Binocular luminance summation in humans with defective binocular vision

Subjects with no functional binocularity (no stereopsis and no peripheral fusion) show much less binocular summation of the pupillary response (about 12%) than normal subjects (29%). However, summation is occasionally quite high in clinically stereoblind subjects. The reasons could be the simultaneous use of nonoverlapping fields of the two eyes in large-angle exotropes, peripheral fusion (common in microstrabismus), a mosaic of inputs from the two eyes in different parts of the visual field (often seen in strabismic alternators), or the incomplete suppression of the deviated eye in subjects with double images. These results suggest that the binocular summation of the pupillary response can be used as a test for the simultaneous use of the two eyes, rather than as an index of functional binocularity and stereopsis.