Abnormal Y/X ratio in the area centralis of the Siamese cat.

The responses of X- and Y-type retinal ganglion cells were recorded in the optic tract of Siamese cats in order to investigate the encounter rate of Y-cells with respect to retinal eccentricity. The percentage of Y-cells in Siamese cats was highest in or near the area centralis and it decreased with eccentricity. This is in contrast to the proportion of Y-cells in normally pigmented cats, which was lowest in the area centralis and increased with eccentricity. Thus, the Y/X ratio in Siamese cats is higher in the area centralis, but significantly lower in the peripheral retina compared to those in normal controls. The lower percentage of Y-cells in the periphery parallels an additional finding that Y-cells exhibiting very high conduction velocities were missing from the Siamese cat optic tract. Finally, the receptive-field center (RFC) size of X-cells in the area centralis was larger in Siamese cats, and the correlation between the RFC size and conduction velocity was weak in these animals. The results are discussed in terms of behavioral deficits in Siamese cat vision.