An exuberant retinocollicular pathway in Siamese kittens: effects of competition and abnormal activity on its maturation.

Retinocollicular pathways were studied in normally pigmented and Siamese adult cats and newborn kittens. In addition, retinocollicular pathways were studied in Siamese cats which were unilaterally enucleated on the day of birth and in Siamese cats which were reared in a stroboscopically illuminated environment. In adult Siamese cats the ipsilateral retinocollicular pathway is spatially less extensive than it is in adult normally pigmented cats. In contrast, the ipsilateral retinocollicular pathway in newborn Siamese kittens is widespread, while that of newborn normally pigmented kittens is restricted, as it is in normally pigmented adults. This comparison indicates that the spatial restriction of the retinocollicular pathway occurs after birth in Siamese cats. After enucleation or stroboscopic rearing the ipsilateral retinocollicular pathway in Siamese cats remains widespread. These results demonstrate the importance of interactions with afferents from other sources and the requirement for appropriate neural activity in the normal maturation of this initially exuberant pathway.