Binocular Luminance Summation in Young Kittens and Adult Strabismic Cats

Normal adult cats show a 35% increase in pupil diameter when one eye is illuminated and the other occluded, rather than when both eyes are illuminated. Young kittens show significantly less binocular luminance summation than adult cats (14%). Since these kittens also show a significantly reduced pupillary response to luminance decrements, the postnatal development of binocular luminance summation reflects an increase in pupillary motility, rather than a development of binocular function. Binocular luminance summation is much reduced in esotropic cats (6%), but not in exotropic cats (33%). This finding casts doubts on the usefulness of binocular luminance summation as a measure of binocular function. Invest Ophthalmol Vis Sci 28: [343][344][345][346][347][348] 1987 Under conditions of steady-state illumination, the pupils are smaller when both eyes are illuminated than when one eye is illuminated and the other occluded. Ten Doesschate and Alpern 1 reported that in two strabismic subjects the illumination of both retinae produced the same effect as the illumination of the fixating retina alone. They concluded, "Whatever region of the nervous system performs the operation of pooling signals from the two retinas to make the pupil contract is relatively impervious to signals from the nonfixating eye when the fixating eye is illuminated to the same extent." 2 Birch and Held 3 capitalized on this reported absence of binocular luminance summation in strabismic adults. They found that in infants younger than 4 months of age, there was no binocular summation in the pupillary response, and that the development of binocular luminance summation paralleled the development of stereopsis as assessed by preferential looking. Birch and Held 3 concluded that, since the binocular luminance summation test requires a minimum of time and cooperation from the infant, it could be used "to assess the integrity of binocular visual pathways even in the presence of large-angle tropias." 3 Attracted by the simplicity of this test, we attempted to apply it in young kittens and adult strabismic cats. We found that a significant development of binocular luminance summation occurs in young kittens. However, since young kittens also show a reduced pupillary response to luminance decrements, we cannot exclude the possibility that this development is related to an increase in pupillary motility, rather than to the development of binocular function. On the other hand, we found that binocular luminance summation is greatly reduced in adult cats raised with a surgical convergent strabismus, but not in divergently strabismic cats. These results suggest that the test of binocular luminance summation is not appropriate as a measure of binocularity (or even stereopsis). This conclusion is supported and extended by a complementary study on human subjects with known deficits of binocular vision. Materials and Methods The experimental arrangement was adapted, with minor modifications, from Birch and Held. 3 All testing was conducted in a dark room. The kittens were held on a platform with the eyes at the entrance of a full-field, uniformly illuminated (55.2 cd/m 2 ) white dome, 48 cm in diameter. This luminance was used, since in pilot experiments we observed that, with the lower luminance (1.92 cd/m 2 ) used by Birch and Held, 3 the kittens' pupils were inconveniently large, occupying most of the exposed surface of the eye. The dome contained an 18 deg aperture through which a video camera monitored the kitten's left eye. The video camera was employed in conjunction with a video recorder and a black and white video monitor. The distance between the kitten's tested eye and the camera was 25 cm. In pilot experiments we observed that patching of one eye produced an irritation of that eye, and that the kittens tended to keep the eye closed after removal of the patch. Therefore, testing was always begun in a binocular-viewing condition. The kitten was adapte