Subjects began each trial by fixating on a fixation point which appeared for 1500 ms (Fig.  1 c).

Single pulse TMS was delivered in randomly selected trials at randomly selected delays measured from the onset of the visual stimulus (Fig.  1 c) to posterior occipital cortex by a Cadwell MES-10 magnetic stimulator through a 7 cm diameter round coil.

We used a tilt discrimination task for this purpose, in which subjects decided if two foveally-presented, parallel black lines appearing within a white square were tilted to the right or left of vertical (Fig.  1 a).

In our version of this task a 5×5 array of black lines was presented within a 3°×3° white square in the center of a black computer screen (Fig.  1 b).

In trials containing an odd-element, it was randomly located in one of four oblique locations, one in each visual field quadrant (Fig.  1 b).