Squint-induced modification of callosal connections in cats

A commissural pathway occurs in the frog, Xenopus, which interconnects the two optic tecta 7,9 11. It develops late in ontogeny and relates points seeing the same region of the visual field, even though one eye may have been rotated or otherwise manipulated prior to metamorphosis. The role of vision in determining the order of the pathway is demonstrated by the finding that adjustments resulting from eye rotation do not occur in animals reared in the dark 11. An interesting parallel is provided in the mammalian nervous system by the callosal connections between the visual cortical areas of the cat. These appear to connect quite late in ontogeny t (although our unpublished studies find a callosal connection somewhat earlier than indicated by Anker and Craggl), and are modified to accommodate the aberration found in the geniculo-cortical pathway of 'Boston' Siamese cats 14,15. The question arises as to what result might be expected if a squint is induced early in development in regular cats. Any variation from normal would not only bear directly on the frog experiments but also add a complication to interpretation of the Siamese cat studies since these animals apparently exhibited marked strabismus. To test this, regular cats (showing no indication of Siamese traits) were subjected to varying degrees of squint by cutting either lateral or medial rectus muscles, of one or both eyes on postnatal days 9-10 (5 animals-4 divergent, 1 convergent), days 44-56 (2 animals, I divergent, 1 convergent), day 84 (1 animal, divergent) and day 112 (1 animal, divergent). After 4 years survival, a large lesion was made involving one lateral gyrus in these cats, as well as in 4 control animals with normal eye alignment. Four days later the cats were perfused with buffered paraformaldehyde. The brains were subsequently sectioned transversely at 26 /~m and the sections stained by Fink Helmet 5 and Nissl methods. Description is restricted in this paper to the callosal projection to area 17. The left lateral gyrus of one further animal was injected with horseradish peroxidase and [aH]proline to define both source and terminal distribution of the callosal connections and the tissue was subsequently treated by standard techniques4,12. In