Properties of X- and Y-type retinal ganglion cells in Siamese cats.

There is ample evidence that the visual system of the Siamese cat is different from common cats. These abnormalities suggest possible retinal origins, although no documentation exists. In the present study, single unit recordings were made from 91 misrouted and 209 normally-routed optic tract fibers in Siamese cats. Electrophysiological responses of the misrouted fibers did not differ from those found in the normally-routed fibers of the Siamese cat with the exception of depressed responses to contrast reversal stimuli. X/Y classification of units and experiments on receptive field center sizes, intensity-response functions, and responses to flicker failed to demonstrate significant differences between the misrouted and normally-routed fibers in Siamese cats. These results were not affected by different degrees of interocular misalignment exhibited by the Siamese cat studied. Response properties of retinal ganglion cells in Siamese cats, however, were found to be quite abnormal when compared with common cats. Only 14% (42/300) of all units studied were Y-cells in Siamese cats in comparison to 35% (60/170) in common cats. The percentage of Y-units also was correlated with the severity of interocular misalignment in Siamese cats, i.e. the greater the misalignment of the eyes, the lower the percentage of Y-cells. Experiments on response to contrast reversal stimuli, intensity-response functions and responses to flicker revealed that the ganglion cells in Siamese cats are not as responsive as those in common cats.