Hemispheric asymmetry in cortical control of memory-guided saccades. A transcranial magnetic stimulation study.

To study the temporal organisation of memory-guided saccade control we used single-pulse transcranial magnetic stimulation (TMS) over the left posteri or parietal (PPC) and prefrontal cortex (PFC) in eight healthy subjects. TM S was applied either following presentation of a visual target, i.e. 160, 2 60, and 360 ms after the flashed point, or during the period of memorisatio n, i.e. between 700 and 1500 ms. or finally 100 ms after extinguishing of t he central fixation point (i.e. 2100 ms after the target presentation). Lat ency of memory-guided saccades and the percentage of error in amplitude (PE A) was measured and compared with results without stimulation. TMS over the left PPC 100 ms after the extinguishing of the central fixatio n point significantly increased memory-guided saccade latency bilaterally. Furthermore, stimulation over the left PFC had a significant effect on the PEA of contralateral memory-guided saccades when applied during the period of memorisation, i.e. between 700 and 1500 ms. In a previous study using identical methodology [13. Muri RM, Vermersch st, Rivaud S, Gaymard B, Pierrot-Deseilligny C. Effects of single-pulse transc ranial magnetic stimulation over the prefrontal and posterior parietal cort ices during memory-guided saccades in humans. Journal of Neurophysiology 19 96;76:2102-2106], we found that TMS over the right PPC increased the contra lateral PEA when applied 260 ms after the flash, the effects on saccade lat ency after right PPC stimulation or on the PEA after right PFC stimulation being similar to those observed here. Taken together, these results show th at (1) a hemispheric asymmetry in the preparation of memory-guided saccade amplitude during the early phase of sensorimotor integration exists, (2) me mory-guided saccade triggering is controlled by PPC on both sides, and (3) PFC on both sides are involved in spatial working memory performance. (C) 2 000 Elsevier science Ltd. All rights reserved.