Early visual cortical processing suggested by transcranial magnetic stimulation.

Single-pulse transcranial magnetic stimulation (TMS) was applied to the occipital pole of healthy subjects while they performed a forced-choice visual letter-identification task. Pulses were applied on the midline but with a left-right asymmetric polarity; pulse application occurred at a variable delay after letter presentation onset; letters were presented in left or right hemifield. Averaging data over subjects and hemifields showed that performance attained local minima at 20 ms and 100 ms; averaging data over subjects and delays showed that performance was biased towards the same hemifield during both delay intervals; averaging data over subjects showed that the hemifield bias progressively decreased from 20 ms to 50 ms. The data are consistent with the possibility that also the earlier delay interval reflects visual cortical processing.