SUPPRESSION OF VISUAL-PERCEPTION BY TRANSCRANIAL MAGNETIC STIMULATION - EXPERIMENTAL FINDINGS IN HEALTHY-SUBJECTS AND PATIENTS WITH OPTIC NEURITIS

The influence of noninvasive magnetic brain stimulation by a magnetic coil (MC) placed over the occiput on perception and correct reporting of a briefly presented set of 3 letters of the alphabet was examined in 15 patients with prolonged VEP latencies due to neuritis of the optic nerve. The results derived from observing these patients were compared to the results obtained from an age-matched control group of 20 healthy voluntary subjects examined under the same experimental conditions. In both groups it was possible to demonstrate that transcranial magnetic stimulation is able to suppress recognition of the letters if applied with a certain delay time after a brief presentation of the visual stimulus. The groups were compared to each other with regard to the delay with which it was possible to demonstrate the most effective suppression. In the healthy subjects, this delay was found between 60 and 100 msec. In the patients, it was prolonged to 80-140 msec. This prolongation was closely related to the VEP latency (P100). Furthermore, visual suppression and the influence on it by different parameters were studied in detail in healthy subjects; the visual suppression depends on visual (e.g., brightness, duration) and magnetic (e.g., intensity) stimulus conditions. The method described seems to be of considerable value in the investigation of basic mechanisms of visual perception. This includes pathophysiological changes caused by optic neuritis and possibly other disorders affecting the visual system.