The binocular depth stereoacuity of the pigeon and its relation to the anatomical resolving power of the eye.

The stereoacuity of the pigeon was measured by finding the smallest depth difference discriminable between two arrays of elements, each array confined to one of two parallel planes in space. For both the human and the pigeon, the depth between the two planes was not discriminable under stationary monocular conditions. The mean binocular depth acuity for five human subjects was 4 arc sec, reflecting hyperacuity. In the pigeon, the binocular stereoacuity was between 0.8-1.8 arc min, allowing each eye to discriminate a visual angle of at most 0.89 min of arc. This value is less than the anatomical resolving power of the retina as predicted by sampling theory. The area of the pigeon retina involved in this binocular depth judgement is likely to be the area dorsalis, a specialised region that does not contain a fovea. The pigeon is a granivorous bird with laterally placed eyes, thus we demonstrate that lifestyle and binocular field width do not predict stereoscopic vision quality.