Temporal and rate representations of time-varying acoustic signals in the auditory cortex of awake primates

Time-varying acoustic stimuli are important features of biologically important sounds. To convey information, humans and animals rely on temporal modulations in the envelope of their speech or vocalizations, and these modulations can range from milliseconds to hundreds of milliseconds and longer. The representation of rapid acoustic transients by the auditory cortex is a fundamental issue that is still unresolved. Auditory cortical neurons have been shown to be limited in their stimulus-synchronized responses, yet the perceptual performances of humans and animals in discriminating temporal variations in complex sounds are better than what existing neurophysiological data in anesthetized animals would predict.; The results of this study show that responses of auditory cortical neurons in an awake animal may account for the perceptual capacity of humans and animals to discriminate fine temporal features. It is shown that there are two largely distinct populations of neurons in the auditory cortex of awake primates: one with stimulus-synchronized discharges that represents slowly occurring sound sequences explicitly with a temporal code, and the other with non-stimulus-synchronized discharges that represents rapidly occurring events implicitly with a rate code. Furthermore, neurons of both populations display selectivity in their discharge rates to temporal features within a short time-window. The results suggest that the combination of temporal and rate codes in the auditory cortex provides a possible neural basis for the wide perceptual range of temporal information.