Acuity-luminance and foveal increment threshold functions in retinitis pigmentosa.

Acuity-luminance functions and foveal increment threshold functions were measured in 20 subjects with retinitis pigmentosa (RP) who had Snellen acuities of 20/40 or better, minimal or no posterior subcapsular cataracts, and no atrophic-appearing foveal lesions. Compared with the results from ten normal subjects, the visual acuities of the RP subjects were reduced at all luminance levels; the acuity deficits were more pronounced at low luminances. Foveal detection thresholds of the RP subjects showed the greatest elevation at low background luminances and approached normal values at high adapting levels. There was a statistically significant correlation (r = 0.79, P less than 0.01) between the visual acuities and absolute thresholds of the RP subjects. The overall pattern of results cannot be explained by a reduced quantal absorption in foveal cones, but it is consistent with the hypothesis that a reduced cone spatial density is the primary mechanism of foveal visual loss in this group of RP subjects.