Temporal Processing

Physiological Mechanisms for Envelope Processing

For a listener to utilize the cues in the temporal envelopes of sound stimuli, these temporal cues need to be coded efficiently and accurately by the auditory system. Envelope processing by neurons at various stages of the ascending auditory pathway has been extensively studied using amplitude-modulated stimuli in laboratory animals. In a typical experiment, a pure-tone stimulus is amplitude-modulated using a sinusoidal function, and the response from an auditory neuron is analyzed in terms of its synchronization to the stimulus envelope. A synchronization index is typically defined to quantify the degree of synchronization between the stimulus and neural response. If the pattern of neural spikes matches well to that of the stimulus envelope (i.e., the sinusoidal amplitude modulation), the result is a high synchronization index; on the other hand, if the pattern of neural spikes is independent of the stimulus envelope, it would lead to a low [Page 1944]synchronization index. The function relating the synchronization index to the amplitude-modulation rate is called the synchronization modulation transfer function. The function relating the firing rate to the amplitude modulation rate is called the rate modulation transfer function.

At the level of the auditory nerve, the synchronization modulation transfer function typically exhibits a low-pass characteristic. That is, the spiral ganglion neurons are able to code the temporal envelope information via synchronized neural activities up to a certain cutoff modulation rate, above which the fidelity of envelope coding degrades as the amplitude modulation rate increases. Across spiral ganglion neurons, the cutoff rate for the synchronization modulation transfer function is higher for neurons tuned to higher frequencies, corresponding to more basal locations in the cochlea. Therefore, the upper rate limit for envelop coding by the spiral ganglion neurons reflects the time constant in peripheral filtering, which is governed by cochlear mechanics. For basal spiral ganglion neurons, the cutoff modulation rate is typically between 600 and 1000 Hz.

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