ACTIVITY LINKED TO EXTERNALLY CUED SACCADES IN SINGLE UNITS RECORDED FROM HIPPOCAMPAL, PARAHIPPOCAMPAL, AND INFEROTEMPORAL AREAS OF MACAQUES

We studied whether target-directed, externally commanded saccadic eye movements (saccades) induced activity in single units in inferotempora l cortex, the hippocampal formation, and parahippocampal gyrus. The mo nkeys first were required to fix their gaze on a small cross presented to the left or right of center on the monitor screen. The cross was e xtinguished, and a random 600-1,000 ms thereafter, a small dot was pre sented for 200 ms. The dot was located either 10 degrees above, below, right, or left of the position on which the fixation cross had been. The monkey made a saccadic eye movement to this dot (in darkness). The neuronal activity around this goal-directed saccade was analyzed. In addition. control conditions were imposed systematically in which simi lar dots were presented, but the monkey's task was to withhold the sac cade. We recorded 290 units from two monkeys. From this group, 134 met two criteria, they did not show visual response in control trials and they had spike rates >2 Hz. These were analyzed further; 53% (71/134) showed modulation related to the target directed saccade, and 29% (39 /134) showed saccadic modulation during spontaneous eye movements. The se two groups were correlated only weakly. Of the units with significa nt saccadic modulation, 17% (12/71) showed significant directional sel ectivity, and 13% (9/71) showed significant position selectivity (P < 0.01). At a lower criterion (P < 0.05), almost one-half (33/71) showed one or the other spatial selectivity. Primates use saccades to acquir e visual information. The appearance of strong saccadic modulation in brain structures previously characterized as mnemonic suggests the pos sibility that the mnemonic circuitry uses an extraretinal signal linke d to saccades to control visual memory processes, e.g., synchronizing mnemonic processes to the pulsatile visual data inflow.