Estimates of visually evoked cortical currents

Measurement of magnetic fields evoked by the onset of checkerboard-like sectorial patterns presented at 16 locations near the center of the visual field appears promising for further studies of cortical retinotopy and visual field defects.

Abstract

We have measured magnetic fields evoked by the onset of checkerboard-like sectorial patterns presented at 16 locations near the center of the visual field. Small stimuli (less than 2 degrees), which, nevertheless, gave sufficiently strong responses to enable source localization, were used to limit cortical activation to a small area, thus simplifying the analysis of the magnetic field data. We focused on optimizing the experimental design: cortical sources could be located from measurements at just one position of our 24-channel magnetometer and with as few as 15-20 repetitions of the stimulus. Minimum-norm-estimate maps calculated from even a single response showed reproducible features of the current distribution, which was, 80-100 msec after the pattern onset, retinotopically organized in the occipital lobe. Since magnetoencephalography can reveal cortical locations with a precision of 2-3 mm, our procedure appears promising for further studies of cortical retinotopy and visual field defects.