Motion thresholds in retinitis pigmentosa.

Minimum displacement thresholds, or dmin, were measured in 29 subjects with retinitis pigmentosa (RP) and 10 subjects with normal vision. The results showed that RP can affect an observer's ability to judge the correct direction of motion in a random-dot pattern. The majority of RP subjects had elevated dmin. They required a larger displacement to perceive the correct direction of motion. Only 5 of the 29 RP subjects had thresholds within two standard deviations of the mean of the normal-observer distribution. Moreover, three RP subjects were unable to detect the correct direction of motion regardless of the displacement magnitude, and four RP subjects consistently reported motion in the opposite direction at small displacements. The results cannot be explained by abnormal temporal processing or a reduction in the effective luminance. There was a statistically significant correlation (r = 0.72, P less than 0.001) between log threshold and log MAR, consistent with the hypothesis that a reduction in the spatial density of the photoreceptors contributes to the motion-threshold elevation. Motion thresholds also were measured in subjects with normal vision under conditions of simulated "photoreceptor" dropout. The results showed that a random elimination of information from over 25% of the image positions significantly elevates motion thresholds. These results also support the spatial-density reduction hypothesis.