The retinothalamic pathways in Siamese cats.

By injecting one lateral geniculate nucleus with large amounts of horseradish peroxidase (HRP), we have determined the retinal distributions of contra- and ipsilaterally projecting retinothalamic ganglion cells in the Siamese cat. In accord with the data of others, we observe that large numbers of temporal ganglion cells, which normally send axons ipsilaterally, instead misproject to the contralateral thalamus. However, in contrast to the expectations raised by previous work, we do not find the Siamese defect to be a simple 20 degrees shift of the naso-temporal decussation line. Rather, there is intermingling of the crossed and uncrossed retinothalamic populations in the temporal retina, with a gradual increase in the proportion of ipsilaterally projecting ganglion cells as one moves temporally. Thus, the Siamese abnormality represents not only a temporal displacement of the retinothalamic decussation line, but also a smearing of the normally rather sharp division between regions of ipsilateral and contralateral projection. Cell size measurements and anterograde transport of H3-proline confirm the HRP findings and suggest differential effects of the Siamese abnormality according to ganglion cell class. In particular, it appears that the large ganglion cells to misproject to a greater degree than the rest of the retinothalamic population.