The effect of expectations on slow oculomotor control—I. Periodic target steps

Abstract

Eye movements were recorded while subjects used saccades to track repetitive target steps at three frequencies (0.25, 0.375 and 0.50 Hz). The eye moved smoothly in the direction of expected target steps at velocities as high as 30′ sec beginning as early as 350 msec before the expected target step. Such anticipatory smooth eye movements were caused by expectations. They were not drifts toward the primary position. The expectation that a target would step and not the expectation that a saccade would be made caused the anticipatory smooth eye movements. Anticipatory smooth eye movements were found on both horizontal and vertical meridians, before small (10′) as well as large (426′) target steps, with a sequence of target steps in the same direction as well as with square-wave target motion and in the presence of textured visual backgrounds. They could not be abolished voluntarily and were found in all subjects—the two experienced subjects who knew the purpose of the experiments and also in the three naive inexperienced subjects. These results suggest that expectations of future target motions have important influences on the activities of the slow oculomotor subsystem.