Physiological Mapping of Human Auditory Cortices with a Silent Event-Related fMRI Technique

Cortical field boundaries of sensory areas can be physiologically defined. The delineation of the human auditory cortical architecture remains incomplete. Here we used systematic variation of pitch and duration of sinusoidal tones to define auditory cortical fields along Heschl's gyrus with a silent, event-related fMRI scanning technique that allowed us to determine spatially small shifts of neuronal responses. Thus, we were able to establish higher-resolution tonotopic maps. Acoustic intervals of two octaves correspond to an average 2-mm anatomical distance along Heschl's gyrus. We also demonstrate that one tonotopic map with a medio-lateral low- to high-frequency gradient is located on the lateral half of Heschl's gyrus, which might correspond to field R in primates. Furthermore, we studied cortical responses to brief (50-ms) transients with fMRI and demonstrate that silent, event-related fMRI is capable of measuring significant blood oxygen level-dependent effect to such brief events in the acoustic domain. Our results add to current knowledge on the number and precise localization of multiple tonotopic maps in human auditory cortex. More specifically, they support the hypothesis that there may be two primary auditory cortical fields with mirror tonotopic organization on Heschl's gyrus in man.