[Right hemispheric dominancy in the auditory evoked magnetic fields for pure-tone stimuli]

It is well known that the auditory evoked N100m response is dominant in the contralateral hemisphere to the stimulated ear: N100m latency is shorter and N100m amplitude is larger in the contralateral hemisphere than the ipsilateral hemisphere. Interhemispheric asymmetry of the N100m responses were reported in terms of the equivalent dipole localization: the right hemispheric N100m dipole is more anterior to the left hemispheric dipole. In the present study, we investigated interhemispheric asymmetry of the auditory evoked P50m and N100m response in terms of signal amplitude. A helmet-shaped whole head magnetoencephalography system was employed to see small but significant differences between two hemispheres. Thirty-seven right-handed healthy subjects, 31 males and 6 females, participated in the study. The helmet-shaped whole head MEG system (CTF Systems-Osaka Gas) consists of an array of 66 MEG sensor sites uniformly distributed over the entire head. The monaural auditory stimuli, 2000 Hz tone burst of 50 msec duration were delivered to the left and then the right ear sequentially, with an intensity of 80 dB SPL at the ear. White noise of 50 dB SPL was delivered to the opposite ear. The inter-stimulus intervals were randomized in the range from 2.5 to 4.7 sec. In a total of 37 subjects, N100m dipole pattern appeared on both hemispheres either left or right ear stimuli. When P50m peak amplitude was compared between contralateral responses, the right-hemispheric response (141.5 +/- 16.3 fT, mean +/- s.e.m.) was slightly larger than the left-hemispheric response (134.3 +/- 10.2 fT). Differences, however, were not significant. When P50m peak amplitude was compared between ipsilateral responses, the right-hemispheric response (135.1 +/- 11.0 fT) was slightly larger than the left-hemispheric response (115.3 +/- 10.9 fT). Differences were not significant, either. When N100m peak amplitude was compared between contralateral responses, the right-hemispheric response (484.4 +/- 28.4 fT) was statistically (p < 0.01) larger than the left-hemispheric response (375.6 +/- 28.0 fT). When N100m peak amplitude was compared between ipsilateral responses, the right-hemispheric response (405.8 +/- 27.9 fT) was statistically (p < 0.02) larger than the left-hemispheric response (315.1 +/- 24.8 fT). Previously, we reported that the right hemispheric latency is shorter than the left hemispheric latency in the auditory evoked field N100m responses. Present study indicated another evidence of the right hemispheric dominancy in the auditory evoked responses, namely the lager amplitude in the right hemispheric response at least for pure-tone stimuli. The whole-head MEG system is suitable to analyze differences of auditory function between two hemispheres.