Remy Allard¹ (remy.allard@umontreal.ca), Sam Zakko¹, Gabriel Ladaa¹, Maryam Rezaei¹, Jocelyn Faubert¹; ¹Universite de Montreal

Predators, including humans, have front-facing eyes enabling binocular vision over a large visual field. Why did evolution sacrifice a wider visual field for more binocular vision in the periphery? Stereoscopic peripheral vision must provide a considerable advantage for predators to track preys. The current study investigated if binocular disparity is more useful when tracking multiple objects or a single object, and if binocular disparity is helpful to track objects that are sometimes occluded. In the single-object tracking condition, participants were asked to track one of three balls rotating around a point at the eccentricity of 17 degrees of visual angle. In the multiple-object tracking condition, three balls were presented in each of the four quadrants and the participant had to simultaneously track one ball per quadrant. The contribution of binocular disparity was evaluated by measuring the maximum speed threshold at which objects could be tracked when the disparity cues were available relative to when they were not. The amplitude of the rotation was 6 cm in 3D space along the meridian and depth axis, and was either 0, 2, 4 or 6 along the eccentricity axis. When the amplitude was 0, the balls rotated along a disk that was aligned with the observer and were sometimes occluded behind another ball. Under occlusion conditions (eccentricity amplitude=0), the contribution of binocular disparity was considerably high when tracking 4 balls (~2x), but weak when tracking only one ball (~1.2x). When the balls were never occluded, disparity cues did not provide any advantage. In sum, binocular disparity was considerably useful only for tracking multiple objects that are sometimes occluded. These results suggest that predators have front-facing eyes to facilitate the tracking of multiple objects that are sometimes occluded.

Acknowledgements: This research was supported by NSERC grants to RA and JF.