Abstract
Spontaneous microsaccades are thought to routinely escape conscious detection. Here, we directly compare awareness of microsaccades with varying levels of conscious control and investigate how stimulus presentation around the time of these minuscule eye movements modulates awareness. In a first experiment, observers had no instructions regarding their eye movements (spontaneous microsaccade condition), except that gaze was restricted to stay within an area of 3 dva in diameter. In a second experiment, participants were instructed to either maintain fixation (involuntary microsaccade condition) or make a microsaccade (voluntary microsaccade condition) of specified direction and amplitude (0.2–1 dva). During the eye-movement interval, we displayed a vertically oriented grating with a rapid temporal phase shift (>60 Hz) that rendered it invisible during stable fixation. The grating could be detected, however, when slowed down on the retina by a microsaccade (active condition) or the replay of a microsaccades’ retinal consequence (replay condition). Stimulus-absent trials served as an additional control. In each trial, we asked observers to report if they perceived the stimulus (visual sensitivity), whether they believed to have generated a microsaccade (eye-movement sensitivity) and how confident they are that a microsaccade was the cause of their percept (causal sensitivity). Visual sensitivity was high for both active and replay condition trials, and comparable for spontaneous, involuntary, and voluntary microsaccades, peaking if the eye movement (or a replay) matched the stimulus direction and speed. Eye-movement sensitivity was overall low, in particular in the presence of a visual stimulus, and lower for spontaneous compared to voluntary and involuntary microsaccades, suggesting that knowledge of voluntary microsaccades increases eye-movement sensitivity. Finally, observers had a poor causal sensitivity regarding the relation between eye movement and stimulus percept. Taken together, our findings suggest that learning control over microsaccades is possible and increases conscious detection even for involuntary microsaccadic eye movements.