Development of eye alignment in cats

Abstract

WHEN the eyelids of newborn kittens open, casual observation of the pupils suggests that the eyes must be divergently misaligned. Pupillary divergence may be quantified by photographing the eyes while the cat is facing a distant light source. The images of this light, formed by reflection at the corneas, coincide approximately with the optical centres of the eyes, and the distance by which the separation of the pupils exceeds the separation of these images is defined as pupillary divergence^1–5. It declines with increasing age until some time during the second month². Although it is reasonable to assume that the visual axes are also divergent before this age, this assumption leads to a paradox because surgical misalignment of the eyes of young kittens is known to disrupt binocular connections in striate cortex⁶. Moreover, development of normal orientation selectivity in kitten cortex may require early coordinated binocular vision⁷. We have examined this problem using a combination of techniques with which we are able to determine the alignment of the visual axes in the alert cat. Our results show that while the pupillary axes are quite divergent during the first few weeks after birth, the visual axes are not divergent. It is thus possible that kittens experience synchronised binocular vision very early in life.