On- and Off-Frequency Forward Masking by Schroeder-Phase Complexes

Forward masking by harmonic tone complexes was measured for on- and off-frequency maskers as a function of masker phase curvature for two masker durations (30 and 200 ms). For the lowest signal frequency (1 kHz), the results matched predictions based on the expected interactions between the phase curvature and amplitude compression of peripheral auditory filtering. For the higher signal frequencies (2 and 6 kHz), the data increasingly departed from predictions in two respects. First, the effects of the masker phase curvature became stronger with increasing masker duration, inconsistent with the expected effects of the fast-acting compression and time-invariant phase response of basilar membrane filtering. Second, significant effects of masker phase curvature were observed for the off-frequency masker using a 6-kHz signal, inconsistent with predictions based on linear processing of stimuli well below the signal frequency. New predictions were generated assuming an additional effect with a longer time constant, consistent with the influence of medial olivocochlear efferent activation on otoacoustic emissions in humans. Reasonable agreement between the predicted and the measured effects suggests that efferent activation is a potential candidate mechanism to explain certain spectro-temporal masking effects in human hearing.     

Binaural masking level difference effects in single units of the guinea pig inferior colliculus.

We have studied the masking effects of a binaurally presented noise on the responses to binaural signals recorded from low-frequency cells in the inferior colliculus of the guinea pig. The spike rates to the masker and signal + masker were compared to quantify masking at different interaural time delays of the noise. The signal was a 50-ms tone burst at best frequency or a 50-ms segment of a synthetic vowel presented at the best interaural delay of the unit tested. At each noise masker delay, the noise level was adjusted to obtain a criterion spike difference. In most cases, the level required was lowest at the best delay for the noise. The mean difference between maximum and minimum masked thresholds across the cell population was very similar to the human psychophysical masking level difference under the same signal and masker conditions. In another series of tests, we measured the effect of the noise masker on the temporal pattern of the discharge to the signal. The signal used was a 500-ms segment of the synthetic vowel. In virtually all cases the addition of a continuous noise masker reduced the discharge rate synchronized to the fundamental frequency of the vowel. The degree of this reduction was dependent on the interaural time delay of the noise masker. For most units, maximum reduction was seen when the vowel and noise had the same interaural time delay. The similarity between the masking which we have shown physiologically and the reported in a variety of human psychophysical experiments suggests that the processing at levels up to and including the inferior colliculus contributes to the psychophysical BMLD.     

Recovery from prior stimulation. II: Effects upon intensity discrimination.

We obtained just-noticeable differences (jnds) for the intensity of pure tones following a forward masker. The masker was a 100-ms burst of narrow-band noise centered at 1000 Hz presented at 90 dB SPL; the pure-tone signal was at 1000 Hz and was 25 ms in duration. The masker-signal delay was 100 ms. Under these conditions, there is no threshold shift for the detection of the pure-tone signal following the forward masker. In contrast with the absence of a forward-masker effect upon detection thresholds, unusually large midlevel (40-60 dB SPL) jnds were observed. These large midlevel jnds were measured as a function of signal delay, revealing that they are not completely recovered to the normal (unmasked) values by 400 ms. We interpret these data as a consequence of the slower recovery of low-spontaneous rate, high-threshold neurons following prior stimulation (Relkin and Doucet, 1990). These experiments may therefore provide psychophysical evidence that the low-spontaneous rate, high-threshold neurons are a necessary physiological component in the coding of the large dynamic range for intensity. In addition, the present data provide evidence that the assumption that the effect of forward masking is limited to 100-200 ms is inappropriate, as this recovery time does not necessarily apply to suprathreshold tasks.     

Steady-state evoked responses to sinusoidally amplitude-modulated sounds recorded in man

Steady-state potentials evoked in response to binaural, sinusoidally amplitude-modulated (AM) pure tones and broadband noise signals were recorded differentially from position F4 and the ipsilateral mastoid on the human scalp. The responses elicited by the AM stimuli were approximately periodic waveforms whose energy was predominantly at the modulation frequency of the stimulus. The magnitude of responses was between 0.1 and 4 microV for modulation frequencies between 2 and 400 Hz imposed on a 1-kHz carrier signal. The magnitude of the responses increased linearly with log modulation depth for low (4 Hz) and high (80 Hz) modulation rates. The response magnitude also increased linearly with the mean intensity of the sound for intensities up to 60 dB above the subject's pure tone threshold; at higher levels the response saturated. The relationship between response magnitude and modulation frequency (the modulation transfer function) was a lowpass function for both pure tone and broadband noise carrier signals. The modulation transfer functions were similar to those obtained from human psychophysical measurements where spectral cues are either unavailable or not used by the subject. The responses also contained a significant component at the second harmonic of the modulation frequency. The magnitude of this component was greatest at modulation rates between 5 and 20 Hz. The responses elicited by ipsilateral and contralateral monaural stimulation were approximately equal in magnitude, and binaural stimulation produced a potential 30% greater than the individual monaural responses. It is suggested that the evoked response represents the entrained neural activity to temporal amplitude fluctuations, and reflects the psychophysically measured performance of the auditory system for the detection and analysis of amplitude modulation.     

Effect of psychophysical backward masking on human brain-stem responses (ABR)

A backward-masking (BWM) paradigm was used to obtain measurements on 4 normal young adults of psychophysical BWM and of the analogous electrophysiological masking from human auditory brain-stem responses. The same stimuli and Ss were used in both experiments. Psychophysical BWM was determined to a 100-musec click masked by a 100-msec white noise after time delays of 1, 5, 10, 25, and 100 msec and at noise masker levels of 50, 65, and 80 db SPL. As previously demonstrated, more psychophysical BWM occurs at short delta ts (1 to 10 msec) than at longer delays and masked thresholds are greater as masker level increases. In the electrophysiological experiment, using delta ts = 1, 5, and 10 msec, the extent to which wave V latency was affected by the following masker was determined for various experimental conditions. Wave V latency increased with a decrease in delta t, for maskers above probe level, except that latency was not significantly affected by any masker level at delta t = 10 msec.     

Amplitude Modulation and Loudness in Cochlear Implantees

The effect of amplitude modulation of pulse trains on the loudness perceived by cochlear implantees was investigated for different overall levels of the signal, modulation depth and the carrier rate of the pulse train. Equally loud and threshold levels were determined for a variety of signal levels, modulation depths and carrier rates in six cochlear implantees. The pattern of results was consistent with the predictions of a previously published loudness model of McKay et al. (J Acoust Soc Am 113:2054–2063, 2003). The degree to which the loudness of modulated stimuli differed from the loudness elicited by an unmodulated pulse train with equivalent average current depended on the modulation depth and the absolute current level of the unmodulated stimulus. The effect of carrier rate on this measure was predictable solely from the effect of rate on absolute current level for equal loudness. The results have important implications for the interpretation of experiments measuring modulation detection that do not control loudness cues. We show that several previously published results regarding the effect of carrier rate and added noise on modulation detection could be reinterpreted in the light of these findings.     

Human auditory steady-state responses to tones independently modulated in both frequency and amplitude

OBJECTIVE: Independent amplitude and frequency modulation (IAFM) of a carrier tone uses two different modulating frequencies, one for amplitude modulation (AM) and one for frequency modulation (FM). This study measured the human steady-state responses to multiple IAFM tones. The first question was whether the IAFM responses could be recorded without attenuation of the AM and FM components. The second question was whether IAFM stimuli would provide a more effective demonstration of responses at intensities near threshold than the responses to AM tones. The third question was whether the responses to multiple IAFM stimuli would relate to the discrimination of words at different intensities. DESIGN: Multiple AM, FM, or IAFM stimuli were presented simultaneously. Responses were recorded between the vertex and the neck and analysed in the frequency domain. The first experiment compared IAFM responses with AM and FM responses. The second experiment compared IAFM responses with AM responses between intensities 20 to 50 dB SPL. The third experiment related the IAFM responses to the discrimination of monosyllabic words at intensities between 20 and 70 dB SPL. RESULTS: Steady-state responses to the individual component of the IAFM stimuli were clearly recognizable although attenuated a little (14%) from the responses to AM or FM alone. Using IAFM stimuli was not different than simply using AM stimuli when trying to recognize responses at low intensities. The number of responses detected during multiple IAFM stimulation and the amplitudes of these responses correlated significantly with word discrimination. CONCLUSIONS: IAFM of a carrier using two different modulating frequencies (one for AM and one for FM) elicits separate AM and FM responses that are relatively independent of each other. These separate responses can be used to detect whether a particular carrier has been processed in the cochlea, but they are not as effective as measuring responses to carriers that have been modulated in both amplitude and frequency at the same modulation frequency (mixed modulation). The detectability of eight different responses (four AM and four FM) to an IAFM stimuli relates well to the ability of subjects to discriminate words. IAFM stimuli therefore show promise as an objective test for assessing suprathreshold hearing.     

Modulation detection interference in listeners with normal and impaired hearing.

Listeners were asked to detect amplitude modulation (AM) of a target (or signal) carrier that was presented in isolation or in the presence of an additional (masker) carrier. The signal was modulated at a rate of 10 Hz, and the masker was unmodulated or was modulated at a rate of 2, 10, or 40 Hz. Nine listeners had normal hearing, 4 had a bilateral hearing loss, and 4 had a unilateral hearing loss; those with a unilateral loss were tested in both ears. The listeners with a hearing loss had normal hearing at 1 kHz and a 30- to 40-dB loss at 4 kHz. The carrier frequencies were 984 and 3952 Hz. In one set of conditions, the lower frequency carrier was the signal and the higher frequency carrier was the masker. In the other set, the reverse was true. For the impaired ears, the carriers were presented at 70 dB SPL. For the normal ears, either the carriers were both presented at 70 dB SPL or the higher frequency carrier was reduced to 40 dB SPL to simulate the lower sensation level experienced by the impaired ears. There was considerable individual variability in the results, and there was no clear effect of hearing loss. These results suggest that a mild, presumably cochlear hearing loss does not affect the ability to process AM in one frequency region in the presence of competing AM from another region.     

Auditory brainstem correlates of basilar membrane nonlinearity in humans

A behavioral measure of the basilar membrane response can be obtained by comparing the growth in forward masking for maskers at, and well below, the signal frequency. Since the off-frequency masker is assumed to be processed linearly at the signal place, the difference in masking growth with level is thought to reflect the compressive response to the on-frequency masker. The present experiment used an electrophysiological analog of this technique, based on measurements of the latency of wave V of the auditory brainstem response elicited by a 4-kHz, 4-ms pure tone, presented at 65 dB SPL. Responses were obtained in quiet and in the presence of either an on-frequency (4 kHz) or an off-frequency (1.8 kHz) pure-tone forward masker. Wave V latency increased with masker level, although the increase was greater for the off-frequency masker than for the on-frequency masker, consistent with a more compressive response to the latter. Response functions generated from the data showed the characteristic shape, with a nearly linear response at lower levels and 4:1 compression at higher levels. However, the breakpoint between the linear region and the compressive region was at about 60 dB SPL, higher than expected on the basis of previous physiological and psychophysical measures.     

Effects of masker frequency and duration in forward masking: further evidence for the influence of peripheral nonlinearity

Forward masking has often been thought of in terms of neural adaptation, with nonlinearities in the growth and decay of forward masking being accounted for by the nonlinearities inherent in adaptation. In contrast, this study presents further evidence for the hypothesis that forward masking can be described as a linear process, once peripheral, mechanical nonlinearities are taken into account. The first experiment compares the growth of masking for on- and off-frequency maskers. Signal thresholds were measured as a function of masker level for three masker-signal intervals of 0, 10, and 30 ms. The brief 4-kHz sinusoidal signal was masked by a 200-ms sinusoidal forward masker which had a frequency of either 2.4 kHz (off-frequency) or 4 kHz (on-frequency). As in previous studies, for the on-frequency condition, the slope of the function relating signal threshold to masker level became shallower as the delay between the masker and signal was increased. In contrast, the slopes for the off-frequency condition were independent of masker-signal delay and had a value of around unity, indicating linear growth of masking for all masker-signal delays. In the second experiment, a broadband Gaussian noise forward masker was used to mask a brief 6-kHz sinusoidal signal. The spectrum level of the masker was either 0 or 40 dB (re: 20 μPa). The gap between the masker and signal was either 0 or 20 ms. Signal thresholds were measured for masker durations from 5 to 200 ms. The effect of masker duration was found to depend more on signal level than on gap duration or masker level. Overall, the results support the idea that forward masking can be modeled as a linear process, preceded by a static nonlinearity resembling that found on the basilar membrane.     

Responses to Diotic, Dichotic, and Alternating Phase Harmonic Stimuli in the Inferior Colliculus of Guinea Pigs

Humans perceive a harmonic series as a single auditory object with a pitch equivalent to the fundamental frequency (F0) of the series. When harmonics are presented to alternate ears, the repetition rate of the waveform at each ear doubles. If the harmonics are resolved, then the pitch perceived is still equivalent to F0, suggesting the stimulus is binaurally integrated before pitch is processed. However, unresolved harmonics give rise to the doubling of pitch which would be expected from monaural processing (Bernstein and Oxenham, J. Acoust. Soc. Am., 113:3323–3334, 2003). We used similar stimuli to record responses of multi-unit clusters in the central nucleus of the inferior colliculus (IC) of anesthetized guinea pigs (urethane supplemented by fentanyl/fluanisone) to determine the nature of the representation of harmonic stimuli and to what extent there was binaural integration. We examined both the temporal and rate-tuning of IC clusters and found no evidence for binaural integration. Stimuli comprised all harmonics below 10 kHz with fundamental frequencies (F0) from 50 to 400 Hz in half-octave steps. In diotic conditions, all the harmonics were presented to both ears. In dichotic conditions, odd harmonics were presented to one ear and even harmonics to the other. Neural characteristic frequencies (CF, n = 85) were from 0.2 to 14.7 kHz; 29 had CFs below 1 kHz. The majority of clusters responded predominantly to the contralateral ear, with the dominance of the contralateral ear increasing with CF. With diotic stimuli, over half of the clusters (58%) had peaked firing rate vs. F0 functions. The most common peak F0 was 141 Hz. Almost all (98%) clusters phase locked diotically to an F0 of 50 Hz, and approximately 40% of clusters still phase locked significantly (Rayleigh coefficient >13.8) at the highest F0 tested (400 Hz). These results are consistent with the previous reports of responses to amplitude-modulated stimuli. Clusters phase locked significantly at a frequency equal to F0 for contralateral and diotic stimuli but at 2F0 for dichotic stimuli. We interpret these data as responses following the envelope periodicity in monaural channels rather than as a binaurally integrated representation.     

A method for removing cochlear implant artifact

When cortical auditory evoked potentials (CAEPs) are recorded in individuals with a cochlear implant (CI), electrical artifact can make the CAEP difficult or impossible to measure. Since increasing the interstimulus interval (ISI) increases the amplitude of physiological responses without changing the artifact, subtracting CAEPs recorded with a short ISI from those recorded with a longer ISI should show the physiological response without any artifact. In the first experiment, N1–P2 responses were recorded using a speech syllable and tone, paired with ISIs that changed randomly between 0.5 and 4 s. In the second experiment, the same stimuli, at ISIs of either 500 or 3000 ms, were presented in blocks that were homogeneous or random with respect to the ISI or stimulus. In the third experiment, N1–P2 responses were recorded using pulse trains with 500 and 3000 ms ISIs in 4 CI listeners. The results demonstrated: (1) N1–P2 response amplitudes generally increased with increasing ISI. (2) Difference waveforms were largest for the homogeneous and random-stimulus blocks than for the random-ISI block. (3) The subtraction technique almost completely eliminated the electrical artifact in individuals with cochlear implants. Therefore, the subtraction technique is a feasible method of removing from the N1–P2 response the electrical artifact generated by the cochlear implant.     

Temporal effects in simultaneous pure-tone masking: effects of signal frequency, masker/signal frequency ratio, and masker level

The effect of the temporal relationship between a pure-tone masker and a pure-tone signal in simultaneous masking was investigated in three experiments. The experiments extend previous work by: studying the temporal effect over a wide range of signal frequencies, studying the change in masking over time for several masker/signal frequency ratios, and studying the growth of masking for a brief signal at different temporal positions within a longer duration masker. In the first experiment, threshold was measured for a 20-ms signal temporally centered in a masker whose duration ranged from 20 ms to continuous. Signal frequency (fs) was 0.5, 1.0, 2.0, 4.0, or 8.0 kHz; masker frequency (fm) was 1.2 fs. For all signal frequencies, the amount of masking decreased as masker duration increased. In the second experiment, threshold was measured for a 20-ms, 1.0-kHz signal as a function of the signal's temporal position within a 400-ms masker whose frequency ranged from 1.0 to 1.25 kHz. For all but the 1.0-kHz masker, for which threshold was almost independent of the signal's temporal position, threshold decreased as signal onset was delayed relative to masker onset, but then increased slightly as the signal approached masker offset. In the final experiment, growth-of-masking functions were measured for a 20-ms, 1.0-kHz signal positioned at the beginning, at the temporal center, or at the end of a 400-ms masker whose frequency was 1.20 or 1.25 kHz. The masking functions generally were steepest for a signal at the onset of the masker and, for a given temporal position, steepest for the 1.20-kHz masker.     

Masking and suppression in auditory nerve fibers of the goldfish, Carassius auratus.

The responses of single fibers of the auditory nerve of the goldfish (Carassius auratus) were recorded in response to two tones of different duration (20 ms 'signals' and 200 ms 'maskers') presented simultaneously or non-simultaneously. A single tone may produce excitation, adaptation, and suppression in auditory nerve fibers. For fibers with characteristic frequencies (CF) in the 200 to 400 Hz range, frequencies well above CF tend to produce suppression. If the net response to the masker tone is excitation, an added excitatory signal tone tends to increment the response in a way predictable from the rate-level function for the masker. A masker can attenuate the response to a signal as a result of a compressive and saturating response to the masker, and as a result of a low signal-to-masker ratio. If the net response to a masker tone is suppression, it effectively subtracts from signal excitation, causing 'suppressive masking.' In non-spontaneous fibers, suppression, additive excitatory effects, and adaptation can be revealed by responses to the signal in the absence of spike responses to the masker. In general, the ability of one tone (the masker) to reduce the response to a second tone (the signal) is greater in non-spontaneous fibers than in spontaneous fibers. These results also show that estimates of the frequency selectivity of many goldfish auditory nerve fibers will depend on whether the response of the fiber is defined by excitation, suppression, or both. The response of many fibers with CF in the 200-400 Hz region, as defined by excitation, can be masked or suppressed by a broad range of frequencies covering the effective hearing range of the goldfish.     

Frequency characteristics of contralateral sound suppression of 40-Hz auditory steady-state response

Sound presented to the contralateral ear suppresses the amplitude of the 40-Hz auditory steady-state response (ASSR). The frequency characteristics of this suppression of the 40-Hz ASSR for amplitude modulated (AM) tones at 1,000 Hz (79-dB SPL) were examined in 12 healthy volunteers (10 males and 2 females, mean age 32.3 years) using contralateral AM tones (500, 1,000, 2,000, and 4,000 Hz) and 1/3 octave-band noise (500, 1,000, 2,000, and 4,000 Hz). The 40-Hz ASSR at 1,000 Hz was suppressed by a relatively wide frequency range of contralateral sound than expected from the known characteristics of psychophysical central masking by contralateral sound: the greatest suppression was obtained with 500- and 1,000-Hz sounds, but considerable suppression was also obtained with 2,000- and 4,000-Hz sounds. Substantial differences in the suppression pattern were not observed between two types of contra-suppressors; i.e., AM tones and 1/3 octave-band noise. Therefore, any sound presented to the contralateral ear, regardless of the frequency, can suppress the 40-Hz ASSR. Moreover, the different frequency characteristics of the contralateral sound effects between the psychophysical central masking and the 40-Hz ASSR would support the idea that the 40-Hz ASSR has an additive role in the processing of auditory signals to simple threshold judgment. Investigation of the type of psychophysical measurement using the AM signal showing similar suppression patterns by the presentation of contralateral sound would be helpful to reveal the functional relevance of ASSRs.     

Responses of neurons in the inferior colliculus of the rat to AM and FM tones

The responses of units in the inferior colliculus of the urethane-anaesthetized rat were recorded extracellularly. They responded to sinusoidal AM and FM tones with a modulation of their spike discharge usually at the same, or occasionally at twice, the modulation rate of the stimulus. The modulation depth of the response initially increased with the modulation depth of the stimulus, hut usually saturated or decreased at higher stimulus depths. The units showed a bandpass tuning to stimulus modulation rate which was independent of modulation depth and, in all cases, the most effective modulation rate was below 120 Hz. The modulated response to temporally varying stimuli could not be predicted from the pure tone discharge patterns or, in some cases, the unit's mean firing rate to modulated tones; temporally varying stimuli gave temporally varying responses. When compared with the data available from units at other levels in the auditory system, the results indicate a trend in which units at successively higher levels in the pathway respond most effectively to progressively lower rates of modulation.     

Across- and Within-Channel Envelope Interactions in Cochlear Implant Listeners

The effects of modulated maskers on detection thresholds of a 50-Hz sinusoidal amplitude modulation (SAM) in a signal carrier were measured in nine cochlear implant (CI) listeners as a function of masker envelope type and for different masker–signal electrode separations. Both signal and masker were 200-ms-long pulse trains, presented concurrently in an interleaved stimulation mode. Masker envelopes were SAM at 20, 50, (0- and -phase re: the signal modulator), and 125 Hz, as well as noise amplitude modulated (NAM), all with a fixed 20% modulation depth. Comparisons were made against steady-state maskers that had an amplitude equal to the mean amplitude of the modulated maskers or to their peak amplitude (SS_peak). Modulation thresholds were larger in the presence of the dynamic maskers versus the SS_peak maskers; however, there was significant intersubject variability in the pattern of results. Effects of relative phase between masker and signal were not consistent across subjects. Envelope masking (the dB difference in modulation detection thresholds between modulated and SS_peak maskers) was generally larger for the lower-modulation-frequency maskers than the 125-Hz masker. The spatial distribution of masked modulation detection thresholds was found to be considerably different from spatial forward-masking patterns obtained in the same subjects. Finally, modulation thresholds measured for a very wide separation between the masker and signal showed significant envelope masking. These results suggest that, as has been shown in acoustic stimulation, central, across-channel temporal processing mechanisms also occur in electrical stimulation.     

Forward Masking in Cochlear Implant Users: Electrophysiological and Psychophysical Data Using Pulse Train Maskers

Electrical stimulation of auditory nerve fibers using cochlear implants (CI) shows psychophysical forward masking (pFM) up to several hundreds of milliseconds. By contrast, recovery of electrically evoked compound action potentials (eCAPs) from forward masking (eFM) was shown to be more rapid, with time constants no greater than a few milliseconds. These discrepancies suggested two main contributors to pFM: a rapid-recovery process due to refractory properties of the auditory nerve and a slow-recovery process arising from more central structures. In the present study, we investigate whether the use of different maskers between eCAP and psychophysical measures, specifically single-pulse versus pulse train maskers, may have been a source of confound.In experiment 1, we measured eFM using the following: a single-pulse masker, a 300-ms low-rate pulse train masker (LTM, 250 pps), and a 300-ms high-rate pulse train masker (HTM, 5000 pps). The maskers were presented either at same physical current (Φ) or at same perceptual (Ψ) level corresponding to comfortable loudness. Responses to a single-pulse probe were measured for masker-probe intervals ranging from 1 to 512 ms. Recovery from masking was much slower for pulse trains than for the single-pulse masker. When presented at Φ level, HTM produced more and longer-lasting masking than LTM. However, results were inconsistent when LTM and HTM were compared at Ψ level. In experiment 2, masked detection thresholds of single-pulse probes were measured using the same pulse train masker conditions. In line with our eFM findings, masked thresholds for HTM were higher than those for LTM at Φ level. However, the opposite result was found when the pulse trains were presented at Ψ level.Our results confirm the presence of slow-recovery phenomena at the level of the auditory nerve in CI users, as previously shown in animal studies. Inconsistencies between eFM and pFM results, despite using the same masking conditions, further underline the importance of comparing electrophysiological and psychophysical measures with identical stimulation paradigms.     

Post-stimulatory suppression, facilitation and tuning for delays shape responses of inferior colliculus neurons to sequential pure tones.

Temporal changes in the excitability of inferior colliculus (IC) neurons will shape their responses to complex stimuli. Single-unit responses of rat IC neurons to the second (probe) of a pair of tones exhibited suppression, facilitation and delay tuned effects. Responses to probe tones were markedly suppressed (by 76% for contralateral stimulation with equal intensity tone pairs) during contralateral and binaural stimulation in 60% of IC neurons. Suppression developed rapidly as a function of the duration of the initial tone, and approached maximum for tones of less than 200 ms. Suppression decreased as the interval between tones increased, and this recovery of responsiveness was often exponential (time constants: mean: 271.4 ms; median: 72.8 ms; n = 47), and independent of the duration and intensity of preceding stimulation. Facilitation of responses to probe tones was observed chiefly in neurons with 'pauser/buildup' response patterns, and decreased as the intertone interval increased. The greatest suppression of responses to probe tones occurred only after intertone intervals of 32 ms (delayed minimum; n = 8) in 11% of IC neurons. Other IC neurons exhibited an increased excitability to probe tones presented 128 ms after stimulation (delayed maximum; n = 7). The latencies of the later neurons' responses were longer (mean: 29.5 ms) than other IC neurons. The role of suppression in sound localization and echo suppression, and the relationship between 'delay tuning' effects and encoding of complex stimuli are discussed.     

Binaural masking-level differences with tonal maskers

The thresholds for monaural and binaural 250-Hz test tones masked by a 250-Hz sinusoidal masker were measured as a function of the duration of the test tone. The signal and masker were presented either in-phase or in-quadrature. Next, the just-noticeable degree of amplitude modulation (AM) and just-noticeable modulation index for frequency modulation (FM) were measured as a function of the rate of modulation. Both sets of results suggest that monaural and binaural time constants have similar values (near 100 ms) and that the hearing system does not seem particularly ‘sluggish’ with the paradigms we used. In a second series of experiments, we again measured the masking of a tone by a tone of the same frequency. Long duration signals were used and we manipulated the phases of the masker and the signal. In order to interpret the results we require no more than the individual just-noticeable difference in level (monaurally about I dB) and the just-noticeable interaural time delay (about 100 μs at our signal frequency).