Representation of con-specific vocalizations in the core and belt areas of the auditory cortex in the alert macaque monkey.

Auditory cortical processing in primates has been proposed to be divided into two parallel processing streams, a caudal spatial stream and a rostral nonspatial stream. Previous single neuron studies have indicated that neurons in the rostral lateral belt respond selectively to vocalization stimuli, whereas imaging studies have indicated that selective vocalization processing first occurs in higher order cortical areas. To test the dual stream hypothesis and to find evidence to account for the difference between the electrophysiological and imaging results, we recorded the responses of single neurons in core and belt auditory cortical fields to both forward and reversed vocalizations. We found that there was little difference in the overall firing rate of neurons across different cortical areas or between forward and reversed vocalizations. However, more information was carried in the overall firing rate for forward vocalizations compared with reversed vocalizations in all areas except the rostral field of the core (area R). These results are consistent with the imaging results and are inconsistent with early rostral cortical areas being involved in selectively processing vocalization stimuli based on a firing rate code. They further suggest that a more complex processing scheme is in play in these early auditory cortical areas.