INTERACTION OF THE 2 FRONTAL EYE FIELDS BEFORE SACCADE ONSET

A normal environment often contains many objects of interest that comp ete to attract our gaze. Nevertheless, instead of initiating a flurry of conflicting signals, central populations of oculomotor neurons alwa ys seem to agree on the destination of the next saccade. How is such a consensus achieved? In a unit recording and microstimulation study on trained monkeys, we sought to elucidate the mechanism through which s accade-related cells in the frontal eye fields (FEF) avoid issuing com peting commands. Presaccadic neuronal activity was recorded in one FEF while stimulating the contralateral FEF with low-intensity currents t hat evoked saccades. When an eye-movement cell was isolated, we determ ined: the movement field of the cell, the cell's response to contralat eral FEF microstimulation. the cell's response when the evoked saccade was in the preferred direction of the cell (using the collision techn ique to deviate appropriately the evoked saccade vector), and the cell 's response to a stimulation applied during a saccade in the cell's pr eferred direction, to reveal a possible inhibitory effect. Complete re sults were obtained for 71 stimulation-recording pairs of FEF sites. T he unit responses observed were distributed as follows. 35% of the cel ls were unaffected, 37% were inhibited, and 20% excited by contralater al stimulation. These response types depended on the site of contralat eral stimulation and did not vary when saccades were redirected by col lision, This invariant excitation or inhibition of cells, seemingly du e to hardwired connections, depended on the angular difference between their preferred vector and the vector represented by the cells stimul ated. By contrast, 8% of the cells were either activated or inhibited depending on the vector of the saccade actually evoked by collision. T hese results suggest that the consensus between cells of oculomotor st ructures at the time of saccade initiation is implemented by functiona l connections such that the cells that command similar movements mutua lly excite each other while silencing those that would produce conflic ting movements. Such a rule would be an effective implementation of a winner-take-all mechanism well suited to prevent conflicts.