Estimates of visually evoked cortical currents.

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.