen. Goldstein (1980) modeledfrequencydiscriminationusingrate responses from low- and high-SRfibersand concludedthat if rateresponses wereimportantfor behavioralperformance, low-SR fibersare necessary for performanceat high levels. At 6000 Hz, a role for phase-locking is not asstronglysupported. First, as mentionedpreviously,the accuracywith whichfiberscanphaselock to a 6000-Hz toneis rather poor to beginwith. Second,Kim et al.'s data suggestthat low-SR fibersmay not be necessaryin codinghigh frequenciesvia a rate-placecode.The "excitationpattern"for high-frequency stimuli transmittedby a populationof high-SR fibers remainssharpevenat high signallevels. The presentresultsthen offersupportto a dual-coding modelof frequencydiscrimination,in whichphaselockingis usedat lower frequenciesand rate-placecodesat higherfrequencies.A similar mechanismhasbeenproposedby others (e.g., Moore, 1973, 1989). However,the role of phaselocking at even6000 Hz cannotbe completely.ruledout, as the largefrequencydifferencelimensobtainedat 6000 Hz in this experimentand othersmay still be compatiblewith the degradedphaselockingat that frequency(Goldsteinand Srulovicz, 1977). ACKNOWLEDGMENTS
evant portion of the excitationpattern shouldresult in a decrementin performance.Such a decrement in perfor-
Nos. DC 00380 and DC 00377. The authors thank Anita
mancewas not observedin this study, whereassucha decrement has beenobservedfor intensitydiscrimination.
Young, SusanPlante and Lauren Forget for assistancein data collection.Dr. Jozef Zwislocki also providedhelpful
The absenceof an effectof the forward masker upon frequencydiscrimination under the same stimulus conditionsasthoseemployedin previousforward-maskedintensity discriminationexperimentsindicatesthat the two tasks are performedby the auditory systemin fundamentally different manners.The resultsof the presentfrequencydiscriminationstudy,alongwith previousintensitydiscrimination studies(Zeng et aT.,1991;ZengandTurner, 1992;Plack and Viemeister,1992), would seemto contradictthe general hypothesisoriginallyput forth by Zwicker (1956) that both intensityand frequencyare similarly encodedby the auditory nerverepresentationof the "excitationpattern" along the cochlea.Additional evidenceagainstsuch a common mechanismhasbeenpresentedby Moore (1992). The finding that a backward masker producesboth a "midlevel hump" in additionto a generalelevationin differencelimens for intensitydiscrimination (Plack and Viemeister, 1992b), whereasthe presentstudy'sfinding that only the parallel elevationis observedfor backward-masked frequencydis3107
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This researchwassupportedin part by NIDCD Grants
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