Effects of neonatal cortical lesions upon retinocollicular projections in the hamster.

Autoradiography and anterograde horseradish peroxidase transport were used to examine retinocollicular projections in normal hamsters and in animals subjected to ablation of the ipsilateral, posterior neocortex at 1, 3, 6, 10, or 120 days of age. The crossed retinotectal projections of all groups were quite similar. There did, however, appear to be a slight increase in the density of the projection to the lower portion of the stratum griseum superficiale in the neonatally brain-damaged hamsters. The uncrossed pathway, on the other hand, was quite abnormal in the neonatally lesioned animals. In normals, the ipsilateral retinocollicular projection consisted almost entirely of a series of patches along the stratum griseum superficiale-stratum opticum border in the rostral one-third of the colliculus. Only a few axons from the ipsilateral eye were observed in the caudal two-thirds of the tectum and these could only be visualized when horseradish peroxidase was used as the tracer. In all of the neonatally brain-damaged hamsters both autoradiography and horseradish peroxidase tracing demonstrated that the ipsilateral retina densely innervated the entire rostrocaudal extent of the colliculus. Retrograde tracing experiments demonstrated that the portion of the temporal retina which gave rise to the uncrossed retinocollicular projection in the normal hamsters was also the source of the expanded projection in the neonatally brain-damaged animals; and, further, that the numbers and areal distributions of ipsilaterally projecting retinal and retinocollicular ganglion cells were similar in the two groups. These findings suggest that, at least in the hamster, normal inputs from the two eyes may not be a sufficient condition for the development of the largely complementary pattern of collicular innervation by the two retinae.