HIGH-PRECISION NEUROMAGNETIC STUDY OF THE FUNCTIONAL-ORGANIZATION OF THE HUMAN AUDITORY-CORTEX

Previous studies have proven that a dipole source analysis of the audi tory evoked field is capable of providing evidence of the tonotopic or ganization 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-dimensio nal reconstruction of the cortical surface derived from magnetic reson ance images. The stimuli were 500-ms tone bursts with frequencies of 2 50, 500, 1,000, and 2,000 Hz (mean intensity of 60 dB SL). The total n umber of stimuli presented per condition was about 3,600 (36 independe nt experiments spread over 4 days). Using special postprocessing techn iques, the relative localization accuracy could be enhanced to such an extent that differences in the dipole locations of 1 mm could be clea rly distinguished. The results suggest that peak Nlm (latency around 1 00 ms) arises from the planum temporale, whereas peak P2m (latency aro und 170 ms) appears to correspond to a center of activity in (or close to) Heschl's gyrus. The tonotopic organization found for the generato r of Nlm was consistent with earlier studies ('the higher the frequenc y the deeper the source'). However, additional findings (time dependen ce of the estimated sources; slightly different tonotopy obtained for field change; dependence of the estimated sources on the estimation te chnique) indicate that multiple areas are involved in the generation o f Nlm. Evidence of a frequency-dependent source location was found als o for P2m.