Eye alignment in kittens.

1. The alignment of the pupillary axes and of the visual axes has been measured in 23 normally reared cats ranging in age from 14 days to adulthood. 2. In agreement with a previous report, we find that pupillary divergence, as measured from photographs, tends to decrease during the first 2 mo of life. 3. The angle between the visual axes of cats of various ages was determined during paralysis by plotting the receptive fields of neurons in cortical area 17, and extrapolation to the angle of alignment of the freely moving animal was accomplished by comparing pupillary photographs taken before and after immobilization. Results obtained by this method reveal that in cats of all ages the visual axes are convergent, and that the average angle of convergence is approximately the same at all ages. 4. We conclude that young kittens may be capable of coordinated binocular vision. Further study will be required to determine whether animals as young as 2 wk are able to align their eyes accurately so as to bring the two retinal images of object space into register. 5. Pupillary divergence decreases during development as a result of changes in the geometry of the eye characterized by a reduction of the angle between the pupillary axis and the visual axis in each eye. This angle changes from around 25 degrees at 14 days to around 16 degrees in adulthood. 6. The role of visual experience in the maintenance of normal eye alignment was investigated by rearing five cats in darkness until the age of 4-7 mo. In three animals, visual axis alignment was within the normal range. The two remaining cats were slightly exotropic. 7. A change occurs during development in the apparent cyclotorsional alignment of the eyes, as determined by measuring the intorsional angle formed by the two slit pupils. This angle increases during the 1st and 2nd mo, assuming a mean value of 14 degrees. In dark-reared cats the increase continues through the 3rd mo, culminating in an abnormally large angle of pupillary intorsion (mean of 24 degrees). The possibility that these changes reflect true shifts in cyclotorsional alignment of the eyes is discussed.