Abstract
The normal coupling of action and perception can be re-calibrated by unusual contexts. After running on a treadmill, subjects experience an illusion of increased optic flow rate when walking on solid ground (Pelah & Barlow, 1996). Despite the importance of this finding, prior techniques for quantifying the illusion were not very precise. In the present study, we sought to quantify the illusion using virtual reality. We found that adaptation to walking on a treadmill in a static virtual environment is sufficient for producing a measurable illusion.
Eleven subjects walked continuously on a treadmill while wearing a stereo HMD depicting a fully textured moving corridor. Subjects made FC judgments about whether the speed of the corridor was too fast or too slow relative to their own speed on the treadmill. A multiple-staircase procedure was used to determine the point at which the visual corridor speed was perceived as corresponding to walking speed (PSE). Each subject then walked on the treadmill for a 2-minute adaptation period during which the virtual corridor was stationary. Again, a staircase method was used to determine PSE, but with a brief period of re-adaptation to the stationary corridor before each trial.
As expected, following adaptation to the stationary corridor the average gain at PSE was reliably less than the average gain before adaptation (by about 10%, p < .01). Although inter-subject variability was high, all but 1 of the 11 subjects showed a decreased PSE following adaptation.
The results show that walking on a treadmill in a static world is sufficient to produce an illusion of increased optic flow during subsequent locomotion. Moreover, we have developed a new technique for measuring changes in a subject's perceived matching point between locomotor activity and optic flow. Further studies of changes in expected gain using this technique may be very useful to understanding the visual variables involved in the recalibration process.
Swarthmore College Faculty Research Grant