Sensorimotor awareness requires intention: Evidence from minuscule eye movements

Microsaccades are tiny eye movements typically occurring spontaneously and without awareness but can also be intentionally controlled with high precision. We used these tiny visual actions to investigate how intention modulates sensorimotor awareness by directly comparing intended (upon instruction), unintended (occurring despite instruction to fixate), and spontaneous microsaccades. In addition, we dissociated the effects of action intention and the actions' visual consequences on awareness. To achieve this, we presented a stimulus at high temporal frequency rendering it invisible during stable fixation. Critically, this stimulus became visible when it slowed down on the retina, either incidentally, due to a microsaccade with comparable direction and speed, or physically, when replaying the retinal consequence of previous microsaccades. Trials without a stimulus were included as control. Participants reported whether they perceived the stimulus (visual sensitivity), whether they believed they had made a microsaccade (microsaccade sensitivity), and their level of confidence that their eye movement behavior was linked to their perception (causality assignment). Visual sensitivity was high for both generated and replayed microsaccades and comparable for intended, unintended, and spontaneous eye movements. Microsaccade sensitivity, however, was low for spontaneous microsaccades, but heightened for both intended and unintended movements. Thus, the intention to saccade or fixate enhances awareness of otherwise undetected eye movements. Visual consequences failed to aid eye movement awareness, and confidence ratings revealed a poor understanding of a causal relationship between eye movement and sensory consequence. These findings highlight the functional relevance of intention in sensorimotor awareness at the smallest scale of visual actions.