Study of the visual evoked magnetic field with the m-sequence technique.

PURPOSE Multifocally stimulated visual evoked magnetic field (VEF) examination with an m-sequence technique (multifocal VEF; mVEF) was studied, and the neural generators at peaks of mVEF were estimated in the visual cortex. METHODS Visual field stimulation was generated by a multifocal testing system with use of the m-sequence technique. The stimulation pattern covered a central area extending from 0.6 degrees to 10 degrees in radius outward from the center of four visual-field quadrants. The stimulation pattern was projected onto a screen by a liquid crystal projector. VEFs of 14 healthy adults were recorded with a 160-channel, whole-head-type magnetoencephalography (MEG) system. The output signals of 16 selected MEG sensors covering the occipital region were recorded for each subject with the multifocal testing system, and the second-order responses were calculated. The analyzed response data files were transferred to the MEG system, a single equivalent current dipole (ECD) was estimated to locate the neural generator, and the localization was superimposed onto the corresponding brain magnetic resonance image of the subject. RESULTS mVEFs showed three peak waves (N75m, P100m, N145m) in 75% of the subjects and two peak waves (N75m, N145m) in 25%. (N, P and m denote negative, positive, and magnetic fields, respectively.) Latencies of the first and the last peak were similar between the two kinds of peak waves. ECD examination showed more than 97% of goodness of fit at all peaks, and the relation between EDCs and the stimulated visual field coincided with a retinotopic organization that fit a cruciform model in all subjects. ECD depths from the occipital pole were similar to the depth expected from the human linear cortical magnification factor model in all subjects. Main neural generators of all mVEF components (N75m, P100m, N145m) were shown in the striate cortex (V1). CONCLUSIONS Testing the VEF with an m-sequence technique showed stable responses to simultaneous stimulation of four visual-field quadrants. Consistency of correlation of the estimated ECD with the known cortical organization of the primary visual cortex confirmed the reliability of this examination. The three mVEF peaks were thought to derive mainly from V1 activity.