Topographic analysis of human cortical potentials preceding self-initiated and visually triggered saccades.

The topography of human brain potentials preceding self-initiated saccadic eye movements was compared with those preceding visually triggered saccades to examine differences in presaccadic activity under different programming requirements. A slow negative shift beginning approximately 650 ms before eye movement and largest in amplitude over the frontal region was recorded only in the self-initiated condition. This potential presumably reflects preparatory activity occurring predominantly within the frontal eye fields. A ramp-like positivity preceding eye movements by 100-250 ms and a sharp positive potential peaking 10-40 ms before saccades were recorded under both conditions. These potentials were of maximum amplitude over the parietal region. In order to examine the extent to which presaccadic enhancement of parietal activity was related to executive or to attentional mechanisms, the presaccadic visual evoked potential components were compared with activity at the same latency when stimuli were counted without saccades or were ignored. Enhancement of parietal potentials was seen both with counting and with saccades, but was greater in the eye movement condition. Occipital enhancement predominated with counting and parietal enhancement with saccades. These findings suggest that attentional and command mechanisms contribute distinct complementary increments in posterior cortical presaccadic activity associated with visually triggered saccades.