MAGNETIC COIL SUPPRESSION OF VISUAL-PERCEPTION AT AN EXTRACALCARINE SITE

Perception of extrafoveal visual targets can be suppressed by magnetic stimulation over the occipital lobes, but the site of interference fo r this and similar phenomena has not been well defined. We modified a previously used technique to determine the locus of magnetic activatio n. Using butterfly stimulus coils of different sizes and electric fiel d profiles, we determined a scalp position of minimum threshold and a level of stimulator output that produced 50% error rates for each coil . Intersection of the corresponding electric field profiles in air and in a saline model head was similar and identified superficial occipit al cortex rather than the primary visual area as the site of perceptua l suppression. Less direct analyses involving the distribution of indu ced electric fields produced the same conclusion. These results sugges t specific hypotheses about the effects of magnetic stimulation on vis ual physiology.