Discrimination of spatial displacements by patients with retinitis pigmentosa

We compared maximum displacement thresholds (Dmax) with minimum displacement thresholds (Dmin) in patients with retinitis pigmentosa (RP) in order to characterize the nature of their visual disability, as well as to assess possible models of foveal vision loss. Thresholds for discriminating the direction of the spatial displacement of random dot patterns were measured in a group of 20 patients with typical RP or Usher syndrome whose visual acuities were 20/40 or better and who had minimal or no clinical evidence of changes in the ocular media. Findings were compared with those from an age-similar group of 15 visually normal subjects. Displacement thresholds were measured using a two-frame random dot cinematogram and a four-alternative forced-choice procedure. Measurements were made at each of three dot contrasts and three dot sizes. For the patients with RP, reducing either the dot contrast or dot size increased Dmin and decreased Dmax such that the range of discriminable displacements became considerably restricted, even at modest reductions in dot contrast or size. This restriction in the displacement thresholds of the patients with RP was correlated significantly with their visual acuity. By comparison, the control subjects showed little change in either Dmin or Dmax under these conditions. These results indicate that patients with RP who have only relatively minor reductions in their visual acuity can have severely compromised motion perception. The pattern of findings suggests that an abnormal contrast response of the foveal cone system is a major determinant of the impaired displacement thresholds of these patients with RP.