Transient and continuous responses of neuromagnetic fields evoked by peripheral nerve stimulation

We measured neuromagnetic fields evoked by stimulation of the ulnar nerve using an rf SQUID with a second-derivative gradiometer. Responses to transient stimulus obtained at various positions of the lateral head were used to calculate location and strength of an equivalent current-dipole source by least-squares fit. The dipole location corresponded to the primary somatosensory area. We also measured continuous responses to repeated stimuli, where lag of phase of a component synchronous to the stimuli was detected. The phase lag was linear to the frequency of the stimuli in a limited frequency range. The slope of the linear variation gave latent time of the synchronous component, which agreed with the time of main peaks in the transient response.