Temporal profile of visual evoked responses to pattern-reversal stimulation analyzed with a whole-head magnetometer

Magnetoencephalography (MEG) has become accepted as a useful method for non-invasively studying brain functions, including visual perception. The present study used MEG to elucidate information processing following pattern-reversal stimulation by analyzing the origins and properties of visual evoked magnetic fields (VEFs). The VEFs of ten healthy adults were recorded in a magnetically shielded room using a 122-channel whole-head magnetometer. The visual stimulation of checkerboard-pattern reversal at 1.7 Hz was presented to the subject's right hemifield. Visual evoked potentials (VEPs) were recorded simultaneously, and 150 responses were each averaged for VEFs and VEPs. For the contrast profile study, pattern-reversal stimuli at five different contrast levels from 96% to 8% were used. In all subjects, the VEFs showed three components with latencies of approximately 95, 120, and 160 ms. The equivalent current dipoles for the first and the third components were located and were oriented close to each other in the left occipital lobe, but these dipoles were separated from that of the second component, which showed an opposite direction. Stimuli at a moderate contrast level markedly reduced the first component, but not the third. These findings indicate that the first and the third components of VEFs appear to originate from anatomically closely situated, almost identical, sources, but that their physiological properties differ.