Binocular luminance summation in infants. A test for stereopsis?

The difference in pupil diameter between monocular and binocular viewing conditions has been found to be significantly different in human infants at an age of onset that is positively correlated with the age of onset for stereopsis. We attempted to use this information on binocular luminance summation (BLS) to devise a simple objective test using a direct ophthalmoscope in a modified two-part swinging-flashlight test. We evaluated 67 subjects with varying degrees of known heterotropia ranging in age from 16 hours to 20 years in an effort to determine a threshold measure of stereo-acuity or degree of heterotropia at which the test result might reproducibly change. We found BLS to be normally absent until about 4 months of age. All subjects older than 4 months manifested summation regardless of the extent of heterotropia, thus making our test unusable. We conclude that BLS is more than an ipsilateral consensual pupillary response, and we concur with others that these visual pathways must undergo maturation beyond the time of birth.