Human Frequency-Following Responses to Binaural … · masking level differences in a brain stem...

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J Am Acad Audiol 16:184–195 (2005)

*Department of Audiology and Speech Sciences, Purdue University

Ananthanarayan Krishnan, Ph.D., Auditory Electrophysiology Laboratory, Department of Audiology and Speech Sciences, 500Oval Drive, Purdue University, West Lafayette, IN 47907; Phone: 765-494-3793; Fax: 765-494-0771; E-mail: [email protected]

Human Frequency-Following Responses toBinaural Masking Level Difference Stimuli

James R. Wilson*Ananthanarayan Krishnan*

Abstract

Binaural masking level difference is the behavioral threshold difference betweena diotic condition (SoNo) and a dichotic condition with a 180° interaural phasedelay of either the signal (SπNo) or the masker (SoNπ ). Threshold disparityis partially related to coincidence-detecting units in the medial superior olivethat are sensitive to low-frequency binaural stimuli with interaural phasedifferences. Previous surface evoked potential studies report significant latencyand amplitude differences to SπNo stimuli with respect to SoNo stimuli in theP1-N1 auditory event related potential, but no study has reported physiologicmasking level differences in a brain stem evoked potential. The humanfrequency-following response (FFR) represents activity from low-frequency,phase locking neural units in the upper brainstem. Unmasked FFRs to 500 Hztone bursts and masked FFRs using a 1.5 kHz low-pass masker were recordedfrom nine normal-hearing adult subjects. Significant reduction in FFR amplitudeoccurred in the SoNo condition, re the So condition, with masker intensitiesnear the psychoacoustic SoNo masking level. Significant FFR amplituderecovery was observed for both the SoNπ and SπNo conditions.These resultssupport the role of phase-locked neural activity in brainstem mechanismsinvolved in perceptual masking release.

Key Words: Auditory evoked potentials, frequency-following response, phaselocking, masking level difference

Abbreviations: ABR = auditory brainstem response; BMLD = binaural maskinglevel difference; dB V = dB volts; FFR = frequency-following response; IPD =interaural phase difference; ITD = interaural time difference; MSO = medialsuperior olive; SπNo = dichotic signal, diotic masker; SoNπ = diotic signal,dichotic masker; SoNo = diotic signal/masker

Sumario

La diferencia binaural del nivel de enmascaramiento (masking level difference)es la diferencia de umbral conductual entre una condición diótica (SoNo) y unacondición dicótica, con un retardo binaural de fase de 180°, para la señal (SπNo)o para el enmascarador (SoNπ). La disparidad en los umbrales está parcialmenterelacionada con las unidades de detección de coincidencia en la oliva medialsuperior, que son sensibles a los estímulos binaurales de baja frecuencia condiferencias interaurales de fase. Estudios previos de potenciales evocados desuperficie reportan diferencias significativas de latencia y amplitud para losestímulos SπNo con respecto a los estímulos SoNo en las ondas P1 – N1 delos potenciales auditivos relacionados con el evento, pero ningún estudio hareportado diferencias fisiológicas de nivel de enmascaramiento en un potencialevocado del tallo cerebral. La respuesta humana de seguimiento de frecuencia(FFR) representa la actividad de las unidades neurales en fase y de bajafrecuencia del tallo cerebral superior. Se registraron las FFR con bursts tonalesno enmascarados de 500 Hz y las FFR enmascarados util izando

Human FFRs to BMLD Stimuli/Wilson and Krishnan

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Binaural masking level difference(BMLD) is the improvement inthreshold for a signal in a masked

dichotic condition (antiphasic condition withan interaural phase difference [IPD] betweenthe signal and masker) relative to the signalthreshold obtained in a diotic maskedcondition (homophasic condition without aninteraural disparity). Psychoacoustic studiesreveal that maximum BMLD occurs whenusing low-frequency pure tones with an IPDof 180° (Hirsch, 1948; Jeffress et al, 1952;Robinson and Jeffress, 1963; Egan, 1965).At 500 Hz, the BMLD with an inverted signalstarting phase (SπNo) is 12–15 dB and isgreater than the 9–12 dB BMLD with aninverted masked (SoNπ) (Moore, 1998). Thereduction of the masking effect was shown byLicklider (1948) to lead to improvements inword recognition in low signal-to-noise ratiolistening conditions. The BMLD has beenclinically used, and test materials remainavailable today (Wilson et al, 2003). ReducedBMLDs have been shown in patients withrecruiting sensorineural loss (Hall et al, 1984;Jerger et al, 1984), Meniere’s disease(Quaranta and Cervellera, 1974), andbrainstem lesions (Olsen and Noffsinger,1976).

The perceptual release from maskingreflected in BMLD presumably reflectsbrainstem binaural processing in the medialsuperior olivary (MSO) neurons (Jenkins andMasterton, 1982). Many units in this nucleushave low characteristic frequencies andreceive bilateral excitatory inputs. Singleunit responses show many ITD (interauraltime difference) sensitive units in which the

maximum discharge rate occurs with aspecific interaural delay, the characteristicdelay (Goldberg and Brown, 1968; Crow et al,1978; Fitzpatrick et al, 1997; Brand et al,2002). Responses from the MSO support thedelay line coincidence-detection model(Jeffress, 1948). This model describes amechanism of brainstem ITD processing bymodeling MSO axonal inputs as delay lines.Goldberg and Brown (1969) showed that thecontralateral ear requires a longer latency toan MSO unit than the ipsilateral ear.Maximum unit output was thereforepredicted for signals with an ITD equal to thecharacteristic delay of the unit allowing foreach monaural input to produce simultaneous(or coincident) unit activation.

Kevanishvili and Lagidze (1987)compared latency and amplitude to the SoNoand SπNo conditions using the auditorybrainstem response (ABR), middle latencyresponse, and the P1, N1, P2 event relatedpotential components using 580 Hz tonebursts with a white noise masker in ninenormal-hearing adult subjects. Significantresults were found for latency and amplitudein the event related potential only. Thisfinding was consistent with Tanis and Teas(1974), who used 250 Hz tone bursts. TheABR represents a synchronized populationresponse to the onset of the stimulus that isprimarily initiated by the 2–4 kHz region ofthe cochlea (Picton et al, 1974; Hecox et al,1976). The psychoacoustic unmasking in the2–4 kHz region is approximately 3 dB. Thisfrequency range may not provide enoughunderlying physiologic unmasking to recorddistinct responses in the scalp-recorded ABR.

enmascaramiento con pasa-bajo de 1.5 kHz en nueve sujetos adultos conaudición normal. Una disminución significativa en la amplitud de las FFRocurrió en la condición SoNo , en relación a la condición So, con intensidadesde enmascaramiento cercanas al nivel de enmascaramiento psicoacústico SoNo. Se observó una recuperación significativa en la amplitud de las FFR tantopara la condición SoNπ como para la SπNo. Estos resultados apoyan elpapel de la actividad neural en fase en el mecanismo del tallo cerebralinvolucrado en la liberación perceptual del enmascaramiento.

Palabras Clave: Potenciales evocados auditivos, respuesta de seguimientofrecuencial, phase-locking, nivel diferencial de enmascaramiento

Abreviaturas: ABR = respuestas auditivas de tallo cerebral; BMLD = diferenciabinaural del nivel de enmascaramiento; FFR = respuesta de seguimientofrecuencial; IPD = diferencia interaural de fase; ITD = diferencia interaural detiempo; MSO = oliva medial superior; SπNo = señal dicótica, enmascaradordiótico; SoNπ = señal diótica, enmascarador sicótico; SoNo =señal/enmascarador diótico

Journal of the American Academy of Audiology/Volume 16, Number 3, 2005

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In contrast, the frequency-following response(FFR) at moderate intensities is primarilygenerated by phase-locked neural activity ina population of neurons with lowcharacteristic frequency (Moushegian et al,1973; Ananthanarayan and Durrant, 1992).Phase locked neural activity has beenrecorded in brainstem pathways generatingthe ABR, FFR, and pathways involved inBMLD perception (Rose et al, 1967; Lavine,1971; Moushegian et al, 1975; Young andSachs, 1979; Sinex and Geisler, 1981; Ehretand Merzenich, 1988; Moller, 2000).

Since the greatest psychoacousticunmasking is found for low-frequency tones,it is possible that phase-locked activityencodes the necessary information involvedin brainstem responses to BMLD conditions.Because of this, the FFR might be a moreeffective scalp-recorded brainstem evokedpotential to examine the output ofcoincidence-detecting (MSO) units and relatedpathways. If unmasking is captured by theFFR, then the response amplitude to themasked antiphasic conditions should begreater than the masked homophasicamplitude. This finding could demonstratethat the phase-locked discharge patternrelays temporal information relevant to theperceptual release from masking.

METHOD

Subjects

A total of 15 adult volunteers ranging inage from 18 to 28 years were used. Eachsubject’s hearing sensitivity by air conductionwas screened bilaterally at 15 dB HL atoctave frequencies from 250 to 2000 Hz andat the interoctaves 750 and 1500 Hz. ThePurdue University Institution Review Boardapproved all methods.

Test Stimuli

All signals and maskers were generatedand controlled by BioSig® software (TuckerDavis Technologies) and delivered throughmagnetically shielded Biologic® insertearphones (Tip-300) encased in Mu metal.Two independent 500 Hz tone bursts with 10msec of cosine squared rise/fall time and 90msec of plateau duration were generated andpresented at a repetition rate of 5.1/sec. The

signal for each ear was then independentlyrouted to separate programmableattenuators. The outputs were fed to a signalmixer and headphone buffer. Homophasicstimuli were created with rarefaction polarity,whereas the lagging stimulus for the SπNoantiphasic condition was generated withcondensation polarity. A low monauralstimulus level of 56 dB SPL (re 20 µPa) waschosen to minimize response contaminationfrom acoustic cross talk and/or acousticreflexes and to allow masker levels to bepresented over a comfortable loudness range.The 1.5 kHz low-pass masker was generatedusing a programmable waveform generatorwhose wide-band noise output was routedthrough three cascaded low-pass filters(Wavetek, 852) with an overall rejection rateof 48 dB/octave. The filter output was thenrouted through an attenuator, split, andsubsequently mixed with the signals. Maskerintensity was manually controlled. The phaseof the left monaural masker was manuallyinverted for the SoNπ condition.

Recording System

Subjects relaxed in a comfortable reclinerduring data acquisition. FFRs were recordeddifferentially between electrodes placed atthe center of the forehead near the hairlineand the seventh cervical vertebrae (C7). Anelectrode on the midline forehead near theeyebrows (Fpz) served as the subject ground.Interelectrode impedances were maintainedat or below 1 kΩ. Analogue bandpass filtersettings were 100–3000 Hz (6 dB/octave roll-off, RC response characteristics) with theartifact rejection level set between 10 and 20µV. Recorded conditions represented twotrials of 1000 sweeps amplified by a factor of200,000 and sampled at 20 kHz. Off-lineprocessing of the response waveforms foreach condition included low-pass digitalfiltering cornered at 1.5 kHz. Amplitudespectra (Hanning window, 8192 pointresolution) for each condition were averagedinstead of response waveforms to avoiddistortion due to differences in waveformlatency.

Response Evaluation

A six-point average around the responsecomponent with three points within 50 Hzbelow and three points within 50 Hz above


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the response peak was used to estimate theresidual noise floor in each recording. TheFFR-BMLD was determined by subtractingthe response amplitude for the antiphasicconditions from that obtained for the SoNocondition. No significant differences in noisefloor amplitude were found for any of theFFR conditions.

Experimental Protocol

FFRs were obtained under the followingexperimental conditions. The maskerintensity necessary to perceptually mask theSoNo, SπNo, and SoNπ conditions wereobtained using subject participation in apseudorandomized task in which the maskerintensity was increased until the subjectactivated a response light to indicate maskingof the signal. FFRs to So and Sπ stimuli wererecorded prior to recording the masked FFRconditions. Only subjects showing unmaskedresponses of 10 dB above the noise floor orgreater were selected for the maskedconditions. FFRs for the SoNo, SπNo, andSoNπ conditions were recorded in apseudorandomized fashion. The perceptualmasker intensity for the SoNo condition wasused in the masked FFR recordings. Amasked FFR-SoNo response was defined as50% reduction in the response amplitude rethe So amplitude. This criterion was selectedbecause FFR unmasking was not evident inpilot experiments when the masker intensityproduced no amplitude reduction or greater

than 50% reduction. If the criterion reductionin FFR amplitude was not achieved with theperceptual masker, the masker intensity wasincreased 3–6 dB until the appropriateamplitude reduction was recorded. In thesecases, antiphasic responses were recordedwith the higher masker intensities in additionto the psychoacoustic SoNo masker level.

RESULTS

Psychoacoustic Data

Nine of the participating subjects metthe experimental protocol outlined above andwere selected for further analysis. Dependentsamples t-tests were used for all statisticalcomparisons unless stated otherwise. Figure1 shows the mean perceptual BMLD for thetwo antiphasic conditions. Both antiphasicconditions required masker intensities thatwere significantly greater than the SoNomasker level (SπNo, t = 11.33, p = 0.000;SoNπ, t = 17.46, p = 0.000). The SoNπ maskerintensity was significantly less than the SπNomasker level (t = 4.10, p = 0.004). The meanBMLD values were 9.53 dB for the SπNocondition and 6.74 dB for the SoNπ condition.This trend is consistent with psychoacousticstudies (Durlach and Colburn, 1978; Blauert,1983) in that the SπNo BMLD is greaterthan the SoNπ MLD. However, theunmasking reported here is less than typicalvalues at 500 Hz of 13 dB for SoNπ and 15

Figure 1. Mean perceptual BMLD for the two antiphasic conditions. Both antiphasic conditions requiredgreater masker intensity than the homophasic condition. Results of dependent samples t-tests comparinggroup masker intensity by condition show significant effects for each comparison.


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dB for SπNo (Moore, 1998). This can beexplained by methodological differences. Theaverage of four ascending trials was used tofind masked threshold instead of a multi-interval forced choice paradigm commonlyused in psychoacoustics. Also, the subjects had

no practice with the task, and data wascollected in one session.

Physiologic Data

Unmasked FFR Responses

Figure 2 displays the mean amplitudespectra of the 500 Hz FFR for the So and Sπconditions. Figure 3 displays the groupunmasked FFR amplitudes for the So and Sπconditions expressed relative to the noisefloor. The Sπ amplitude is significantly lessthan the So amplitude (t = 2.99; p = 0.027).This finding is consistent with results frombinaural interaction experiments for boththe ABR and FFR. As the interaural time orphase difference increases away from a dioticsignal, the binaural amplitude decreases(Gerull and Mrowinski, 1984; Furst et al,1985; Jones and Van der Poel, 1990;Sontheimer et al, 1985; Krishnan andMcDaniel, 1998; Ballachanda andMoushegian, 2000; Reidel and Kollmeier,2002).

Masked FFR Responses

Reduction in response amplitude ispresent for the SoNo condition in both theamplitude spectrum (Figure 4) and themasked FFR amplitudes expressed relativeto the noise floor (Figure 5). The SoNoamplitude is reduced by 52% re the So

Figure 2. Grand averaged FFR spectra for the So (A)and Sπ (B) unmasked conditions; dB V = dB volts.

Figure 3. Mean FFR amplitudes for the So and Sπ unmasked conditions. So amplitude is significantly greaterthan Sπ amplitude; dB V = dB volts.


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condition, which is significant (t = 12.00, p < 0.001). Criterion FFR-SoNo amplitudereduction occurred in five of nine subjectswith the psychoacoustic SoNo maskerintensity. Among the remaining four subjects,the psychoacoustic masker level caused FFR-SoNo reductions of 26% (S3), 28% (S5), 30%(S6), and 4% (S8). S3 and S6 required anadditional 6 dB and S5 and S8 an additional

9 dB of the masker intensity to reach theFFR-SoNo criterion.

Unmasking for the 500 Hz componentwas observed in both antiphasic conditions inthe amplitude spectrum (Figures 4 and 5).The effect occurred in six of nine subjects forthe SoNπ condition and four of nine subjectsfor the SπNo condition. Response recovery atthe 1000 Hz and 1500 Hz harmoniccomponents was present in a few subjects, butthis finding was not common. The SoNπamplitude recovered by 67.27% re the SoNoamplitude (t = 4.21, p = 0.008). The SπNoamplitude recovered by a smaller percentage,29.38%, but the effect remained significant(t = 2.64, p = 0.046).

A comparison of SoNπ and SπNoresponse recovery was found to be significant(t = 4.08, p = 0.027). This finding is oppositeto the psychoacoustic data because the SoNπunmasking was greater than SπNounmasking. However, the SπNo amplituderecovery might be affected by the unmaskedresults in which the Sπ amplitude was foundto be smaller than the So amplitude.Therefore, the FFR unmasking was expressedas a percent recovery relative to unmaskedlevels instead of percentage recovery re SoNoby dividing masked amplitude by theunmasked amplitude (SoNπ /So; SπNo/Sπ).When this adjustment was made, the percentrecovery for SoNπ is 66.99% and 60.92% forSπNo. This difference is nonsignificant (t =0.218, p = 0.836) and remains in contrast tothe psychoacoustic data.

Comparison of Psychoacoustic andPhysiologic Unmasking

Another aspect of interest to this studywas the nature of the relationship betweenperceptual and electrophysiologic BMLD.

Table 1: Comparison of the Psychoacoustic and Physiologic MLD

Antiphasic Condition: SoNπ Antiphasic Condition: SπNoSubject Psychoacoustic FFR Subject Psychoacoustic FFRS3 4.05 2.66 S3 4.37 1.60 S2 5.55 8.76 S8 6.20 0.06S8 5.78 S1 6.62 2.94S1 6.05 5.02 S2 10.22 4.90S5 6.90 1.80 S4 10.50 S9 7.87 S7 10.83 0.20S6 7.98 8.23 S5 11.85 S4 8.18 3.55 S9 12.45 5.78

Mean 6.74 5.00 Mean 9.53 2.58SE 0.49 0.97 SE 1.01 0.80

Note: Subjects are ranked by psychoacoustic MLD from lowest to highest.

Figure 4. Grand averaged FFR spectra for the SoNπ(A) and SπNo (B) conditions. In each panel the dashedline represents the FFR spectrum for the SoNo con-dition; dB V = dB volts.


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The amount of FFR unmasking wascalculated in a fashion similar to thepsychoacoustic unmasking, by subtractingthe amplitude difference of either antiphasiccondition with the SoNo condition. Table 1compares the psychoacoustic and physiologicBMLDs for each of the nine subjects. Themean FFR-BMLD for SoNπ is 5.00 dB, andthe mean FFR-BMLD for SπNo is 2.58 dB.For SoNπ , the greatest FFR-BMLD (8.76 dB)is observed for subject S2, who shows thesmallest perceptual BMLD. In contrast,subject S5 shows larger perceptual BMLDthan S2 but has a smaller FFR-BMLD. Also,S8 has the largest perceptual BMLD but didnot show physiologic unmasking. Given this

variability, no significant differences werefound between the psychoacoustic andphysiologic BMLDs for the SoNπ condition(t = 1.137, p = 0.307). Similar relationshipsexist for SπNo. Subjects S2 and S7 havenearly identical perceptual BMLDs with a 4.7dB difference in FFR results. Thepsychoacoustic and physiologic SπNo BMLDdifferences are significant (t = 5.055, p =0.015). This difference might be due to thesmaller unmasked Sπ amplitude.

During data collection, it was observedthat subjects with the strongest unmaskedFFR amplitude had the smallest amount ofphysiologic BMLD, even though theperceptual BMLD between subjects with high

Figure 5. Mean FFR amplitudes for the unmasked and masked conditions. Note the large amplitude reduc-tion for the SoNo condition and its recovery for both antiphasic conditions reflecting physiologic unmasking.The results of dependent samples t-tests are given; dB V = dB volts.

Figure 6. Mean FFR amplitudes for masked conditions from subjects with an So unmasked amplitude of 15dB or less (n = 3). Significant unmasking is present for both antiphasic conditions; dB V = dB volts.


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and low unmasked amplitudes were notsignificantly different for both the SoNπ (t= 0.93, p = 0.385) and SπNo (t = 1.18, p =0.277) conditions. To explore the effects ofSo amplitude on FFR-BMLD, subjects weredivided into two subgroups, one representingthose with FFR amplitudes of 15 dB or lessin unmasked conditions and the secondincluding subjects with FFR unmaskedamplitudes equal to or greater than 16 dB.Each group contains three of the six subjectsshowing FFR-BMLD.

The mean FFR amplitudes for thesubgroup with smaller (15 dB or less)unmasked amplitudes are plotted in Figure6. Significant FFR-BMLD is present in bothantiphasic conditions with magnitudes of7.34 dB for SoNπ (p = 0.025) and 4.54 dB forSπNo (p = 0.033).

Also, the BMLD magnitude for the SoNπcondition is significantly larger than thatobserved for the SπNo condition (p = 0.036).Figure 7 shows the mean FFR amplitudes forthe subgroup with larger (16 dB or greater)unmasked amplitude. While a small FFR-BMLD is present for both antiphasicconditions, statistical significance wasattained for only the SoNπ (2.67 dB, p =0.034) condition. Independent samples t-testrevealed significant differences in FFR-BMLDbetween the two subgroups for the SoNπcondition (t = 3.66, p = 0.022) but not for theSπNo condition (t = 1.75, p = 0.222). TheSπNo result could be due to the small samplesize of three subjects in each subgroup andthe smaller Sπ unmasked amplitude. The

finding of smaller physiologic release in thesubgroup having stronger unmaskedamplitudes perhaps reflects a limitation of the50% masking criterion. The amount ofmasker intensity necessary to mask a strongresponse increases as the FFR amplitudeincreases. For example, the criterion FFRamplitude reduction for subject S5 is 11.38dB, whereas the same proportion for subjectS2 is 6.66 dB due to a weaker response. Threeof the four subjects (S4, S5, S8) requiringmasker levels higher than the psychoacousticvalue have So FFR amplitudes greater than16 dB.

DISCUSSION

The results of this study relate primarilyto the nature of binaural interaction in theunmasked So and Sπ conditions and itsconsequences on the characteristics of thephysiologic FFR-BMLD. However, an attemptis made to compare the perceptual and FFR-BMLD data in an effort to relate our FFR-BMLD to perceptual BMLD.

Binaural Interaction in the UnmaskedSo and Sπ Response

Binaural interaction in the ABRresponses correlates well with the perceptualfusion of a lateralized stimulus. Specifically,largest interaction component is seen for amidline source with amplitude decreasingas the fused image is lateralized to one ear.In fact, the ABR interaction component is

Figure 7. Mean FFR amplitudes for masked conditions from subjects with an So unmasked amplitude of 16dB or greater (n = 3). Significant unmasking is present for only the SπNo condition; dB V = dB volts.

absent for interaural disparities that produceseparate images of the binaural stimuli at thetwo ears. The So-Sπ amplitude differencecould therefore reflect different amounts ofbinaural interaction. Previous FFR studiesshow binaural amplitude reductions withincreasing ITD within the localization range(0.0 to 0.67 msec) (Clark et al, 1997;Ballachanda and Moushegian, 2000). Inaddition, the reduced Sπ amplitude mayresult from a relatively smaller MSOsubpopulation stimulated by the dichoticsignal relative to a signal with no ITD. Thedelay line model maintains that binaurallyinnervated MSO units represent ITDsmediating the release from masking.However, MSO single-unit data suggests thatthe majority of ITD sensitive units have delaylines within the physiologic range (Kuwadaand Yin, 1983; Carr and Konishi, 1990;Fitzpatrick et al, 1997). Therefore, a minorityof the proportion of available delay linescould be tuned to ITDs outside the physiologicrange, which could lead to reduced responseamplitudes.

Frequency-Following ResponseCorrelates of Binaural Masking LevelDifference

The demonstration of FFR amplitudeenhancement under dichotic stimulusconditions that produce perceptual binauralmasking level difference suggests that thephase-locked neural activity in a populationof neural elements generating the FFR mayindeed preserve information about neuralactivity relevant to BMLD. Similar releasefrom masking reflecting BMLD has beenreported for binaural neurons at the level ofmedial superior olive (Langford, 1984) andinferior colliculus (Caird et al, 1991; McAlpineet al, 1996; Jiang et al, 1997; Palmer et al,1999; Palmer et al, 2000). The results of thesephysiologic studies are consistent with cross-correlation models of BMLD (Jeffress, 1948;Colburn, 1977). Specifically, these modelsassume the existence of brainstem neuralnetworks consisting of coincidence detectorsthat receive inputs from a series of delaylines, each sensitive to a specific interauraltime/phase difference (It should be notedhere that for tones, a phase shift is equivalentto a time shift). It is believed that theseneural networks perform a cross-correlation

on the neural activity in a single tunedchannel.

Specifically, for the SπNo condition,BMLD results from an asymmetry in themodifications to the responses to the No noiseas a result of adding the So or Sπ tones (Jianget al, 1997). That is, the No noise generatesa peak of activation in neurons with bestdelays close to zero ITD, and the addition ofSo tone at masked threshold produces onlya small increase in the amplitude of thispeak. In contrast, at the same S/N ratio, theSπ signal produces a larger decrease in thepeak amplitude. Since increases anddecreases in discharge are equally detectableto achieve the same detectability, the Sπsignal can be reduced in level, giving asubstantial BMLD. Also, the reduction inactivity caused by the Sπ signal is believedto disrupt or desynchronize the response tothe No noise (Palmer et al, 1999), therebymaking it a less effective masker.

For the SoNπ condition, Nπ noise is anineffective masker because it produces aminimum or trough in activation centered onzero ITD. The addition of So signal at zeroITD raises the discharge rate of neurons withbest delays near zero ITD. This represents adesynchronization effect of the noise in theSoNπ condition. The net effect is an increasein the number of coincident spikes deliveredto the coincidence detector. That is, the Sosignal produces a larger change to the Nπnoise response than to the No noise response.However, this change in neural activity levelfor the SoNπ condition is still smaller thanthat observed for the SπNo condition. Theresult is a smaller BMLD for NπSo than forNoSπ condition (Palmer et al 2000).

Comparison of Perceptual and FFR-BMLD

One goal of this study was to examine ifa direct correlation exists between theperceptual and physiologic unmasking. Thiscomparison is complicated by the observationthat larger unmasked FFR amplitudes tendedto yield smaller physiologic unmasking. Thismay be due to the fact that the largerunmasked FFRs required greater maskerintensity to achieve criterion reduction inamplitude. This is turn influences themagnitude of unmasking by affecting thedegree to which spatial tuning in thebrainstem can separate the processing of


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signal and masker. However, no significantdifferences in psychoacoustic BMLD existsbetween the experimental group and the sixsubjects with below-criteria FFR unmaskedamplitude by independent samples t-tests(SoNπ p = 0.103; SπNo p = 0.654). All of thesix subjects excluded had criterion levelunmasked FFR-So amplitudes with a signallevel increase of 5 to 10 dB, and the same wasfound in four of the six subjects with the Sπstimulus. Therefore, the absent FFR to the56 dB SPL signal is more likely due to FFRamplitude variability occurring at lowsensation levels than to differences in therole of phase-locked activity in brainstemBMLD mechanisms.

In this study, the FFR-BMLD correlatesto the psychoacoustic MLD more accuratelyfor the SoNπ condition than the SπNocondition that is partially explained throughamplitude reductions resulting fromdecreased perceptual fusion. One way tofurther examine the SπNo BMLD would beto reduce the ITD of the stimulus. Thepsychoacoustic BMLD increases along anIPD continuum peaking at 180° (Hirsch,1948; Jeffress et al, 1952; Robinson andJeffress, 1963). The opposite might happento the FFR-BMLD in that the magnitude ofmasking release might decrease withincreasing IPD due to decreasing binauralinteraction. Therefore, at an IPD less than180°, the increased physiologic maskingrelease associated with increased unmaskedFFR amplitude could correlate moreaccurately with the decreased psychoacousticBMLD.

Considerations for Further Research

The SπNπ condition was not utilized inthis study. This condition could be useful insetting the masker level before recordingresponses in antiphasic conditions. Since theSo amplitude is larger than Sπ, the maskerintensity that reduces the SoNo condition by50% (re So) may reduce the SπNπ amplitudeby greater than 50% (re Sπ). This overmaskingcould be a factor contributing to the lesserunmasking seen for the SπNo. Perhapsgreater SπNo unmasking would occur ifdifferent masker intensities were used forSo and Sπ signals while holding the 50%criterion constant.

The magnitude of FFR BMLD is affectedby parametric factors that must be considered

during the experimental design. Among theseinclude the stimulus frequency and intensity,masker bandwidth, and response noise floor.The perceptual BMLD is maximal with astimulus frequency around 250 Hz anddecreases to 3 dB above 1500 Hz (Blauert,1983). In pilot experiments, 250 Hz stimuliwere used with a small group of subjectsunder the assumption that the greater thepsychoacoustic BMLD, the greater the FFR-MLD. Higher stimulus levels were necessarydue to the approximately 10 dB increase inFFR noise floor in this frequency region. Thisresulted in uncomfortably high masker levelsfor some subjects. On the other hand, thenoise floor at 1000 Hz is about 10 dB less thanthe noise floor at 500 Hz. This region providesan opportunity to use lower signal andmasker intensities. However, smaller FFR-BMLD values than those presented in theresults herein could occur since the perceptualunmasking at 1000 Hz is reduced comparedto lower signal frequencies. Finally, as statedabove, pilot data indicates that unmaskedresponses needed to be at least 10 dB abovethe noise floor to show unmasking. No upperlimit was set on unmasked amplitude. Addingthis to the protocol could be useful in futurestudies since the subjects who had responseamplitudes from 10 to 15 dB seemed to showgreater unmasking than those with largerunmasked amplitudes.

CONCLUSIONS

Past studies have shown that the ABR issensitive to ITD stimuli, but significant

unmasking with BMLD stimuli has not beenpreviously reported in a brainstem-evokedpotential. The ABR is evoked by neuralactivity synchronized to the stimulus onset.This discharge pattern may not be theprimary type of neural activity involved inbinaural processing related to the perceptualrelease from masking in the caudal brainstempathways. However, at a more rostrallocation, neural activity related to the BMLDis recoded to onset activity. This is supportedby studies showing unmasking in the surfacerecorded event related potential. To theauthor’s knowledge, no previous study hasused the frequency-following response withBMLD stimuli. This approach allows for ananalysis of the role of temporally preciseneural phase locking in brainstem BMLDmechanisms that involve binaurally


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innervated, ITD sensitive units in the MSO.The results from this study demonstrate thatunmasking is present in the scalp recordedFFR from adult subjects showing a normalpsychoacoustic BMLD. The physiologic MLDwas greater for SoNπ condition, which is incontrast to psychoacoustic MLD studies. Thisresult is related, in part, to differences inbinaural interaction between the diotic anddichotic signals.

Acknowledgment. This research was completed inpartial fulfillment of the Master of Science degree inaudiology from Purdue University. Portions of thisstudy were presented at the 2003 26th Mid-WinterMeeting of the Association for Research inOtolaryngology in Daytona Beach, FL. We thank Dr.Elizabeth Strickland and Dr. Chris Weber-Fox forinsight and comments throughout the project.

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