Effects of discrepant optic flow during walking on the perceived visual and proprioceptive straight ahead in egocentric space

Abstract

Previous research has shown that discrepant optic flow experienced during walking with prism glasses can shift both the visual and proprioceptive straight ahead (SA) in egocentric space. Here by having participants walk in an immersive virtual environment, we examined how adding optic flow information in the scene affected such shifts and whether such shifts increased with prolonged exposure to discrepant optic flow. Nineteen participants wore a head-mounted display (44°Hx34°V) and walked toward a red post target placed on a textured ground or a doorway on the back wall of a room. The target and the doorway were both at 8 m. The room display provided denser flow and motion parallax information than did the textured ground display. Participants’ visual heading specified by optic flow was displaced by ±10° from their physical walking direction, causing discrepant optic flow. We measured participants’ VSA and PSA before walking, after 10 trials, 20 trials, and 30 trials of walking, respectively. For VSA measurement, the experimenter moved a light spot on a wall and participants judged when it was at their SA. For the PSA measurement, participants were blindfolded and used a laser pointer to point at their SA. We found a significant shift in PSA for both the room and the textured ground displays. The shift in PSA was larger than that in VSA for both displays, and the shift in VSA was significant for the room (about 2°) but not the textured ground display. The shifts in both PSA and VSA increased with the number of trials tested for the room but not the textured ground display. We conclude that discrepant optic flow during walking recalibrates PSA in egocentric space more than VSA, and this recalibration increases with the exposure to discrepant optic flow when the display contains rich optic flow information.