EFFECTS OF SINGLE-PULSE TRANSCRANIAL MAGNETIC STIMULATION OVER THE PREFRONTAL AND POSTERIOR PARIETAL CORTICES DURING MEMORY-GUIDED SACCADESIN HUMANS

1. We used single-pulse transcranial magnetic stimulation (TMS) to exp lore the temporal organization of the cortical control of memory-guide d saccades in eight humans. The posterior parietal cortex (PPC) or the dorsolateral prefrontal cortex (DPFC). which are both known to be inv olved in the control of such saccades, were stimulated on the right si de at different time intervals after the presentation of a Bashed late ral visual target. The memorization delay was 2,000 ms. Single pulses were applied at 160, 260, and 360 ms after the Bashed target, during t he period of 700 and 1,500 ms. and finally at 2,100 ms, i.e., 100 ms a fter the extinguishing of the central fixation point. The effects of T MS were evaluated by calculating the percentage of error in amplitude (PEA) and latency of memory-guided saccades. The PEA was determined fo r the primary saccade (motor aspect) and the final eye position, i.e., after the end saccade (mnemonic aspect). Stimulation over the occipit al cortex at the same time intervals served as control experiments. 2. After PPC stimulation, a significant increase in the PEA of the prima ry saccade and final eye position existed for contralateral saccades, compared with the PEA without stimulation, when stimulation was applie d 260 ms after target presentation, but not at other time intervals. T here was no significant effect on ipsilateral saccades. Latency was si gnificantly increased bilaterally when stimulation was performed 2,100 ms after target presentation. 3. After prefrontal stimulation, a sign ificant increase in the PEA of the primary saccade and final eye posit ion existed for contralateral saccades, when stimulation was applied b etween 700 and 1,500 ms after target presentation, but not at other ti me intervals. There was no significant effect on ipsilateral saccades. Latency was not affected by prefrontal TMS at any stimulation times. 4. Occipital stimulation resulted in no significant effect on the PEA and latency of ipsilateral or contralateral saccades, in particular in cluding the application at 260 ms after target presentation or during the memorization phase. 5. From these results it may be concluded that the observed effects of TMS on saccade accuracy were specific to the stimulated region and specific to the stimulation time. The PPC seems to be involved in the preparation of saccade amplitude, during the ear ly phase of the paradigm, i.e., the sensorimotor processing period, wh ereas the DPFC could play a role during the later phase of the paradig m, i.e., the memorization period. Therefore in humans these results su pport the experimental findings suggesting that sensorimotor integrati on is controlled by the PPC and spatial memory by the DPFC. Furthermor e, our results suggest that the PPC, although not the DPFC, plays a ro le in saccade triggering.