Pigeon's eyes converge during feeding: evidence for frontal binocular fixation in a lateral-eyed bird.

Pigeons' head-in-space and eye-in-head positions were simultaneously recorded (EOG) in video during feeding. A linear correlation was found between the experimentally measured eye convergence (sum of both EOGs ) and grain parallax (calculated from interocular and eye-to-grain distances). This graded convergence of the eyes on approaching a target is considered evidence of frontal binocular fixation. Since such convergence prevails when the animal is forced to rely on frontal monocular or on lateral vision, a central mechanism controlling frontal fixation in the pigeon must be presupposed .