The present results demonstrated that peripheral vision can perceive the backscroll illusion. The probabilities observed in the far periphery were as high as those in the center. Monocular viewing lowered the probability in the far temporal retina. This result can be explained by the constraints of input level. That is, the density of retinal cells in the temporal field is smaller than that in the nasal field (Curcio et al.,
1990; Jonas et al.,
1992). Recognition of walk was also poor in the far temporal retina. However, the current data were not strongly correlated with eccentricity. This could be due to the ceiling effect caused by the large stimulus displays. The involvement of higher level processing was also probable.
The results of the binocular condition suggested some anisotropy. The probabilities of the backscroll illusion increased when a person appeared to walk away from the fovea. However, the results of the backward walk conditions implied that a foveofugally facing person had deteriorated recognition of walking action. This result is paradoxical if the recognition of object locomotion determines the backscroll illusion. However, this finding of poorer recognition in the backward walk conditions can be explained by the interference caused by the facing direction of the person walking, which can determine the direction of locomotion in everyday life because a backward gait is rare. It was possible, therefore, that shape information dominated motion information in walk recognition. This argument is also supported by the occurrence of illusion from the static figures in the current and previous experiments (Fujimoto & Sato,
2006). In addition, a foveofugal walker might require attention, for instance, to prepare tracking eye movements; otherwise, he or she will gradually fade away from the visual field in everyday situations. We therefore suggest that there are high-level factors that affect the backscroll illusion in peripheral vision.
Further investigations are required to more fully understand the backscroll illusion in peripheral vision. One possible idea for future studies is the use of a smaller display, which should worsen the recognition of walking and alter the appearance of the illusion. A smaller display would also allow us to formulate a magnification factor and make comparisons with other types of perceptual phenomena in peripheral vision.
Although neural mechanisms underlying the backscroll illusion are unclear, the most likely candidate is networks among motion-specific areas in the occipital–temporal cortex of the brain. Several studies have consistently reported that human bodily movements activated areas around the superior temporal sulcus (for review, see Puce & Perrett,
2003). On the other hand, motion components of a counterphase grating are extracted in the primary visual (V1) area and integrated in the middle temporal (MT+, the human homologue to MT/MST in macaque monkeys) area (Heeger, Boynton, Demb, Seidemann, & Newsome,
1999). Thus, neural connections among those areas probably have a relation to the backscroll illusion. A recent study using monkeys as subjects showed that different parts of MT receive inputs from different brain areas according to representation of the visual field (Palmer & Rosa,
2006). Central and near-peripheral representations receive inputs from various visual areas, whereas far periphery receives inputs exclusively from V1, MST, and the retrosplenial cortex, which probably plays a role in visual information processing for rapid reactions such as orienting or postural actions. The current results were slightly linked to such a neural organization. If further investigations find that the backscroll illusion is differently perceived between far and other visual fields, it may provide clues to the understanding of neural mechanisms.
This study is the first to confirm that far peripheral vision can recognize complex human motion quite robustly, to the extent that it induces apparent motion in its background image. Ikeda et al. (
2005) reported that peripheral vision is poor at recognizing biological motion. However, this may be attributed to their use of artificial stimulus settings, in which a fragmented point-light figure was embedded in dynamic point-light noise. By contrast, in this study, we used realistic animations. Moreover, the largeness of our stimuli was likely advantageous to recognition. There is no doubt that peripheral vision is inferior to central vision in various aspects. However, this inferiority is relative. Our data indicate that peripheral vision has the ability to see a moving person at an ordinary distance. It is therefore important to make considerations for the actual viewing conditions of everyday life when investigating the recognition of natural objects or events.