Introduction: Self-motion relative to the stationary environment produces a globally consistent pattern of visual motion on the retina known as optic flow. Local motion signals inconsistent with the global flow are generated by objects moving relative to the scene. The nervous system must therefore parse retinal image motion to estimate object motion and self-motion separately. Here we investigate whether simultaneous vestibular self-motion facilitates this parsing process.

Methods: Experiments were conducted using a motion platform and attached visual display. There were two conditions, Visual-only and Combined (Visual/Vestibular), and trials for these conditions were interleaved. The visual stimulus consisted of a 3D starfield and a spherical object located to the left of the fixation point; both the starfield and the object were composed of randomly placed, limited-lifetime, frontoparallel triangles rendered in stereo. On each trial, the visual stimulus (and motion platform on Combined trials) simulated an earth-horizontal translation of the subject relative to the world and simultaneous vertical displacement of the spherical object upwards or downwards. Subjects were asked to discriminate the direction of object movement. Object displacement (and velocity) was varied from trial to trial according to a staircase procedure and psychometric functions (cumulative Gaussian) were fit to estimate discrimination thresholds. This procedure was repeated for forward, lateral (rightward), and two intermediate heading angles.

Results & Conclusions: Repeated measures ANOVA with condition (Visual-only or Combined) and heading direction as factors revealed a significant effect of condition (p=0.03) and heading direction (p<0.001) and a significant interaction (p=0.02). Thresholds were reduced in the Combined condition relative to the Visual-only; optic flow parsing was facilitated by congruent vestibular stimulation. Thresholds were lowest for lateral heading (laminar flow) and highest for forward movement (radial flow), probably because of the geometry of the optic flow pattern. Finally, vestibular facilitation was greatest for lateral heading but negligible for forward movement.