Phosphenes and transient scotomas induced by magnetic stimulation of the occipital lobe: their topographic relationship

Transcranial magnetic stimulation (TMS) of the visual cortex is known to in duce phosphenes and is able to suppress visual perception. To address the t opographic relationship of phosphenes and transient scotomas, the visual fi eld of 10 normal subjects was investigated using a perimetric approach. The central visual field (diameter: 20 degrees) was tested at 32 sites. Percep tual thresholds were determined by presenting I ms test spots flashed with varying intensity in random order. TMS was applied with a focal figure-of-e ight coil placed over the inion. All subjects perceived phosphenes, mostly restricted to one of the lower quadrants within the visual held. In 13 out of 15 investigations, a magnetic stimulus triggered 100 ms after the visual target resulted in a relative scotoma with threshold changes of 8 dB or mo re. In 9 of 13 investigations, scotomas coincided spatially with sketches o f phosphenes made by subjects in a separate test. Scotomas covered only a s mall percentage of the total visual field, which may explain the failure of previous studies to find perceptual suppression with the focal coil. The p resent result demonstrates that phosphenes evoked during TMS can serve as a guide for optimal visual stimulus alignment in neuropsychological experime nts. (C) 1999 Elsevier Science Ltd. All rights reserved.