EFFECTS OF SINGLE PULSE TRANSCRANIAL MAGNETIC STIMULATION OVER THE PREFRONTAL AND POSTERIOR PARIETAL CORTICES DURING MEMORY-GUIDED SACCADES IN HUMANS. Muri, Rene M., Anne-Isabelle Vermersch, Sophie Rivaud, Bertrand Gaymard, Charles Pierrot-Deseilligny. Service de Neurologie et INSERM U 289, H[circumflex]opital de la Salp[circumflex]etri[grave]ere, 47 Boulevard de l'H[circumflex]opital, F-75651 Paris CEDEX 13 , France and Department of Neurology, University of Bern, Inselspital, CH-3010 Bern , Switzerland.
APStracts 3:0119N, 1996.
SUMMARY AND CONCLUSIONS
1. We used single pulse transcranial magnetic stimulation (TMS) to explore the temporal organization of the cortical control of memory-guided saccades in 8 humans. The posterior parietal cortex (PPC) or the dorsolateral prefrontal cortex (DPFC), which are both known to be involved in the control of such saccades, were stimulated on the right side at different time intervals after the presentation of a flashed lateral visual target. The memorization delay was 2000 ms. Single pulses were applied at 160 ms, 260 ms, and 360 ms after the flashed target, during the period of 700 ms and 1500 ms, and finally at 2100 ms, i.e. 100 ms after the extinguishing of the central fixation point. The effects of TMS were evaluated calculating the percentage of error in amplitude (PEA) and latency of memory-guided saccades. The PEA was determined for the primary saccade (motor aspect) and the final eye position, i.e. after the end saccade (mnemonic aspect). Stimulation over the occipital cortex at the same time intervals served as control experiment. 2. After PPC stimulation, a significant increase in the PEA of the primary saccade and final eye position existed for contralateral saccades, compared to the PEA without stimulation, when stimulation was applied 260 ms after target presentation, but not at other time intervals. There was no significant effect on ipsilateral saccades. Latency was significantly increased bilaterally when stimulation was performed 2100 ms after target presentation. 3. After prefrontal stimulation, a significant increase in the PEA of the primary saccade and final eye position existed for contralateral saccades, when stimulation was applied between 700 ms and 1500 ms after target presentation, but not at other time 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 ipsi- or contralateral saccades, in particular including 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 early phase of the paradigm, i.e. the sensorimotor processing period, whereas the DPFC could play a role during the later phase of the paradigm, i.e. the memorization period. Therefore, in humans these results support the experimental findings suggesting that sensorimotor integration is controlled by the PPC and spatial memory by the DPFC. Furthermore, our results suggest that the PPC, however not the DPFC, plays a role in saccade triggering. 

Received 23 February 1996; accepted in final form 21 May 1996.
APS Manuscript Number J143-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 17 June 96