Synchrony Suppression in Complex Stimulus Responses of Biophysical Model of the Cochlea.

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A minimal biophysical model of the cochlea is used to investigate the validity of the hypothesis that a single compressive nonlinearity at the hair cell level can explain some of the synchrony suppression phenomena in cochlear response to complex stimuli. The dependencies of the model responses on the amplitudes and frequencies of two-tone stimuli resemble in many respects the behavior of the experimental data, and can be traced to explicit biophysical parameters in the model. Most discrepancies between theory and experiment stem from simplifications in parameters of the minimal model that play no direct role in the hypothesis. The analysis and simulations predict further results which, pending experimental verification, may provide a more direct test of the influence of the compressive nonlinearity on the relative amplitudes of the synchronous response components, and hence of its role in synchrony suppression. For instance, regardless of the overall absolute levels of a two-tone stimulus applied to this type of model, the ratio of the amplitudes at the input, and the ratio of the corresponding responses at the output, remain approximately constant and equal (the output ratio changes by at most 6 dB in favor of the stronger tone). Other nonlinear responses to multi- tonal stimuli can also be reproduced such as 'spectral edge enhancement' (Horst et al. [l985]), Springer-Verlag) and some aspects of three-tone suppression (Javel et al. [l983]), Monash University Press). In contrast to the complex behavior of the synchrony suppression with increasing intensity, and the resulting drastic influences of the compressive nonlinear on the response measures on the auditory-nerve (e.g. average rate and synchrony profiles), the percepts of complex sounds are relatively stable. This suggests that the invariant response measures are more likely used in the encoding and CNS extraction of the spectrum of complex stimuli such as speech.