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 for this and similar phenomena has not been well defined. We modified a previously used technique to determine the locus of magnetic activation. Using butterfly stimulus coils of different sizes and electric field 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 occipital cortex rather than the primary visual area as the site of perceptual suppression. Less direct analyses involving the distribution of induced electric fields produced the same conclusion. These results suggest specific hypotheses about the effects of magnetic stimulation on visual physiology.