TMS over the left posterior parietal cortex prolongs latency of contralateral saccades and convergence

This study explored the role of the left posterior parietal cortex (PPC) in saccades, vergence, and combined saccade-vergence movements by means of transcranial magnetic stimulation (TMS) in humans.

TMS was applied over the left PPC for 80, 90, or 100 ms after target onset. In a control experiment, TMS was applied over the primary motor cortex at 90 ms after target onset.

Relative to no TMS trials, TMS over the left PPC increased the latency of only certain types of eye movements: saccades to right, convergence, and convergence combined with rightward saccades. It had no such effect on saccades to the left, divergence, saccades combined with divergence, or left saccades combined with convergence. TMS over the vertex had no effect on the latency of any type of eye movement. Thus, the effects of TMS on latency are both area- and eye-movement specific. TMS of the left PPC or the motor cortex did not alter the accuracy of any type of eye movement. The results are in contrast to those of prior studies of the right PPC, which is involved in the initiation of any type of eye movement.

The findings suggest that the right PPC is involved primarily in the processing of fixation disengagement, whereas the left PPC provides the signal that is also necessary to initiate eye movements and could occur in parallel with the process (model of Findlay and Walker). Such a functional role of the left PPC seems to be topographically organized and concerns a saccade to the right and convergence alone or combined with a saccade to the right.