The Neural Control of Visually Guided Eye Movements
C. Cortical Mechanisms of Visually Guided Saccadic Eye Movements

To study target selection, paired targets were presented and monkeys had to make saccadic eye movements to one or the other of the targets to be rewarded. This is shown in the top of Figure 12. The temporal asynchrony with which the targets appeared was varied randomly from trial to trial. Normal monkeys tend to select the left and right targets with equal probability when they appear simultaneously. Unilateral removal of the frontal eye fields causes a major, long-lasting deficit in target selection; the target appearing in the contralateral hemifield has to be presented much earlier than the other target to yield equal probability choices. By contrast, removal of the medial eye fields has only a minor effect on target choice and the execution of sequences of eye movements.