Linear vection in virtual environments can be strengthened by discordant inertial input

Visual and gravitoinertial sensory inputs are integrated by the central nervous system to provide a compelling and veridical sense of spatial orientation and motion. Although it's known that visual input alone can drive this perception, questions remain as to how vestibular/ proprioceptive (i.e. inertial) inputs integrate with visual input to affect this process. This was investigated further by combining sinusoidal vertical linear oscillation (5 amplitudes between 0m and +/-0.8m) with two different virtual visual inputs. Visual scenes were viewed in a large field-of-view head-mounted display (HMD), which depicted an enriched, hi-res, dynamic image of the actual test chamber from the perspective of a subject seated in the linear motion device. The scene either depicted horizontal (+/-0.7m) or vertical (+/-0.8m) linear 0.2Hz sinusoidal translation. Horizontal visual motion with vertical inertial motion represents a 90 degrees spatial shift. Vertical visual motion with vertical inertial motion whereby the highest physical point matches the lowest visual point and vice versa represents a 180 degrees temporal shift, i.e. opposite of what one experiences in reality. Inertial-only stimulation without visual input was identified as vertical linear oscillation with accurate reports of acceleration peaks and troughs, but a slight tendency to underestimate amplitude. Visual-only (stationary) stimulation was less compelling than combined visual+inertial conditions. In visual+inertial conditions, visual input dominated the direction of perceived self-motion, however, increasing the inertial amplitude increased how compelling this non-veridical perception was. That is, perceived vertical self-motion was most compelling when inertial stimulation was maximal, despite perceiving "up" when physically "down" and vice versa. Similarly, perceived horizontal self-motion was most compelling when vertical inertial motion was at maximum amplitude. "Cross-talk" between visual and vestibular channels was suggested by reports of small vertical components of perceived self-motion combined with a dominant horizontal component. In conclusion, direction of perceived self-motion was dominated by visual motion, however, compellingness of this illusion was strengthened by increasing discordant inertial input. Thus, spatial mapping of inertial systems may be completely labile, while amplitude coding of the input intensifies the percept.