Magnetic stimulation of visual cortex: Factors influencing the perception of phosphenes

Using transcranial magnetic stimulation of occipital cortex, the authors studied the stimulus parameters that generate phosphenes in healthy volunteers. Single pulses or trains of stimuli readily elicited phosphenes in all subjects. The threshold current needed to elicit perception of phosphenes was essentially the same for stimulus trains from 250 msec to 2000 msec in length, but increased dramatically for trains of shorter duration. The effect of stimulus frequency was variable, with each subject having a distinctive 'frequency tuning curve,' but overall, the threshold current necessary to produce phosphenes decreased as frequency of stimulation increased. Using paired pulses, the perceptual threshold was flat for interstimulus intervals between 2 msec and 100 msec, but increased rapidly as the interstimulus interval was increased above 100 msec. Stimulation of sites lateral to the midline elicited phosphenes in the contralateral visual field. Phosphenes were dominant in the lower and peripheral aspects of the visual fields. The findings are discussed in relation to similar studies of electrical stimulation of somatosensory cortex.