(2015) to manipulate motion parallax of a visual target on a monitor. In the current study, we used the same tracking method as Stewart et al. The butterfly's position was estimated using the triangulation method two cameras continuously tracked the butterfly and its 3D position was calculated from the combination of 2D positions on the camera images. (2015) manipulated motion parallax of a visual target on a monitor by automatically tracking the flying position of a butterfly and moving the target according to its current position. In recent years, computer-based trackers have enabled the tracking of fast and complex movements and displaying of the motion parallax cue based on these movements ( Sobel, 1990 Van der Willigen et al., 2002 Stewart et al., 2015). The locusts were induced to undershoot toward the target when it moved against their self-motion, which signifies motion parallax of nearer position. In pioneering work by Wallace (1959), a platform was manually moved with or against the peering movement of a locust before jumping. To experimentally manipulate self-generated motion parallax, it is necessary to track the subject's head position and move a visual target with respect to the changing viewpoint of the subject ( Poteser and Kral, 1995 Goodale et al., 1990 for review, see Kral, 2003). The aim of the current study was to investigate whether and how pigeons use motion parallax depth cues obtained via head movements in two cognitive contexts: visuo-motor control and visual size perception. Given the evidence that frontal-eyed owls use motion parallax for depth information, it is likely that lateral-eyed pigeons also use motion parallax because they are considered to depend less on binocular depth cues with their narrow binocular visual field ( Martin and Young, 1983 Martinoya et al., 1981), and behavioral and physiological evidence of stereopsis is scarce in pigeons compared with owls and raptors ( McFadden and Wild, 1986 Martinoya et al., 1988). Fux and Eilam (2009) observed that, in a more naturalistic situation, owls move their head before attacking prey they suggested that owls get depth information via self-generated motion parallax. When faced with novel displays in which the depth structure was defined only by self-generated motion parallax, the owls successfully extracted depth information by spontaneously moving their head, and they transferred learning from the binocular to the motion parallax cue. In one study on owls ( Van der Willigen et al., 2002), birds were first trained to discriminate concave and convex figures of random dot stereograms on a monitor, with only binocular depth cues. However, there is little behavioral evidence that birds use motion parallax obtained via head movements for visual depth information. Pigeons also move their heads when flying in the presence of obstacles ( Ros et al., 2017) and when landing on a perch ( Davies and Green, 1988), suggesting that they utilize motion parallax for flight motor control. The authors suggested that the forward-thrust movement amplifies relative motion in retinal images, serving as a motion parallax depth cue. Davies and Green (1988) showed that, when running, pigeons swing their head even though this no longer stabilizes retinal images. Whereas head movements in the hold phase are for visual stabilization, it is proposed that head movements in the thrust phase have another visual function ( Frost, 1978 Davies and Green, 1988 Troje and Frost, 2000). These results indicate that motion parallax via head movements modulates pecking motor control in pigeons, suggesting that head movements of pigeons have the visual function of accessing motion parallax depth cues. By contrast, motion parallax did not affect how the pigeons classified target sizes, implying that motion parallax might not contribute to size constancy in pigeons. Pecking motor control was affected by the manipulation of motion parallax: when the motion parallax signified the target position farther than the monitor surface, the head position just before pecking to target was near the monitor surface, and vice versa. To manipulate motion parallax of the target, we changed the target position on the monitor according to the bird's head position in real time using a custom-built head tracker with two cameras. We trained pigeons to peck a target on a touch monitor and to classify it as small or large. This study investigated whether self-generated motion parallax modulates pecking motor control and visual size perception in pigeons ( Columba livia). Although it has been proposed that birds acquire visual depth cues through dynamic head movements, behavioral evidence on how birds use motion parallax depth cues caused by self-motion is lacking.
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