Coding of pitch and amplitude modulation in the auditory brainstem: One common mechanism?

The neural mechanisms underlying the perception of pitch and amplitude modulation (AM) are still unclear. Current computer models designed to understand the functional mechanisms underlying the extraction of periodicity share a relatively detailed implementation of cochlear processing, but they diverge in their implementation of the neural processing strategy. Specifically, the modulation filterbank model of Dau, Kollmeier, and Kohlrausch (1997) recruits modulation filters not unlike the cochlear band-pass filters. A current model of pitch perception (Meddis and O'Mard 1997) recruits autocorrelation to extract periodicity from the information provided by the auditory nerve. Both these models lack a direct physiological correlate. Cariani and Delgutte (1996a,b) have shown that autocorrelation of auditory-nerve activation results in a good estimate of the perceived pitch for a comprehensive set of stimuli. Here, we investigate the hypothesis that sustained-chopper (Chop-S) units in the ventral cochlear nucleus (VCN) may play an important role in pitch and AM processing. Specifically, populations of Chop-S units with different best frequencies (BFs) and chopping periods (CPs) may serve to create a temporal place code of periodicity which, possibly at the level of the inferior colliculus (IC), is transformed into a rate-place code. This hypothesis is evaluated using a refined computer model of Chop-S units receiving their input from a state-of-the-art computer model of the auditory periphery.