Dynamic Corridor Illusion in Pigeons: Humanlike Pictorial Cue Precedence Over Motion Parallax Cue in Size Perception

Abstract

Depth information is necessary for perceiving the real size of objects at varying visual distances. To investigate to what extent this size constancy present in another vertebrate class, we addressed the two questions using pigeons: (a) whether pigeons see a corridor illusion based on size constancy and (b) whether pigeons prioritize pictorial cues over motion parallax cues for size constancy, like humans. We trained pigeons to classify target sizes on a corridor. In addition, we presented a dynamic version of corridor illusion in which the target and corridor moved side by side. Target speed was changed to manipulate motion parallax. With the static corridor, pigeons overestimated the target size when it was located higher, indicating that pigeons see a corridor illusion like humans. With the dynamic corridor, the pigeons overestimated the target size depending on target position, as in the static condition, but target speed did not affect their responses, indicating that the pictorial precedence also applies to pigeons. In a follow-up experiment using the same stimulus, we confirmed that humans perceive object size based on pictorial cues. These results suggest that size constancy characteristics are highly similar between pigeons and humans, despite the differences in their phylogeny and neural systems.

Motion parallax via head movements modulates visuo-motor control in pigeons

“…If the size constancy works perfectly, perceived sizes changed at the ratios of 65:75 and 85:75 for closer and farther conditions, respectively. These ratios correspond to the manipulation of pictorial cues in our previous study (115:135 and 155:135;Hataji et al, 2020). These ratios also correspond to the discrimination thresholds 65.7:73.9 and 73.9:84.9, which are calculated from the ratios of target size when the proportion of large key choices was 25% and 50%, and when it was 50% and 75%, respectively.…”

“…This indicates that pigeons use motion parallax selectively for visuo-motor control and not for size constancy. We previously found that this species' visual size perception was not affected by motion parallax caused via relative object motion (Hataji et al, 2020). The difference between our previous study and this one is that motion parallax in the stimuli was object produced or selfgenerated.…”

“…However, this does not ensure that the change in virtual depth ±10 mm is sufficient to induce size constancy in pigeons, because size constancy does not perfectly correct perceived size. In our previous study, which showed size constancy with pictorial depth cues (Hataji et al, 2020), the ratio of perceptual change of target size to the size change predicted by the pictorial cue manipulation was below 1.0 (0.92 and 0.95 when the pictorial cue was far and near, respectively). If the contribution of motion parallax to the size constancy is smaller than the pictorial cue, then the motion parallax manipulation of ±10 mm is not sufficient.…”

“…Our data suggest that size constancy does not work with the motion parallax cue regardless of whether it is object produced or self-generated. Our earlier study (Hataji et al, 2020) demonstrated size constancy based on pictorial depth cues in pigeons. Given that this might not be due to depth information but to two-dimensional structures inherent in these cues, such as repulsion and attraction mechanisms (Fujita et al, 1991(Fujita et al, , 1993, it seems plausible that size constancy does not function with motion parallax depth cue.…”

“…When two objects of identical visual angular size were presented on a picture with pictorial depth cues, primates perceived the object at a farther position as larger than the object at a nearer position. We previously demonstrated that pigeons also have the size constancy function (Hataji et al, 2020). Pigeons were trained to classify the size of visual targets as 'large' or 'small' against a background corridor drawing.…”