VISUALIZATION OF THE INFORMATION-FLOW THROUGH HUMAN OCULOMOTOR CORTICAL REGIONS BY TRANSCRANIAL MAGNETIC STIMULATION

We investigated the topography of human cortical activation during an antisaccade task by focal trans-cranial magnetic stimulation (TMS). We used a figure-eight shaped coil, with the stimulus intensity set just above the threshold for activation of the hand motor areas but weak e nough not to elicit blinks. TMS was delivered at various time interval s(80, 100, and 120 ms) after target presentation over various sites on the scalp while the subjects performed the antisaccade task. It was p ossible to elicit a mild but significant delay in saccade onset over 1 ) the frontal regions (a region 2-4 cm anterior and 2-4 cm lateral to hand motor area) and 2) posterior parietal regions (6-8 cm posterior a nd 0-4 cm lateral to hand motor area) regardless of which hemisphere w as stimulated. The frontal regions were assumed to correspond to a cor tical region including the frontal eye fields (FEFs), whereas the pari etal regions were assumed to represent a wide region that includes the posterior parietal cortices (PPCs). The regions inducing the delay sh ifted from the posterior parietal regions at an earlier interval 80 ms )to the frontal regions at a later interval (100 ms), which suggested an information flow from posterior to anterior cortical regions during the presaccadic period. At 120 ms, the effect of TMS over the frontal regions still persisted but was greatly diminished. Erroneous prosacc ades to the presented target were elicited over a wide cortical region including the frontal and posterior parietal regions, which again sho wed a forward shift with lime. However, the distribution of effective regions exhibited a clear contralateral predominance in terms of sacca de direction. Our technique provides a useful method not only for dete cting the topography of cortical regions active during saccadic eye mo vement, but also for constructing a physiological map to visualize the temporal evolution of functional activities in the relevant cortical regions.