Caloric vestibular stimulation induces vestibular circular vection even with a conflicting visual display presented in a virtual reality headset.

This study explored visual-vestibular sensory integration when the vestibular system receives self-motion information using caloric irrigation. The objectives of this study were to (1) determine if measurable vestibular circular vection can be induced in healthy participants using caloric vestibular stimulation and (2) determine if a conflicting visual display could impact vestibular vection. In Experiment 1 (E1), participants had their eyes closed. Air caloric vestibular stimulation cooled the endolymph fluid of the horizontal semi-circular canal inducing vestibular circular vection. Participants reported vestibular circular vection with a potentiometer knob that measured circular vection direction, speed, and duration. In Experiment 2 (E2), participants viewed a stationary display in a virtual reality headset that did not signal self-motion while receiving caloric vestibular stimulation. This produced a visual-vestibular conflict. Participants indicated clockwise vection in the left ear and counter-clockwise vection in right ear in a significant proportion of trials in E1 and E2. Vection was significantly slower and shorter in E2 compared to E1. E2 results demonstrated that during visual-vestibular conflict, visual and vestibular cues are used to determine self-motion rather than one system overriding the other. These results are consistent with optimal cue integration hypothesis.

Perception of smooth and perturbed vection in short-duration microgravity.

Successful adaptation to the microgravity environment of space and readaptation to gravity on earth requires recalibration of visual and vestibular signals. Recently, we have shown that adding simulated viewpoint oscillation to visual self-motion displays produces more compelling vection (despite the expected increase in visual-vestibular conflict experienced by stationary observers). Currently, it is unclear what role adaptation to gravity might play in this oscillation-based vection advantage. The vection elicited by optic flow displays simulating either smooth forward motion or forward motion perturbed by viewpoint oscillation was assessed before, during and after microgravity exposure in parabolic flight. During normal 1-g conditions subjects experienced significantly stronger vection for oscillating compared to smooth radial optic flow. The magnitude of this oscillation enhancement was reduced during short-term microgravity exposure, more so for simulated interaural (as opposed to spinal) axis viewpoint oscillation. We also noted a small overall reduction in vection sensitivity post-flight. A supplementary experiment found that 1-g vection responses did not vary significantly across multiple testing sessions. These findings: (i) demonstrate that the oscillation advantage for vection is very stable and repeatable during 1-g conditions and (ii) imply that adaptation or conditioned responses played a role in the post-flight vection reductions. The effects observed in microgravity are discussed in terms of the ecology of terrestrial locomotion and the nature of movement in microgravity.

Shape constancy measured by a canonical-shape method.

Shape constancy is the ability to perceive that a shape remains the same when seen in different orientations. It has usually been measured by asking subjects to match a shape in the frontal plane with an inclined shape. But this method is subject to ambiguity. In Experiment 1 we used a canonical-shape method, which is not subject to ambiguity. Observers selected from a set of inclined trapezoids the one that most resembled a rectangle (the canonical shape). This task requires subjects to register the linear perspective of the image, and the distance and inclination of the stimulus. For inclinations of 30° and 60° and distances up to 1m, subjects were able to distinguish between a rectangle and a trapezoid tapered 0.4°. As the distance of the stimulus increased to 3m, linear perspective became increasingly perceived as taper. In Experiment 2 subjects matched the perceived inclination of an inclined rectangle, in which the only cue to inclination was disparity, to the perceived inclination of a rectangle with all depth cues present. As the distance of the stimulus increased, subjects increasingly underestimated the inclination of the rectangle. We show that this pattern of inclination underestimation explains the distance-dependent bias in taper judgments found in Experiment 1.