Eye movements help capture optic-flow information necessary to perceive visually our self motion. Visual and vestibular systems control compensatory eye movements that serve to stabilize the retinal images we capture. We examined the role that these eye movements may play in generating visual illusions of self motion (or vection). Observers viewed radially expanding optic-flow displays while performing lateral translational head oscillations at 1 Hz. Simulated viewpoint changes in these displays were synchronized with head movements, either in an ipsilateral (minimal sensory conflict) or a contralateral (high sensory conflict) direction. In control conditions, the observer viewed purely radial displays. Vection-onset latency and overall vection strength ratings were recorded, as well as horizontal eye movements. Vection onsets and strength ratings were significantly greater when the observer's head movements were incorporated into the visual displays. However, vection strength ratings were very similar for both ipsilateral and contralateral active display oscillation. Surprisingly, the non-ecological contralateral viewpoint oscillation actually induced vection earlier, despite the relatively small eye-in-head rotations coordinating gaze in these conditions. Our results support the view that compensatory eye movements are controlled through cooperative visual and vestibular interactions, and show that linear vection is highly robust against large sensory conflicts.
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