The organization of tonotopic fields in human auditory cortex was investigated using functional magnetic resonance imaging. Subjects were presented with stochastically alternating multi-tone sequences in six different frequency bands, centered at 200, 400, 800, 1600, 3200, and 6400 Hz. Two mirror-symmetric frequency gradients were found extending along an anterior-posterior axis from a zone on the lateral aspect of Heschl's gyrus (HG), which responds preferentially to lower frequencies, toward zones posterior and anterior to HG that are sensitive to higher frequencies. The orientation of these two principal gradients is thus roughly perpendicular to HG, rather than parallel as previously assumed. A third, smaller gradient was observed in the lateral posterior aspect of the superior temporal gyrus. The results suggest close homologies between the tonotopic organization of human and nonhuman primate auditory cortex.
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