Auditory Processing in Primate Brains

Abstract

The auditory system of humans and other primates resembles those of other mammals at early stages of processing, in that the mechanisms of the transformation of sound pressure waves into movements of the basilar membrane and stimulation of the auditory receptor cells are similar, as are the functions of brain stem nuclei as they use differences in the intensities and timing of sounds to the two ears to extract information about the location of sound sources, and use activation along the receptor surface and phase timing of neuron spikes to signal sound frequencies. At the cortical level, auditory cortex is organized somewhat differently than other mammals, with a core of three primary areas, termed A1, R (rostral area), and RT (rostrotemporal area). These areas distribute information to a surrounding belt of seven or eight secondary areas, named by location relative to core areas. The belt areas in turn project to a lateral parabelt auditory region, divided into rostral and caudal segments. Both monkeys and humans appear to have a similar arrangement of core, belt, and parabelt areas. Overall, the core areas have neurons more selective for stimulus parameters such as sound frequency, while belt and especially parabelt areas have neurons more responsive to the meaning of sounds. More rostral core, belt, and parabelt regions appear to be stepwise more specialized for voices and communication sounds, while more caudal areas are thought to be more involved in localizing sounds for attention and action. Neurons in even core areas are influenced by visual stimuli and direction of gaze, and the caudal belt areas are responsive to both somatosensory and auditory stimuli. Belt areas provide auditory information to parietal cortex, polysensory cortex of the temporal lobe, and several locations of frontal cortex, including a lateral prefrontal region where neurons are responsive to vocalization and faces. Some auditory connections even reach visual areas. Humans have major specializations of the left cerebral hemisphere for language and speech. Keywords: auditory cortex; cochlea; inferior colliculus; thalamus; multisensory