Abstract
Perceiving the direction of self-motion is typically a multisensory process. The most effective cue to detect the observer's heading direction is the visual optic-flow pattern. In the initial period of body movement, however, the vestibular sense is another effective cue to detect the direction in which one's body started to move. Here I report our recent research on the perception of self-motion, 1) when observers are passively experiencing their real somatic motion in different body posture, and 2) when observers are actively moving their bodies forward and backward. Previously, we reported that when upright observers passively experience real linear oscillatory somatic motion (leftward/rightward or forward/backward) while viewing orthogonal visual optic flow patterns (translating or expanding/contracting), their perceived body motion direction is intermediate to those specified by visual and vestibular information individually (Sakurai et al., 2002, ACV; 2003, ECVP; Sakurai et al., 2010, VSS; Kubodera et al., 2010, APCV). We then generalized those findings exploring other visual/vestibular combinations, investigating when the vertical axis of body coordinates is orthogonal to the gravity axis. Observers lay supinely and reported their perceived direction of self-motion, experiencing real upward/downward or leftward/rightward motion in body coordinates, while viewing orthogonal optic-flow patterns that were phase-locked to the swing motion. The results are very similar to our previous reports, but for combinations of body motion with visual expanding/contracting optic-flow, some observers' judgments were vision-only or vestibular-only, suggesting that multimodal integration in this context is an either-or process for these observers. Compared to our previous reports, one possible reason for this weighted combination failure is the discrepancy between body coordinates and gravity coordinates (Sakurai et al., 2011, ECVP). We recently extended those studies to active somatic motion, measuring the angular shift in body direction after active body motion while viewing synchronized orthogonal optic flow. Observers viewed the stimulus of translating leftward (rightward) random-dots for 30 seconds through a face-mounted display, while actively stepping forward and backward such that their forward body movement was synchronized with the random-dot translational motion. Observers' body direction was measured before and after each trial, and was compared with that of control condition (random noise). Translational optic flow induced shifts in body direction that were opposite to shifts in perceived direction with passive viewing in our previous reports. Observers may have compensated their body motion in response to perceived direction shifts similar to those we reported for passive viewing.