Stimulus dependencies of the gerbil brain-stem auditory-evoked response (BAER). I: Effects of click level, rate, and polarity

Three experiments evaluating the effects of various stimulus manipulations on the click-evoked gerbil brain-stem auditory-evoked response (BAER) are reported. In experiment 1, click polarity and level were covaried. With increasing click level, there is a parallel decrease in the latency of the first five BAER peaks (i-v) and an increase in BAER peak amplitudes. Mean wave i amplitude was greater for rarefaction than condensation clicks at high click levels; mean wave v amplitude was greater for condensation clicks at higher click levels. Experiment 2 covaried click rate and polarity. The latency of the BAER peaks increased with increasing click repetition rate. This rate-dependent latency increase was greater for the later BAER peaks, resulting in an increase in the i-v interval with increasing click rate. As rate increased, the amplitudes of waves i and v decreased monotonically, whereas the amplitudes of waves ii-iv were largely uninfluenced by click rate. As in experiment 1, mean wave i amplitude was greater for rarefaction clicks, whereas mean wave v amplitude was greater for condensation clicks. The magnitude of these polarity dependencies on waves i and v amplitude decreased with increasing click rate. Experiment 3 evaluated the effects of click polarity on BAERs to high-intensity (100 dB pSPL) clicks presented at a rate of 10 Hz. In eight of ten gerbils evaluated, wave i amplitude was greater to rarefaction clicks, and, in all ten animals, wave v amplitude was greater to condensation clicks. The effects of click level and rate on BAER peak amplitudes, latencies, and interwave intervals are reminiscent of stimulus dependencies reported for the human BAER. The effects of click polarity on the amplitudes of waves i and v of the gerbil BAER have also been reported for the human BAER.     

Brain-stem auditory-evoked responses elicited by maximum length sequences: effect of simultaneous masking noise

The effects of masking noise on wave V of the brain-stem auditory-evoked response (BAER) obtained to pseudorandom pulse sequences are evaluated in two experiments. In the first experiment, the level of broadband noise was covaried with minimum pulse interval (rate) using maximum length sequence analysis (MLSA). Both increasing noise level and decreasing minimum pulse interval decrease wave V amplitude and increase wave V latency. A nonadditivity of rate and noise level was observed such that, at the shortest interpulse intervals, simultaneous background noise produced virtually no latency change and minimal amplitude change, for the noise levels tested. In a second experiment, high-pass masking was performed to assess the feasibility of derived-band techniques using maximum length sequence analysis (MLSA) and to compare the frequency regions responsible for the BAER using MLSA versus conventional averaging. Results of experiment 2 showed that reliable responses across high-pass masker cutoff frequency could be obtained in normal-hearing listeners. The frequency specificity of the MLSA-based responses was nearly identical to that obtained by conventional averaging, although both amplitude and latency of wave V were affected by the high-pass masker cutoff and minimum pulse interval values. These studies suggest that the neuronal populations and frequency regions responsible for the BAER are virtually the same for MLSA and conventional averaging.     

The effect of broadband noise on the human brainstem auditory evoked response. II. Frequency specificity

A series of experiments evaluated the effects of broadband noise (ipsilateral) on wave V of the brainstem auditory evoked response (BAER) elicited by tone bursts or clicks in the presence of high-pass masking noise. Experiment 1 used 1000- and 4000-Hz, 60-dB nHL tone bursts in the presence of broadband noise. With increasing noise level, wave V latency shift was greater for the 1000-Hz tone bursts, while amplitude decrements were similar for both tone-burst frequencies. Experiment 2 varied high-pass masker cutoff frequency and the level of subtotal masking in the presence of 50-dB nHL clicks. The effects of subtotal masking on wave V (increase in latency and decrease in amplitude) increased with increasing derived-band frequency. Experiment 3 covaried high-pass masker cutoff frequency and subtotal masking level for 1000- and 4000-Hz tone-burst stimuli. The effect of subtotal masking on wave V latency was reduced for both tone-burst frequencies when the response-generating region of the cochlear partition was limited by high-pass maskers. The results of these three experiments suggest that most of the wave V latency shift associated with increasing levels of broadband noise is mediated by a place mechanism when the stimulus is a moderate intensity (60 dB nHL), low-frequency (1000 Hz) tone burst. However, the interpretation of the latency shifts produced by broadband noise for 4000-Hz tone-burst stimuli is made more complex by multiple technical factors discussed herein.     

Stimulus dependencies of the gerbil brain-stem auditory-evoked response (BAER). III: Additivity of click level and rate with noise level

Two experiments were performed that evaluated the effects of ipsilateral-direct broadband noise maskers on the gerbil brain-stem auditory-evoked response (BAER) to click stimuli. In experiment 1, clicks were presented at 27 Hz at levels including 70, 80, 90, and 100 dB pSPL. Noise conditions included a no-noise control, and included noise levels varying in 10-dB increments from 20 dB SPL to a maximum noise level of 50, 60, 70, and 80 dB SPL for click levels of 70, 80, 90, and 100 dB pSPL, respectively. Gerbil BAER peaks were labeled with small roman numerals to distinguish them from human BAER peaks. The dependent variables included waves i and v latencies and amplitudes. Peak latencies increased and peak amplitudes decreased with decreasing click level and increasing noise level. To a first approximation, peak latencies and amplitudes showed changes with increasing noise level that were similar across click level. With increasing click level, there was little or no effect on the i-v interval. There was an increase in the i-v interval with increasing noise level. In experiment 2, click level was held constant at 90 dB pSPL, and click rates included 15, 40, 65, and 90 Hz. For each click rate, noise conditions included a no-noise control, and noise levels included 20, 30, 40, 50, 60, and 70 dB SPL. With increasing click rate and noise level, there was an increase in peak latencies, an increase in the i-v interval, and a decrease in peak amplitudes. The magnitude of peak latency and amplitude shifts with increasing click rate was dependent on noise level. Specifically, the magnitude of rate-dependent changes decreased with increasing level of broadband noise. These data are compared to human BAER experiments, and are found to be in fundamental agreement.     

The effect of broadband noise on the human brain-stem auditory evoked response. III. Anatomic locus

The effects of broadband noise on the brain-stem auditory evoked response (BAER) are reported for two experiments. Experiment 1 used a high-pass subtractive-masking technique and covaried derived bandwidth and continuous broadband noise level. Comparison of responses to half-octave wide derived bands in the presence of within-band noise showed that wave V latency changes were greater than could be explained on the basis of shifts in the cochlear region responsible for generating the response. The magnitude of within-band noise-induced wave V latency shift was independent of the frequency separation of the masker cutoffs. In experiment 2 the effects of noise level and rate on waves I, III, and V of the BAER were evaluated. Peak latencies increased and peak amplitudes decreased with increasing noise level and rate. Higher noise levels and rates produced an increased central (I-V) conduction time in which the wave III-V increase was greater than the wave I-III increase. Together, these results are most consistent with the hypothesis that a nonplace, central auditory mechanism produces most of the noise-induced latency shifts in normal-hearing adults.     

Rise-fall time effects on the brainstem auditory evoked response: mechanisms

Experiments were conducted to assess the contribution of place mechanisms to the effect of rise--fall time on wave V of the human brainstem auditory evoked response (BAER). Noise bursts of 4- and 10-ms duration were presented at various rise-fall times (0, 1, 2, and 5 ms). Subtractive high-pass masking techniques were used to determine the effect of rise time as a function of derived-band frequency. In general, increasing rise time prolonged wave V latency but did not affect amplitude. Rise-time effects did not depend on derived-band frequency and similar effects were seen in the unmasked conditions. In addition, narrowing the derived band did not alter the observed effects on latency and amplitude. Signal envelope showed no effects on traveling wave velocity. These results suggest that place mechanisms contribute little to changes in the BAER associated with rise--fall time.     

The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies.

Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V's of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response (Nmax) were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. Nmax decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.     

The effect of broadband noise on the human brainstem auditory evoked response. I. Rate and intensity effects

A series of experiments investigated the effects of continuous broadband noise (ipsilateral) on wave V of the click-evoked brainstem auditory evoked response (BAER). In general, a broadband noise masker increases the latency and decreases the amplitude of wave V. Varying both click and noise intensity, it was found that noise levels above about 40 dB SPL increase the latency and decrease the amplitude of wave V, regardless of click intensity. The effects of noise on wave V amplitude appear constant across click intensity, whereas the effects of a constant noise level on wave V latency decrease at higher click intensities. Both masking and adaptation increase wave V latency, but their combined effects are occlusive: rate-induced wave V latency shift decreases in the presence of continuous broadband noise. The clinical and theoretical implications of these findings are discussed.     

Place specificity of multiple auditory steady-state responses

Auditory steady-state responses (ASSRs) were elicited by simultaneously presenting multiple AM (amplitude-modulated) tones with carrier frequencies of 500, 1000, 2000, and 4000 Hz and modulation frequencies of 77, 85, 93, and 102 Hz, respectively. Responses were also evoked by separately presenting single 500- or 2000-Hz AM tones. The objectives of this study were (i) to determine the cochlear place specificity of single and multiple ASSRs using high-pass noise masking and derived-band responses, and (ii) to determine if there were any differences between single- and multiple-stimulus conditions. For all carrier frequencies, derived-band ASSRs for 1-octave-wide derived bands ranging in center frequency from 0.25 to 8 kHz had maximum amplitudes within a 1/2 octave of the carrier frequency. For simultaneously presented AM tones of 500, 1000, 2000, and 4000 Hz, bandwidths for the function of derived-band ASSR amplitude by derived-band center frequency were 476, 737, 1177, and 3039 Hz, respectively. There were no significant differences when compared to bandwidths of 486 and 1371 for ASSRs to AM tones of 500 or 2000 Hz presented separately. Results indicate that ASSRs to moderately intense stimuli (60 dB SPL) reflect activation of reasonably narrow cochlear regions, regardless of presenting AM tones simultaneously or separately.     

The effect of broadband noise on the human brain-stem auditory evoked response. IV. Additivity of forward-masking and rate-induced wave V latency shifts

The additivity of forward masking and repetitive stimulation effects on wave V of the brain-stem auditory evoked response (BAER) was investigated. The effects of repetitive stimulation were evaluated for a stimulus train (called the adaptation series), with a 12.5-ms within-train interclick interval. The forward masker was a 100-ms, 80-dB SPL broadband noise with forward-masker intervals ranging from 12.5-87.5 ms. Forward masking and repetitive stimulation increased the latency of wave V of the BAER. The combined forward masking/adaptation series produced less wave V latency shift than the summed individual effects. Forward masking reduced wave V amplitude at brief forward masker intervals, while repetitive stimulation did not affect wave V amplitude. Wave V amplitude was decreased for the combined forward masking/adaptation series, and the time course of amplitude recovery of the combination was prolonged compared to the forward masking alone condition. The nonadditivity of forward masking and rate effects on wave V latency is similar to that found for repetitive stimulation and simultaneous masking [Burkard and Hecox, J. Acoust. Soc. Am. 74, 1204-1213 (1983)]. These findings are consistent with the position that forward masking and rate effects on wave V latency are produced by overlapping mechanisms.     

Maturation of the traveling-wave delay in the human cochlea

The maturation of the traveling-wave delay in the human cochlea was investigated in 227 subjects ranging in age from 29 weeks conceptional age to 49 years by using frequency specific auditory brain-stem responses (ABRs). The derived response technique was applied to ABRs obtained with click stimuli (presented at a fixed level equal to 60-dB sensation level in normal hearing adults) in the presence of high-pass noise masking (slope 96 dB/oct) to obtain frequency specific responses from octave-wide bands. The estimate of traveling-wave delay was obtained by taking the difference between wave I latencies from adjacent derived bands. It was found that the traveling-wave delay between the octave band with center frequency (CF) of 11.3 kHz and that with CF of 5.7 kHz decreased (about 0.4 ms on average) in exponential fashion with age to reach adult values at 3-6 months of age. This decrease was in agreement with reported data in kitten auditory-nerve fibers. The traveling-wave delays between adjacent octave bands with successive lower CF did not change with age.     

Frequency specificity of chirp-evoked auditory brainstem responses

This study examines the usefulness of the upward chirp stimulus developed by Dau et al. [J. Acoust. Soc. Am. 107, 1530-1540 (2000)] for retrieving frequency-specific information. The chirp was designed to produce simultaneous displacement maxima along the cochlear partition by compensating for frequency-dependent traveling-time differences. In the first experiment, auditory brainstem responses (ABR) elicited by the click and the broadband chirp were obtained in the presence of high-pass masking noise, with cutoff frequencies of 0.5, 1, 2, 4, and 8 kHz. Results revealed a larger wave-V amplitude for chirp than for click stimulation in all masking conditions. Wave-V amplitude for the chirp increased continuously with increasing high-pass cutoff frequency while it remains nearly constant for the click for cutoff frequencies greater than 1 kHz. The same two stimuli were tested in the presence of a notched-noise masker with one-octave wide spectral notches corresponding to the cutoff frequencies used in the first experiment. The recordings were compared with derived responses, calculated offline, from the high-pass masking conditions. No significant difference in response amplitude between click and chirp stimulation was found for the notched-noise responses as well as for the derived responses. In the second experiment, responses were obtained using narrow-band stimuli. A low-frequency chirp and a 250-Hz tone pulse with comparable duration and magnitude spectrum were used as stimuli. The narrow-band chirp elicited a larger response amplitude than the tone pulse at low and medium stimulation levels. Overall, the results of the present study further demonstrate the importance of considering peripheral processing for the formation of ABR. The chirp might be of particular interest for assessing low-frequency information.     

Auditory brain stem responses from human adults and infants: restriction of frequency contribution by notched-noise masking

The frequency contribution to the click-evoked ABR wave V was examined in adults and 3-month-old infants through the use of notch-filtered broadband noise. Notch center frequencies were set at 1.0, 4.0, and 8.0 kHz. Responses were obtained at 20, 40, and 60 dBnHL during the simultaneous presentation of each notched-noise masker as well as in an unmasked condition. The ABR wave V was analyzed for absolute latency and amplitude, as well as latency and amplitude changes resulting from the introduction of masking. Analyses showed wave V latency and amplitude values to be similar for adults and infants within the 1.0-kHz notch. Differences between adult and infant groups were observed as the notch was shifted to the high frequencies. Further, latency and amplitude shifts resulting from the introduction of masking noise produced differential effects on infant responses when compared to adults.     

Intelligibility of speech in noise at high presentation levels: effects of hearing loss and frequency region

These experiments examined how high presentation levels influence speech recognition for high- and low-frequency stimuli in noise. Normally hearing (NH) and hearing-impaired (HI) listeners were tested. In Experiment 1, high- and low-frequency bandwidths yielding 70%-correct word recognition in quiet were determined at levels associated with broadband speech at 75 dB SPL. In Experiment 2, broadband and band-limited sentences (based on passbands measured in Experiment 1) were presented at this level in speech-shaped noise filtered to the same frequency bandwidths as targets. Noise levels were adjusted to produce approximately 30%-correct word recognition. Frequency bandwidths and signal-to-noise ratios supporting criterion performance in Experiment 2 were tested at 75, 87.5, and 100 dB SPL in Experiment 3. Performance tended to decrease as levels increased. For NH listeners, this "rollover" effect was greater for high-frequency and broadband materials than for low-frequency stimuli. For HI listeners, the 75- to 87.5-dB increase improved signal audibility for high-frequency stimuli and rollover was not observed. However, the 87.5- to 100-dB increase produced qualitatively similar results for both groups: scores decreased most for high-frequency stimuli and least for low-frequency materials. Predictions of speech intelligibility by quantitative methods such as the Speech Intelligibility Index may be improved if rollover effects are modeled as frequency dependent.     

Edge effects on frequency discrimination of tones presented in low- and high-pass noise backgrounds

Previous research has indicated that frequency discrimination performance is poorer for tones presented near the sharp spectral edge of a low-pass noise than for tones presented near the edge of a high-pass noise, or for tones in the same low-pass noise with high-pass noise added [Emmerich et al., J. Acoust. Soc. Am. 80, 1668-1672 (1986)]. The present study extends these findings in order to investigate how the steepness of the spectral edges of low- and high-pass maskers influences the discriminability of tones presented near these edges. Frequency discrimination was measured in each of three high- and low-pass noise backgrounds (which differed in the steepness of their filter skirts). The following results were obtained: (1) In the low-pass noise background, frequency discrimination performance improved as the filter skirt became more gradual; (2) in the high-pass noise background, performance first improved and then became poorer as the filter skirt became shallower; and (3) performance in low-pass noise was poorer than that in high-pass noise for the two steepest slopes employed (96 and 72 dB/oct) but not for the shallower slope (36 dB/oct). Results are discussed in the context of lateral suppression and edge pitch effects, and of a trade-off between possible edge effects and masking.     

Masking patterns in the bullfrog (Rana catesbeiana). II: Physiological effects

Responses of individual eighth-nerve fibers in the bullfrog (Rana catesbeiana) were measured to tone bursts at best frequency against a background of continuous, broadband masking noise. These data were used to calculate critical masking ratios to describe the fibers' responses to tones embedded in noise. In the frequency response range of the amphibian papilla (100-1000 Hz), critical ratios increase with tone frequency. Critical ratios of basilar papilla fibers (1000-2000 Hz) are generally higher than those of amphibian papilla fibers. Critical ratios are also significantly related to fiber threshold such that fibers with high thresholds, regardless of their best frequencies, have higher critical ratios and are thus less selective to signals embedded in noise. Critical ratios based on neural responses show a somewhat different frequency-dependent trend than do critical ratios based on psychophysical data presented previously for this species [A. M. Simmons, J. Acoust. Soc. Am. 83, 1087-1092 (1988a)]. In addition, these neural critical ratios do not appear to be level independent, as are psychophysical critical ratios. The data suggest that frequency selectivity of hearing in the bullfrog as measured behaviorally is probably not mediated solely by spectral filtering in the auditory periphery.     

Correcting Low-Frequency Phase Distortion in Electroglottograph Waveforms

Dynamic high-pass filtering with a -3 dB frequency that is a factor of ten or more below the voice fundamental frequency has a negligible effect on the amplitudes of the Fourier components of an EGG waveform. However, such a filter can significantly distort the waveform due to distortion in the phase or time alignment of these Fourier components. Such high-pass filtering can be introduced purposefully to stabilize the waveform by attenuating low-frequency noise, or may be an undesired effect of using an amplification or data acquisition system designed for acoustic signals. For a given voice fundamental frequency, the amount of distortion depends greatly on the order or attenuation characteristics of the filter and on the type of EGG waveform. Both a high-order filter and a breathy voice tend to increase the amount of distortion. If the characteristics of the high-pass filter are known, there are a number of digital filter techniques that can be used to reduce the phase distortion. However, it is shown that a relatively simple analogue network can also be used to obtain a correction that suffices for most applications. If the precise characteristics of the filter are not known, the response to a square wave can be used to adjust the compensator parameters for an optimal correction.     

Physiological responses to the pulsation threshold paradigm. I: Pulsation threshold patterns do not reproduce physiological rate profiles of high-pass and low-pass noise maskers

This article compares psychophysical measures of human processing of acoustic stimuli with one neurophysiological representation (normalized discharge rate profiles) of those stimuli. Psychophysical pulsation threshold patterns (PTPs) were derived for high-pass and low-pass noise maskers. Spectral features of both maskers are clearly evident in the PTPs. However, while the representation of high-pass noise in the PTPs becomes sharper with increasing masker level, the representation of low-pass noise degenerates as masker level is increased. One assumption that has been used previously to interpret pulsation threshold data is that PTPs reflect the profile of activity in primary neural elements in response to the masking stimulus. To investigate this hypothesis, normalized-rate profiles of responses to both maskers were derived from populations of auditory-nerve fibers in cats. Normalized-rate profiles do not exhibit the same behavior as PTPs for high-pass noise maskers in that the neural representation of the band edge degenerates as sound level increases. Furthermore, the distinction between the passband and the stop band is lost in the neural rate profiles, whereas the distinction improves in the high-pass noise PTPs.     

Signal detection in temporally modulated and spectrally shaped maskers.

The first part of this paper presents several experiments on signal detection in temporally modulated noise, yielding a general approach toward the concept of comodulation masking release (CMR). Measurements were made on masked thresholds of both long- and short-duration, narrow-band signals presented in a 100% sinusoidally amplitude-modulated (SAM) noise masker (modulation frequency 32 Hz), as a function of masker bandwidth from 1/3 oct up to 13/3 octs, while the masker band was geometrically centered at signal frequency. With the short-duration signals placed in the valley of the masker, a substantial CMR (i.e., a decrease of masked threshold with increasing masker bandwidth) was found, whereas for the long-duration signals CMR was smaller. Furthermore, investigations were carried out to determine whether CMR changes when the bandwidth of the signals, consisting of bandpass impulse responses, is increased. The data indicate that substantial CMR remains even when all masker bands contain a signal component, thus minimizing across-channel differences. This finding is not in line with current models accounting for the CMR phenomenon. The second part of this paper concerns signal detection in spectrally shaped noise. Also investigated was whether release from masking occurs for the detection of a pure-tone signal at a valley or a peak of a simultaneously presented masking noise with a sinusoidally rippled power spectrum, when this masker was preceded and followed by a second noise (temporal flanking burst) with an identical spectral shape as the on-signal noise. Similar to CMR effects for temporal modulations, the data indicate that coshaping masking release (CSMR) occurs when the signal is placed in a valley of the spectral envelope of the masker, whereas no release from masking is found when the signal is placed at a peak of the spectral envelope of the masker. The implications of these experiments for measures of spectral and temporal resolution are discussed.     

High-synchrony cochlear compound action potentials evoked by rising frequency-swept tone bursts

The auditory compound action potential (CAP) represents synchronous VIIIth nerve activity. Clicks or impulses have been used in the past to produce this synchrony under the assumption that the wide spectral spread inherent in transient signals will activate a large portion of the cochlear partition. However, the observation that only auditory nerve units tuned above 3 kHz contribute to synchronous activity in the N1P1 complex of the CAP [Dolan et al., J. Acoust. Soc. Am. 73, 580-591 (1983)] suggests that temporal delays imposed by the traveling wave result in an asynchronous pattern of VIIIth nerve activation. In order to determine if units tuned below 3 kHz could be recruited into the CAP response, the present study uses tone bursts of exponentially rising frequency to hypothetically activate synchronous discharges of VIIIth nerve fibers along the length of the cochlear partition. The equations defining the frequency sweeps are calculated to be the inverse of the delay-line characteristics of the guinea pig cochlear partition. The resultant sweeps theoretically cause a constant phase displacement of a large portion of the cochlear partition at one time. Compound action potentials recorded in response to the rising frequency sweeps were compared to CAPs evoked by corresponding falling frequency sweeps and clicks. Analysis of the CAP waveforms showed narrower N1 widths and larger N1 and P1 amplitudes for rising sweeps when compared to falling sweeps. This is consistent with the hypothesis of increased synchrony. A further test of the hypothesis was made by using high-pass masking noise to evaluate the contributions of discrete cochlear locations to the CAP ("derived" CAP). Latency functions of the derived CAPs for clicks and falling frequency sweeps showed progressive increases in latency as the cutoff frequency of the high-pass filter was lowered. The latency of the derived CAP for these stimulus conditions reflects traveling wave delays [Aran and Cazals, "Electrocochleography: Animal studies," in Evoked Electrical Activity in The Auditory Nervous System (Academic, New York, 1978)]. In contrast, derived CAPs obtained from rising sweeps showed no change in latency for any cutoff frequencies, indicating a constant delay of response for fibers with different characteristic frequencies (CFs). These results support the theoretical premise underlying the derivation of the rising sweep: Spectral energy with the appropriate temporal organization, dictated by basilar membrane traveling wave properties, will increase CAP synchrony.