Analysis and Measurement of Neuromagnetic Fields Elicited by Stimulations of Sensory Nerves

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We have studied the neuromagnetic fields which are evoked by peripheral nerve and auditory stimulations. The measurement system we have developed consists of a mapping apparatus which includes a 2nd-order gradiometer and SQUID, a somatosensory and auditory stimulator, and a signal processor for averaging and amplitude and phase detection.In the somatosensory evoked fields (SEF), responses have two main peaks at 80 and 160 ms after the stimulus, while responses of the auditory evoked fields (AEF) have main peaks at 100 and 180 ms latencies. Estimation of parameters of a current-dipole source based on a least-squares fit to the mapping measured data has indicated that the dipole is located at somatosensory and auditory cortexes for the SEF and AEF, respectively. These locations agree well with MRI observation. The dipole moments are 5-30 nAm.In the steady state SEF responses to repeated stimuli, phase lag exhibits a linear dependence on the frequency of the stimuli. The linear relation gives three values of the time lag, two of which agree with the latencies of the main peaks in the transient response. The shortest time lag is attributed to the latency of the early component.In the AEFs with tone burst and short verbal sounds like/a/and/ka/, responses to the verbal sounds have similar peaks to the tone burst response. Estimated moments of the current-dipole sources for the tones and verbal sounds are nearly the same or larger in the right hemisphere than in the left hemisphere. There seems a slight difference in the location of the dipoles of the tones and verbal sounds in the left hemisphere.