The Evolution of Auditory Cortex in Humans

Abstract

The brains of humans and monkeys show striking anatomical similarities, from the partitioning of their cerebral cortex into lobes to the functional specialization of particular cortical regions. The auditory cortical system, originating in the superior temporal cortex (STC), is organized into core and belt regions and into ventral and dorsal pathways in both species. A fundamental question with regard to the evolution of auditory cortex, which culminates in the evolution of speech, language, and music in humans, is whether human and monkey brains show principal differences in their organization (eg, new pathways or new areas appearing perhaps as the result of a genetic mutation), or whether species differences are largely of a quantitative nature. There is little doubt about a similar role of the ventral auditory pathway (anterolateral STC to ventral inferior frontal cortex) in both humans and monkeys in the decoding of spectrally complex sounds, which some authors have referred to as auditory object recognition. This function includes the decoding of vocalizations and speech sounds (“speech perception”) and their ultimate linking to meaning (“semantics”). The originally presumed role of the auditory dorsal pathway (posterior STC to inferior parietal cortex and onto premotor and dorsal prefrontal regions) in spatial processing, by analogy to the visual dorsal pathway, has recently been generalized into one of sensorimotor integration and control, which includes the processing of not only space and motion but also sequences in space and time. Thus it seems that at least the basic layout of auditory cortex in humans and nonhuman primates is quite similar, although some refinement in one or several of these structures and their connectivity seems likely.