[Examination of the visual system with transcranial magnetic stimulation]

The influence of transcranially applied magnetic stimuli on the function of the afferent (sensory) and efferent (motor) parts of the visual system have been discussed. Excitatory (positive) phenomena are subjective photic sensations (phosphenes) which can be elicited by transcranial magnetic stimulation over occipital parts of the skull. The phosphenes appear on the left or right side of the visual field depending upon the direction of the coil currents, which determines whether the visual cortex of the right or the left hemisphere is activated. The configuration of the phosphene fields hints at an excitation of the primary visual cortex (Brodmann's area 17). However, magnetic brain stimulation also produces inhibitory (negative) phenomena. When strong magnetic field pulses are applied over the primary visual cortex, foveally presented visual stimuli cannot be identified even when no phosphenes are perceived at the same time. Depending on the position of the stimulation coil, this suppression of perception can be restricted to visual stimuli presented on the right, or left of, above or below the fixation point. No generation or disturbance of eye movements by transcranial magnetic stimulation has been reported before, except for a delay of saccades within a reaction time paradigm.