High-precision neuromagnetic study of the functional organization of the human auditory cortex.

Previous studies have proven that a dipole source analysis of the auditory evoked field is capable of providing evidence of the tonotopic organization of the human auditory cortex. To explore the nature of the estimated dipoles in greater detail, a single subject was extensively studied, and the estimated sources were registered in a three-dimensional reconstruction of the cortical surface derived from magnetic resonance images. The stimuli were 500-ms tone bursts with frequencies of 250, 500, 1,000, and 2,000 Hz (mean intensity of 60 dB SL). The total number of stimuli presented per condition was about 3,600 (36 independent experiments spread over 4 days). Using special postprocessing techniques, the relative localization accuracy could be enhanced to such an extent that differences in the dipole locations of 1 mm could be clearly distinguished. The results suggest that peak N1m (latency around 100 ms) arises from the planum temporale, whereas peak P2m (latency around 170 ms) appears to correspond to a center of activity in (or close to) Heschl's gyrus. The tonotopic organization found for the generator of N1m was consistent with earlier studies ("the higher the frequency the deeper the source"). However, additional findings (time dependence of the estimated sources; slightly different tonotopy obtained for field change; dependence of the estimated sources on the estimation technique) indicate that multiple areas are involved in the generation of N1m. Evidence of a frequency-dependent source location was found also for P2m.