Sensorimotor awareness requires intention: Evidence from minuscule eye movements
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Microsaccades are tiny eye movements that are thought to occur 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, unintended, and spontaneous microsaccades. In addition, we dissociated the effects of action intention and the actions’ visual consequences on awareness. In 80% of all trials, we presented a stimulus at high temporal frequency rendering it invisible during stable fixation. Critically, the stimulus became visible when a microsaccade in the same direction caused it to slow down on the retina (generated microsaccade condition; 40% of trials) or when the microsaccades’ visual consequence was replayed (replayed microsaccade condition; 40% of trials). 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 eye movements, showing that 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.
Significance statement
While eye movements are among the most frequent human actions, they are rarely perceived consciously, despite causing sweeping changes in retinal inputs. Here investigate how intention can modulate awareness of even the smallest human actions: microsaccades. We developed a novel paradigm that allowed us to dissociate the role of action intention and an action’s sensory consequence for awareness, two factors that previous research has typically confounded. Our data provide strong evidence that observers can detect small eye movements reliably and demonstrates that sensitivity towards microsaccades was neither driven by an eye movement’s motor component nor its visual consequences alone. Instead, we find that intention opens a gate to sensorimotor awareness, even for actions typically too small to be perceived.
