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 auditory perception. Common temporal features include sound envelope fluctuations, often modeled in the laboratory by amplitude modulation (AM), and starts and stops in ongoing sounds, which are frequently approximated by hearing researchers as gaps between two sounds or are investigated in forward masking experiments. The auditory system has evolved many neural processing mechanisms for encoding important temporal features of sound. Due to rapid progress 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 article. 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 illustrative examples of how the system encodes key temporal features of sound. This report, following a systems analysis approach, starts with findings in the auditory nerve and proceeds centrally through the cochlear nucleus, superior olivary complex and inferior colliculus. Some general principles can be seen when reviewing this entire field. For example, as one ascends the central auditory system, a neural encoding shift occurs. An emphasis on synchronous responses for temporal coding exists in the auditory periphery, and more reliance on rate coding occurs as one moves centrally. In addition, for AM, modulation transfer functions become more bandpass as the sound level of the signal is raised, but become more lowpass in shape as background noise is added. In many cases, AM coding can actually increase in the presence of background noise. For gap processing or forward masking, coding for gaps changes from a decrease in spike firing rate for neurons of the peripheral 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 suppression effects become quite long (echo suppression), and in some stimulus configurations enhancement to a second sound can take place.