Microsaccadic modulation in goal-directed reaching.

Previous work has demonstrated that microsaccades play a key role in both preparation for upcoming sensory stimuli and visual attention. Microsaccadic modulation prior to a predictable event is observed with visual, tactile, and auditory stimuli, suggesting a common function across sensory modalities. This study investigated how microsaccade modulation relates to sensorimotor control in goal-directed reaching. We designed a task in which participants positioned their hand (represented as a cursor) at a start position and then fixated a target, which changed color after a variable delay. Depending on the condition, in response to this color change (go cue), participants waited for the trial end (cue monitoring), watched a cursor move with biological motion from the hand start position to the target (cursor tracking), or reached to the target either with (reach visible) or without (reach invisible) vision of the hand. We found that the rate of microsaccades was consistently reduced in anticipation of the visual cue. Microsaccade reduction immediately before the go cue was greatest in conditions that required active hand movement, suggesting that movement preparation added attentional load to the sensorimotor system. Following the go cue, microsaccades remained reduced in all conditions, indicating that not only external events but also self-generated movements are associated with event prediction. Yet, the reduction in microsaccade rate was not greater in the movement conditions, suggesting that sensorimotor control processes that are involved in predicting the sensory consequences of motor commands neither enhance nor reduce microsaccade rate.