Subcortical neural coding mechanisms for auditory temporal processing

Biologically relevant sounds such as speech, animal vocalizations and music have distinguishing temporal features that are utilized for effective audi tory perception. Common temporal features include sound envelope fluctuatio ns, often modeled in the laboratory by amplitude modulation (AM), and start s and stops in ongoing sounds. which are frequently approximated by hearing researchers as gaps between two sounds or are investigated in forward mask ing experiments. The auditory system has evolved many neural processing mec hanisms for encoding important temporal features of sound. Due to rapid pro gress made in the field of auditory neuroscience in the past three decades, it is not possible to review all progress in this field in a single articl e. The goal of the present report is to focus on single-unit mechanisms in the mammalian brainstem auditory system for encoding AM and gaps as illustr ative examples of how the system encodes key temporal features of sound. Th is report, following a systems analysis approach, starts with findings in t he auditory nerve and proceeds centrally through the cochlear nucleus, supe rior olivary complex and inferior colliculus. Some general principles can b e seen when reviewing this entire field. For example, as one ascends the ce ntral auditory system, a neural encoding shift occurs. An emphasis on synch ronous responses for temporal coding exists in the auditory periphery, and more reliance on rate coding occurs as one moves centrally. In addition. fo r AM, modulation transfer functions become more bandpass as the sound level of the signal is raised, but become more lowpass in shape as background no ise is added. In many cases, AM coding can actually increase in the presenc e of background noise. For gap processing or forward masking, coding for ga ps changes from a decrease in spike firing rate for neurons of the peripher al auditory system that have sustained response patterns, to an increase in firing rate for more central neurons with transient responses. Lastly, for gaps and forward masking, as one ascends the auditory system, some suppres sion effects become quite long (echo suppression), and in some stimulus con figurations enhancement to a second sound can take place. (C) 2001 Elsevier Science BN. All rights reserved.