Analogues of simple and complex cells in rhesus monkey auditory cortex.

Receptive fields (RFs) of neurons in primary visual cortex have traditionally been subdivided into two major classes: "simple" and "complex" cells. Simple cells were originally defined by the existence of segregated subregions within their RF that respond to either the on- or offset of a light bar and by spatial summation within each of these regions, whereas complex cells had ON and OFF regions that were coextensive in space [Hubel DH, et al. (1962) J Physiol 160:106-154]. Although other definitions based on the linearity of response modulation have been proposed later [Movshon JA, et al. (1978) J Physiol 283:53-77; Skottun BC, et al. (1991) Vision Res 31(7-8):1079-1086], the segregation of ON and OFF subregions has remained an important criterion for the distinction between simple and complex cells. Here we report that response profiles of neurons in primary auditory cortex of monkeys show a similar distinction: one group of cells has segregated ON and OFF subregions in frequency space; and another group shows ON and OFF responses within largely overlapping response profiles. This observation is intriguing for two reasons: (i) spectrotemporal dissociation in the auditory domain provides a basic neural mechanism for the segregation of sounds, a fundamental prerequisite for auditory figure-ground discrimination; and (ii) the existence of similar types of RF organization in visual and auditory cortex would support the existence of a common canonical processing algorithm within cortical columns.