7.2 A1 and the cortical tonotopic map

Primary auditory cortex (A1) sits on Heschl’s gyrus on the superior temporal plane, mostly hidden inside the Sylvian fissure. It is the cortical entry point of the auditory pathway.

A1 inherits the tonotopic organization that has been preserved all the way from the cochlea: neurons are arranged so that nearby cells respond to nearby frequencies, and a continuous map of frequency runs across the cortical surface. Modern imaging (fMRI tonotopy, electrocorticography, direct intracranial recording in patients) has confirmed and refined this map in humans, finding multiple parallel tonotopic gradients — A1 plus several adjacent fields — covering the superior temporal plane.

The tonotopic map of A1 is the cortical echo of the basilar membrane’s Greenwood function. A 1 kHz tone, which peaks at a specific place on the cochlea, drives a specific place in A1 most strongly. The mapping is preserved by an unbroken chain of place-preserving projections.