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Abstract:
Transcranial magnetic stimulation (TMS) of the visual cortex is known to induce phosphenes and is able to suppress visual perception. To address
the topographic relationship of phosphenes and transient scotomas, the visual field of 10 normal subjects was investigated using a perimetric approach. The
central visual field (diameter: 20 degrees) was tested at 32 sites. Perceptual thresholds were determined by presenting I ms test spots flashed with varying
intensity in random order. TMS was applied with a focal figure-of-eight 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 more. In 9 of 13 investigations, scotomas coincided spatially with sketches of phosphenes made by subjects in a
separate test. Scotomas covered only a small percentage of the total visual field, which may explain the failure of previous studies to find perceptual suppression
with the focal coil. The present result demonstrates that phosphenes evoked during TMS can serve as a guide for optimal visual stimulus alignment in
neuropsychological experiments. (C) 1999 Elsevier Science Ltd. All rights reserved.
