Auditory nerve representation of vowels in background noise

1983 ◽  
Vol 50 (1) ◽  
pp. 27-45 ◽  
Author(s):  
M. B. Sachs ◽  
H. F. Voigt ◽  
E. D. Young
Keyword(s):  
Steady State ◽  
Auditory Nerve ◽  
Background Noise ◽  
Discharge Rate ◽  
Nerve Fibers ◽  
Noise Levels ◽  
Stimulus Component ◽  
Signal Noise ◽  
Formant Frequencies ◽  
Second Formant

Responses of auditory nerve fibers to steady-state vowels presented alone and in the presence of background noise were obtained from anesthetized cats. Representation of vowels based on average discharge rate and representation based primarily on phase-locked properties of responses are considered. Profiles of average discharge rate versus characteristic frequency (CF) ("rate-place" representation) can show peaks of discharge rate in the vicinity of formant frequencies when vowels are presented alone. These profiles change drastically in the presence of background noise, however. At moderate vowel and noise levels and signal/noise ratios of +9 dB, there are not peaks of rate near the second and third formant frequencies. In fact, because of two-tone suppression, rate to vowels plus noise is less than rate to noise alone for fibers with CFs above the first formant. Rate profiles measured over 5-ms intervals near stimulus onset show clear formant-related peaks at higher sound levels than do profiles measured over intervals later in the stimulus (i.e., in the steady state). However, in background noise, rate profiles at onset are similar to those in the steady state. Specifically, for fibers with CFs above the first formant, response rates to the noise are suppressed by the addition of the vowel at both vowel onset and steady state. When rate profiles are plotted for low spontaneous rate fibers, formant-related peaks appear at stimulus levels higher than those at which peaks disappear for high spontaneous fibers. In the presence of background noise, however, the low spontaneous fibers do not preserve formant peaks better than do the high spontaneous fibers. In fact, the suppression of noise-evoked rate mentioned above is greater for the low spontaneous fibers than for high. Representations that reflect phase-locked properties as well as discharge rate ("temporal-place" representations) are much less affected by background noise. We have used synchronized discharge rate averaged over fibers with CFs near (+/- 0.25 octave) a stimulus component as a measure of the population temporal response to that component. Plots of this average localized synchronized rate (ALSR) versus frequency show clear first and second formant peaks at all vowel and noise levels used. Except at the highest level (vowel at 85 dB sound pressure level (SPL), signal/noise = +9 dB), there is also a clear third formant peak. At signal-to-noise ratios where there are no second formant peaks in rate profiles, human observers are able to discriminate second formant shifts of less than 112 Hz. ALSR plots show clear second formant peaks at these signal/noise ratios.

Effects of continuous noise backgrounds on rate response of auditory nerve fibers in cat

1984 ◽  
Vol 51 (6) ◽  
pp. 1326-1344 ◽  
Author(s):  
J. A. Costalupes ◽  
E. D. Young ◽  
D. J. Gibson
Keyword(s):  
Auditory Nerve ◽  
Noise Level ◽  
Background Noise ◽  
Dynamic Range ◽  
Discharge Rate ◽  
Nerve Fibers ◽  
Noise Levels ◽  
High Noise ◽  
Continuous Noise ◽  
Auditory Nerve Fibers

This study describes the effects of broadband background noise on the average discharge rate to best-frequency (BF) tones of auditory nerve fibers in the cat. The effects of exposure to long-term continuous noise are compared to the effects of noise gated on and off simultaneously with test tones. Addition of background noise causes a shift of the dynamic portion of tone-evoked rate versus level functions to higher tone intensities. The shift occurs at a mean rate of 0.61 dB of shift for each 1-dB increment in noise level. The rate of shift is independent of best frequency and spontaneous discharge rate. The noise level at which the shift begins is frequency dependent and is consistent with the frequency-dependent bandwidths of auditory nerve fiber tuning curves. The adjustment of the dynamic range shows many similarities to two-tone suppression. Therefore, it is most likely that it is caused by suppression of the response to the BF test tone by energy present in the noise at surrounding frequencies. At high noise levels, the ability of auditory nerve fibers to respond to test tones is limited by the rate response to the noise. As noise level increases, the discharge rate it evokes approaches a fiber's saturation rate and ultimately eliminates the fiber's ability to respond to test tones. Low spontaneous rate fibers, which have been shown to have higher thresholds and wider dynamic range (17,29), are significantly more resistant to saturation by high noise levels. Exposure to broadband noise prior to onset of test tones produces an overall decrement in response rate. This phenomenon is similar to the effects of short-term adaptation (32) and seems to develop independently of the shift of dynamic range. At high noise levels, previous exposure to the noise produces a small dynamic range shift. This effect is similar to that produced by suppression but is smaller. The effect is occluded in continuous noise backgrounds by the adjustment of sensitivity produced by suppression.

Lateral suppression and inhibition in the cochlear nucleus of the cat

1994 ◽  
Vol 71 (2) ◽  
pp. 493-514 ◽  
Author(s):  
W. S. Rhode ◽  
S. Greenberg
Keyword(s):  
Cochlear Nucleus ◽  
Auditory Nerve ◽  
Background Noise ◽  
Sound Pressure ◽  
Dynamic Range ◽  
Discharge Rate ◽  
Nerve Fibers ◽  
Range Shift ◽  
Rate Information ◽  
Noisy Background

1. The ability of cells in the cochlear nucleus (CN) to encode frequency information in the presence of background noise on the basis of "place/rate" information was investigated by measuring the threshold, magnitude, and extent of lateral suppression in the ventral and dorsal CN of the anesthesized cat. The suppression regions were delineated through the use of "masked" response areas (MRAs). The MRA is a family of isointensity curves derived from the average discharge rate in response to a tone of variable frequency and sound pressure level in the presence of a concurrently presented broadband, quasi-flat-spectrum noise. Tonal stimuli of sufficient intensity are often effective in significantly reducing the average discharge rate of CN neurons over a wide frequency range. 2. Most units in the CN exhibit prominent lateral suppressive sidebands, but the variability in threshold, magnitude, and extent of suppression is large. Primary-like and onset units of the ventral CN manifest the least suppression and have the highest suppression thresholds. Pauser/buildup units in the dorsal division and choppers distributed throughout the CN show the largest amount of suppression and have the lowest suppression thresholds. 3. Auditory nerve fibers manifest some degree of lateral suppression, particularly fibers of low and medium spontaneous rate. However, in few instances are the threshold, magnitude, and extent comparable with that observed among the majority of chopper and pauser/buildup units. For this reason the lateral suppression observed among the latter unit types is unlikely to originate entirely from cochlear processes, but rather is likely to reflect largely neural mechanisms intrinsic to the CN. In contrast, the MRAs of most primary-like and onset units suggest that the suppression behavior of most of these cells originates mostly, if not entirely, in the cochlea and auditory nerve. 4. A primary consequence of lateral suppression is to preserve the sharp frequency selectivity of CN neurons at moderate to high sound pressure levels, particularly in background noise. In this fashion lateral suppressive mechanisms potentially enhance the representation of spectral information on the basis of place/rate information relative to that in the auditory nerve under noisy background conditions. 5. Lateral suppressive mechanisms probably underlie the dynamic range shift seen in the presence of a simultaneously presented noise. This mechanism may be crucial for preserving the ability to perceive signals in a noisy background.

Effect of electrical stimulation of the crossed olivocochlear bundle on auditory nerve response to tones in noise

1987 ◽  
Vol 57 (4) ◽  
pp. 1002-1021 ◽  
Author(s):  
R. L. Winslow ◽  
M. B. Sachs
Keyword(s):  
Auditory Nerve ◽  
Noise Level ◽  
Dynamic Range ◽  
Response Rate ◽  
Discharge Rate ◽  
Nerve Fibers ◽  
Range Shift ◽  
Auditory Nerve Fiber ◽  
Auditory Nerve Fibers ◽  
Rate Suppression

The discharge rates of single auditory-nerve fibers responding to best-frequency (BF) tones of varying level presented simultaneously with fixed level broadband noise were recorded with and without electrical stimulation of the crossed olivocochlear bundle (COCB). In the absence of COCB stimulation, monotonic increases in noise level produce monotonic increases in the low-level noise-driven response rate of auditory nerve fibers. As a result of adaptation, these increases in noise-driven response rate produce monotonic decreases in saturation discharge rate. At high noise levels, these compressive effects may eliminate the differential rate response of auditory nerve fibers to BF tones. COCB stimulation can restore this differential rate response by producing large decreases in noise-driven response rate and large increases in saturation discharge rate. In backgrounds of quiet, COCB stimulation is known to shift the dynamic range of single auditory nerve fiber BF tone responses to higher stimulus levels. In the presence of background noise, COCB stimulation produces upward shift of dynamic range, which decreases with increasing noise level. At high noise levels, COCB-induced decompression of rate-level functions may occur with little or no dynamic range shift. This enables auditory nerve fibers to signal changes in tone level with changes in discharge rate at lower signal-to-noise ratios than would be possible otherwise. Broadband noise also produces upward shift of the dynamic range of single auditory nerve fiber BF tone response. Noise-induced dynamic range shift of BF tone response was measured as a function of noise level with and without COCB stimulation. COCB stimulation elevates the threshold of noise-induced dynamic range shift. This shift is thought to result from two-tone rate suppression. Increases in the threshold of noise-induced shift due to COCB stimulation therefore suggests an interaction between the mechanism of two-tone rate suppression and the mechanism by which COCB stimulation produces dynamic range shift. These interactions were further investigated by recording auditory nerve fiber rate responses to fixed-level BF excitor tones presented simultaneously with fixed-frequency variable level suppressor tones. Rate responses were recorded with and without COCB stimulation. Experimental results were quantified using a phenomenological model of two-tone rate suppression presented by Sachs and Abbas.

Neural mechanisms of tone-on-tone masking: patterns of discharge rate and discharge synchrony related to rates of spontaneous discharge in the chinchilla auditory nerve

1986 ◽  
Vol 56 (6) ◽  
pp. 1763-1780 ◽  
Author(s):  
D. G. Sinex ◽  
D. C. Havey
Keyword(s):  
Auditory Nerve ◽  
Average Rate ◽  
Discharge Rate ◽  
Quantitative Agreement ◽  
Nerve Fibers ◽  
Spontaneous Discharge ◽  
Probe Tone ◽  
Distortion Products ◽  
Auditory Nerve Fibers ◽  
Psychophysical Studies

Responses of chinchilla auditory nerve fibers to brief probe tones in the presence of a fixed tonal masker were obtained. The stimulus conditions were analogous to those that have been used in many psychophysical experiments. The relation between previously described response properties of auditory nerve fibers and features of psychophysical tone-on-tone masking was examined. In psychophysical studies, a fixed narrowband masker produces a characteristic pattern of masked thresholds, which becomes broad and asymmetrical at high masker levels. In the present experiment 1, a 5,000-Hz masker was presented at 30, 50, and 70 dB SPL. Masked thresholds based on the average rate of response to probe tones were estimated for single auditory nerve fibers. The lowest of these masked thresholds formed a pattern similar to the psychophysical masking pattern, becoming broader and more asymmetrical as the masker was increased to 70 dB SPL. The masked thresholds of fibers with low and medium rates of spontaneous discharge (SR) were as low as or lower than the masked thresholds of fibers with high SRs. In certain frequency regions, masked thresholds based on responses to cochlear distortion products were lower than the masked thresholds of any fiber responding to the probe tone; this result is also similar to previous psychophysical observations. In experiment 2, responses of chinchilla auditory nerve fibers to probe tones in the presence of a masker at 1,000 Hz and 50 dB SPL were studied. Probe tone thresholds in the presence of this masker have been measured psychophysically in chinchillas. Thus the relation between behavioral and neural masked thresholds in the same species could be examined. Masked thresholds were estimated from average discharge rate responses and also from discharge synchrony. Good quantitative agreement was observed between the probe tone levels at which changes in average discharge rate were observed and the chinchilla's behavioral masked thresholds. For fibers matched for characteristic frequency, the masked thresholds based on average discharge rate of high-SR fibers tended to be elevated compared with the thresholds of medium-SR fibers. Changes in discharge rate synchronized to the probe tone occurred at levels lower than the chinchilla's behavioral masked thresholds. If discharge synchrony can be used for detection, the code would appear to be based on the relative synchrony to the probe tone and to the masking tone. Low synchrony masked thresholds were obtained from fibers with all SRs.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonlinear Modeling of Auditory-Nerve Rate Responses to Wideband Stimuli

2005 ◽  
Vol 94 (6) ◽  
pp. 4441-4454 ◽  
Author(s):  
Eric D. Young ◽  
Barbara M. Calhoun
Keyword(s):  
Auditory Nerve ◽  
Transfer Functions ◽  
Discharge Rate ◽  
Nonlinear Modeling ◽  
Nerve Fibers ◽  
Second Order ◽  
Spectral Processing ◽  
Spectrum Shape ◽  
Level Function ◽  
Auditory Nerve Fibers

The spectral selectivity of auditory nerve fibers was characterized by a method based on responses to random-spectrum-shape stimuli. The method models the average discharge rate of fibers for steady stimuli and is based on responses to ≈100 noise-like stimuli with pseudorandom spectral levels in 1/8- or 1/16-octave frequency bins. The model assumes that rate is determined by a linear weighting of the spectrum plus a second-order weighting of all pairs of spectrum values within a certain frequency range of best frequency. The method allows prediction of rate responses to stimuli with arbitrary wideband spectral shapes, thus providing a direct test of the degree of linearity of spectral processing Auditory-nerve fibers are shown to rely mainly on linear weighting of the stimulus spectrum; however, significant second-order terms are present and are important in predicting responses to random-spectrum shape stimuli, although not for predicting responses to noise filtered with cat head-related transfer functions. The second-order terms weight the products of levels at identical frequencies positively and the products of different frequencies negatively. As such, they model both curvature in the rate versus level function and suppressive interactions between different frequency components. The first- and second-order characterizations derived in this method provide a measure of higher-order nonlinearities in neurons, albeit without providing information about temporal characteristics.

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