Eye movements in darkness modulate self-motion perception

During self-motion humans typically move the eyes to maintain fixation on the stationary environment around them. These eye movements could in principle be used to estimate self-motion, but their impact on perception is unknown. We had participants judge self-motion during different eye movement conditions in the absence of full-field optic flow. In a two alternative forced choice (2-AFC) task, participants indicated whether the second of two successive passive lateral whole-body translations was longer or shorter than the first. This task was employed in two experiments. In the first (n=8), eye movements were constrained differently in the two translation intervals by presenting either a world-fixed or body-fixed fixation point or no fixation point at all (allowing free gaze). Results show that perceived translations were shorter with a body-fixed than world-fixed fixation point. A linear model indicated that eye movement signals received a weight of approximately 25 percent for the self-motion percept. This model was independently validated in the trials without a fixation point (free gaze). In the second experiment (n=10), gaze was free during both translation intervals. Results show that the translation with the larger eye movement excursion was judged to be larger more often than chance, based on an oculomotor choice probability analysis. We conclude that eye movement signals influence self-motion perception, even in the absence of visual stimulation.