Visual hemifield mapping using transcranial magnetic stimulation coregistered with cortical surfaces derived from magnetic resonance images

The perception of a visual stimulus can be inhibited by occipital transcran ial magnetic stimulation. This visual suppression effect has been attribute d to disruption in the cortical gray matter of primary visual cortex or in the fiber tracts leading to V1 from the thalamus. However, others have sugg ested that the visual suppression effect is caused by disruption in seconda ry visual cortex. Here the authors used a figure-eight coil, which produces a focal magnetic field and a Quadropulse stimulator to produce visual supp ression contralateral to the stimulated hemisphere in five normal volunteer subjects. The authors coregistered the stimulation sites with magnetic res onance images in these same subjects using optical digitization. The stimul ation sites were mapped onto the surface of the occipital lobes in three-di mensional reconstructions of the cortical surface to show the distribution of the visual suppression effect. The results were consistent with disrupti on of secondary visual cortical areas.