Limits of the pigeon's binocular field and direction for best binocular viewing

The divergence of the optic axes has been used as a crude index of binocular field width in animals (Hughes, 1977); however, such estimates do not account for refraction in the cornea. More direct estimates have been based on transcleral illumination (Rochon-Duvigneaud, 1922) or upon ophthalmological examination in animals which possess well defined fixation areas (Hughes, 1977). In granivorous species, such as the pigeon, the binocular field has been estimated as 24” wide, with a total field of 340”-342” (Walls, 1967). In the course of a study of depth perception in the pigeon, we needed to have a more accurate information on the binocular overlap in the bird’s visual field to know where to place our testing cues. It occurred to us that the binocular field might be determined by analizing the shape of the animal’s pupils as seen by a photographic camera from different visual field directions. This method would take into account both the position of the eyes and the refraction of light at the air-cornea interface. If the pigeon’s eye possesses rotational symmetry, this method could also serve to detect the best direction for binocular viewing as considered from an optical point of view. Lateral-eyed species such as the pigeon have almost separated uniocular fields, the binocular overlap being defined as the intersection of the nasal-in this case peripheral-portions of both uniocular fields, frontal head features playing a minor role. Rays coming from different parts of the binocular overlap always form large angles with the axes of the eyes. For this reason a continuous and strong variation of light and the degree of optical aberrations may be expected in the binocular field. The transition from binocular to monocular viewing in these lateral-eyed species would therefore be gradual, so that a qualification of binocularity seemed pertinent. Good optical conditions for binocular fusion are present all along the saggital plane. Nevertheless there is only one direction in this plane where the quantity of light is maximal and the degree of optical aberrations minimal: the direction defined by the intersection of the sagittal plane and the plane containing both optical axes. It is this direction which we shall refer to as the direction of “best binocularity”.