The effect of gain adaptation on perception and posture.

Consistent motion simulation in Virtual Reality (VR) applications is challenging due to constraints on tracking technologies and locomotion or interaction paradigms that scale motion. In this study, we measured (a) the point of subjective stationarity (PSS) during active self-motion, (b) postural sway in the dark, and (c) visually-induced postural sway during quiet stance, both before and after adapting to different gains between physical self-motion and the motion portrayed in the virtual environment. Participants adapted to each of three adaptation gain levels in separate blocks: normal, reduced, and increased, in which observers' physical motion was scaled and displayed as the virtual motion. We measured PSS during active self-motion and postural sway during quiet stance in both left-right and front-back directions in separate sessions. We found that the PSS measured during self-motion did not vary with adaptation gain. However, postural sway elicited by visual perturbation was modulated after adapting to non-unity gains. We also measured baseline postural sway prior to adaptation and found that exposure to virtual motion under unity (normal) gain increased the postural variability along the left-right direction, when tested without visual feedback (in dark). Collectively these results suggest that while observers adapt to gain, active self-motion provides sufficient somatosensory feedback to counteract the shift in perceived motion. As a result, PSS remains consistent across all gain manipulations in our experiment setup. In contrast, postural responses during quiet stance did not recalibrate immediately after motion gain perturbation was removed suggesting that they operate independently of perceptual mechanisms.