Cortical processing of temporal modulations

Temporal modulations are fundamental components of human speech and animal communication sounds. Understanding their representations in the auditory cortex is a crucial step towards our understanding of brain mechanisms underlying speech processing. While modulated signals have long been used as experimental stimuli, their cortical representations are not completely understood, particularly for rapid modulations. Known physiological data do not adequately explain psychophysical observations on the perception of rapid modulations, largely due to slow stimulus-synchronized temporal discharge patterns of cortical neurons. In this article, we summarize recent findings from our laboratory on temporal processing mechanisms in the auditory cortex. These findings show that the auditory cortex represents slow modulations explicitly using a temporal code and fast modulations implicitly by a discharge rate code. Rapidly modulated signals within a short-time window (similar to20-30 ms) are integrated and transformed into a discharge rate-based representation. The findings also indicate that there is a shared representation of temporal modulations by cortical neurons that encodes the temporal profile embedded in complex sounds of various spectral contents. Our results suggest that cortical processing of sound streams operates on a "segment-by-segment" basis with a temporal integration window on the order of similar to20-30 ms. (C) 2002 Elsevier Science B.V. All rights reserved.