Abstract
Backscroll illusion is apparent motion perceived in backgrounds of movie images that present locomotive objects such as people, animals, or vehicles (Fujimoto & Sato, in press, Vision Research). Here we report that the backscroll illusion can occur in far peripheral vision at retinal eccentricity of more than 30 deg. This was confirmed by psychophysical experiments in which we presented a walking person in profile against an ambiguously moving background of vertical counterphase grating. This stimulus, which subtended 15 × 15 deg of visual angle, was projected on a hemispheric screen and positioned at horizontal eccentricity between 0 and 50 deg at intervals of 10 deg. The eccentricity was changed randomly trial by trial and stimulus duration was as short as 360 ms, so that observers could not move their eyes to the stimulus effectively. Six observers viewed the stimulus monocularly or binocularly and reported a perceptual impression for the grating in a three alternative forced choice procedure: drifting left, drifting right or flickering. Results showed that the grating appeared to move in the opposite direction of walking in more than 77% of trials on average at 0–40 deg eccentricity. The percept was stronger in peripheral vision than in central vision. Even at 50 deg eccentricity, the illusion occurred at 60% levels. Additional experiments confirmed that walking action could be recognized from the far peripheral stimulation. Our findings suggest that the visual system uses high-level object-centered motion signals to disambiguate low-level retinocentric motion signals in the whole visual field.