Neuromagnetic measurements of the human primary auditory response

The spatio-temporal organization of the human primary auditory response (N19 and P30) was determined by analyzing magnetic fields from the right hemisphere evoked by monaural clicks to the left ear. The magnetic response consisted of peaks corresponding to electrical N19 (N19m) and P30 (P30m) within 40 msec after stimulation. However, the onset of electrical N19 occurred 5-9 msec earlier than that of the magnetic counterpart. Furthermore, the relative amplitude of N19 and P30 (N19/P30) was larger (more than 1) than that (about 0.5) of N19m and P30m (N19m/P30m). The findings suggest an additional contribution from a subcortical source for electrical N19. The estimated equivalent sources for N19m and P30m were localized at the postero-medial part of Heschl's gyrus (the primary auditory cortex). Furthermore, the dipole orientation of N19m was directed postero-ventrally and that of P30m was in the opposite direction, which is compatible with the anatomy of the supratemporal primary auditory cortex. These results resolve a long standing controversy of cortical vs. subcortical structures as the generators of the N19.