Biomedical subjects

Michael G Heinz

Publications and source records attributed to Michael G Heinz.

4 recordsLinked to original sources

PubMed2005

Auditory-nerve rate responses are inconsistent with common hypotheses for the neural correlates of loudness recruitment.

A number of perceptual phenomena related to normal and impaired level coding can be accounted for by the degree of compression in the basilar-membrane (BM) magnitude response. However, the narrow dynamic ranges of auditory-nerve (AN) fibers complicate these arguments. Because the AN serves as an information bottleneck, an improved understanding of the neural coding of level may clarify some of the limitations of current hearing aids. Here three hypotheses for the neural correlate of loudness recruitment were evaluated based on AN responses from normal-hearing cats and from cats with a noise-induced hearing loss (NIHL). Auditory-nerve fiber rate-level functions for tones were analyzed to test the following hypotheses: Loudness recruitment results from steeper AN rate functions after impairment. This hypothesis was not supported; AN rate functions were not steeper than normal following NIHL, despite steeper estimated BM responses based on the AN data. Loudness is based on the total AN discharge count, and recruitment results from an abnormally rapid spread of excitation after impairment. Whereas abnormal spread of excitation can be observed, steeper growth of total AN rate is not seen over the range of sound levels where recruitment is observed in human listeners. Loudness of a narrowband stimulus is based on AN responses in a narrow BF region, and recruitment results from compression of the AN-fiber threshold distribution after impairment. This hypothesis was not supported because there was no evidence that impaired AN threshold distributions were compressed and the growth of AN activity summed across BFs near the stimulus frequency was shallower than normal.Overall, these results suggest that loudness recruitment cannot be accounted for based on summed AN rate responses and may depend on neural mechanisms involved in the central representation of intensity.

Acoustic Stimulation↗

PubMed2003

Response growth with sound level in auditory-nerve fibers after noise-induced hearing loss.

People with sensorineural hearing loss are often constrained by a reduced acoustic dynamic range associated with loudness recruitment; however, the neural correlates of loudness and recruitment are still not well understood. The growth of auditory-nerve (AN) activity with sound level was compared in normal-hearing cats and in cats with a noise-induced hearing loss to test the hypothesis that AN-fiber rate-level functions are steeper in impaired ears. Stimuli included best-frequency and fixed-frequency tones, broadband noise, and a brief speech token. Three types of impaired responses were observed. 1) Fibers with rate-level functions that were similar across all stimuli typically had broad tuning, consistent with outer-hair-cell (OHC) damage. 2) Fibers with a wide dynamic range and shallow slope above threshold often retained sharp tuning, consistent with primarily inner-hair-cell (IHC) damage. 3) Fibers with very steep rate-level functions for all stimuli had thresholds above approximately 80 dB SPL and very broad tuning, consistent with severe IHC and OHC damage. Impaired rate-level slopes were on average shallower than normal for tones, and were steeper in only limited conditions. There was less variation in rate-level slopes across stimuli in impaired fibers, presumably attributable to the lack of suppression-induced reductions in slopes for complex stimuli relative to BF-tone slopes. Sloping saturation was observed less often in impaired fibers. These results illustrate that AN fibers do not provide a simple representation of the basilar-membrane I/O function and suggest that both OHC and IHC damage can affect AN response growth.

Acoustic Stimulation↗

PubMed2003

Quantifying the information in auditory-nerve responses for level discrimination.

An analytical approach for quantifying the information in auditory-nerve (AN) fiber responses for the task of level discrimination is described. A simple analytical model for ANT responses is extended to include temporal response properties, including the nonlinear-phase effects of the cochlear amplifier. Use of simple analytical models for AN discharge patterns allows quantification of the contributions of level-dependent aspects of the patterns to level discrimination. Specifically, the individual and combined contributions of the information contained in discharge rate, synchrony, and relative phase cues are explicitly examined for level discrimination of tonal stimuli. It is shown that the rate information provided by individual AN fibers is more constrained by increases in variance with increases in rate than by saturation. As noted in previous studies, there is sufficient average-rate information within a narrow-CF region to account for robust behavioral performance over a wide dynamic range; however, there is no model based on a simple limitation or use of AN information consistent with parametric variations in performance. This issue is explored in the current study through analysis of performance based on different aspects of AN patterns. For example, we show that performance predicted from use of all rate information degrades significantly as level increases above low-medium levels, inconsistent with Weber's Law. At low frequencies, synchrony information extends the range over which behavioral performance can be explained by 10-15 dB, but only at low levels. In contrast to rate and synchrony, nonlinear-phase cues are shown to provide robust information at medium and high levels in near-CF fibers for low-frequency stimuli. The level dependence of the discharge rate and phase properties of AN fibers are influenced by the compressive nonlinearity of the inner ear. Evaluating the role of the compressive nonlinearity in level coding is important for understanding neural encoding mechanisms and because of its association with the cochlear amplifier, which is a fragile aspect of the ear believed to be affected in common forms of hearing impairment.

Animals↗

PubMed2002

Quantifying the implications of nonlinear cochlear tuning for auditory-filter estimates.

The relation between auditory filters estimated from psychophysical methods and peripheral tuning was evaluated using a computational auditory-nerve (AN) model that included many of the response properties associated with nonlinear cochlear tuning. The phenomenological AN model included the effects of dynamic level-dependent tuning, compression, and suppression on the responses of high-, medium-, and low-spontaneous-rate AN fibers. Signal detection theory was used to evaluate psychophysical performance limits imposed by the random nature of AN discharges and by random-noise stimuli. The power-spectrum model of masking was used to estimate psychophysical auditory filters from predicted AN-model detection thresholds for a tone signal in fixed-level notched-noise maskers. Results demonstrate that the role of suppression in broadening peripheral tuning in response to the noise masker has implications for the interpretation of psychophysical auditory-filter estimates. Specifically, the estimated psychophysical auditory-filter equivalent-rectangular bandwidths (ERBs) that were derived from the nonlinear AN model with suppression always overestimated the ERBs of the low-level peripheral model filters. Further, this effect was larger for an 8-kHz signal than for a 2-kHz signal, suggesting a potential characteristic-frequency (CF) dependent bias in psychophysical estimates of auditory filters due to the increase in strength of cochlear nonlinearity with increases in CF.

Auditory Perception↗