Representation of speech categories in the primate auditory cortex.

A "ventral" auditory pathway in nonhuman primates that originates in the core auditory cortex and ends in the prefrontal cortex is thought to be involved in components of nonspatial auditory processing. Previous work from our laboratory has indicated that neurons in the prefrontal cortex reflect monkeys' decisions during categorical judgments. Here, we tested the role of the superior temporal gyrus (STG), a region of the secondary auditory cortex that is part of this ventral pathway, during similar categorical judgments. While monkeys participated in a match-to-category task and reported whether two consecutive auditory stimuli belonged to the same category or to different categories, we recorded spiking activity from STG neurons. The auditory stimuli were morphs of two human-speech sounds (bad and dad). We found that STG neurons represented auditory categories. However, unlike activity in the prefrontal cortex, STG activity was not modulated by the monkeys' behavioral reports (choices). This finding is consistent with the anterolateral STG's role as a part of functional circuit involved in the coding, representation, and perception of the nonspatial features of an auditory stimulus.