Visual reorientation illusion (VRI) is a phenomenon in which one’s perception of body positioning (e.g. standing vs. lying down) or motion (e.g., moving vs. static) disagrees with reality. It can be induced by tricking the brain with false sensory data: rotating a room 360º about a horizontal axis, for example, or tilting a room 90º with familiar objects (e.g. a lamp) whose orientation disagrees with gravity.
Beyond the purely scientific interest in understanding how differing and contrary signals are interpreted by the brain, being able to reliably produce (or combat) sensory illusions has practical applications in transportation and the future of immersive gaming and virtual experiences. In the present study, published in PLoS ONE, scientists from York University in Canada took a closer look at how the brain weighs contradictory sensory signals.
The series of three studies, involving a total of 72 participants, used supine (lying on one’s back) and supine-like (head tilted back), prone (lying face-down) and prone-like (head tilted forward), and standing positions to manipulate sensations of gravity, while a VR helmet provided corollary or contradictory visual stimuli, in the form of either a hallway (strong up-down signals) or starfield (ambiguous up-down signals).
The authors were primarily interested in understanding how contradictory motion signals are weighted and interpreted. Participants thus “moved” through their virtual environment relative to an indicated point, and were asked to estimate their displacement.
The authors were able to provide evidence for one of two competing hypotheses: the additive hypothesis and the cognitive hypothesis. Indeed, mis-estimation of displacement was much stronger in the structured, hallway environment, and was mediated by the experience of a VRI.
On the other hand, when motion agreed with gravity (e.g., a supine person experiencing “forward” motion), the effect was found not to be additive, but instead to depend on whether visual signals dominated over vestibular (gravitational) signals.
This tendency to prioritize visual signals over posture enables us to better understand how the human brain aggregates (or dismisses) conflicting sensory information and why certain situations lead to reweighting one as more important than the other. Given that humans experience sensory illusions on a daily basis (while driving, for example), understanding these phenomena can lead to safer environments, and has obvious practical applications for entertainment.
“On Earth, the brain has to constantly decide whether a given acceleration is due to a person’s movements or to gravity. This decision is helped by the fact that we normally move at right angles to gravity. But if a person’s perception of gravity is altered by the visual environment or by removing gravity, this distinction becomes much harder,” said study author Laurence Harris.
“The findings reported in this paper could be helpful when we land people on the Moon again, on Mars, or on comets or asteroids, as low-gravity environments might lead some people to interpret their self-motion differently — with potentially catastrophic results.”
The study, “When gravity is not where it should be: How perceived orientation affects visual self-motion processing,” was authored by Meaghan McManus and Laurence R. Harris.