Eye Design for Depth and Distance Perception in the Pigeon: An Observer Orientated Perspective
Abstract
The perception of the distance of objects with respect to an observer (egocentric distance) and the perception of the relative distance (depth) between external points was found to be optimised in the pigeon visual system according to the optical and retinal constraints of the eye. Each of these perceptual capacities is mediated by different binocular mechanisms in the frontal field, both of which appear to be designed for a stationary world. This is particularly evident in the egocentric distance estimation that occurs during the reaching movement when pecking. Here both the saccadic nature of the head movement and the convergence eye movements appear to allow constant retinal stimulation. This system of vergence signal alone is inadequate for depth perception which is instead mediated by retinal disparity. Stereopsis in the pigeon appears to be more effective for pattern decoding than for absolute spatial perception.