Pupillary measures of binocular luminance summation in infants and stereoblind adults.

Preverbal infants and children with ocular misalignments do not perform well on standard clinical measures of binocular function which require accurate alignment of the eyes. In 1983, Birch and Held described a pupillary measure of binocular luminance summation that could be used to assess binocular function in strabismic infants. They reported a correlation between the onset age of stereopsis and the onset age of binocular luminance summation, with both emerging at the end of the fourth postnatal month. Using a similar pupillary technique, we measured significant levels of binocular luminance summation in infants as young as 2 mo of age, as well as in stereoblind adults. In addition, the infant's pupillary system, in comparison to the adult's, showed reduced magnitudes and increased latencies to luminance increments. The sensitivity of the pupillary system to luminance increments provided a better predictor of binocular luminance summation (Pearson r = 0.88) than did stereoscopic performance (Pearson r = 0.43). These data suggest that developmental changes in the pupillary system itself, rather than factors intrinsic to binocular vision, may have been the source of the correlation between binocular luminance summation and stereopsis reported by Birch and Held.