Temporal properties of letter identification in retinitis pigmentosa.

The effect of stimulus duration on the visual acuity of individuals with retinitis pigmentosa (RP) was assessed by using Sloan letters of 100% contrast and durations ranging from 15 ms to 3.8 s. In addition, contrast thresholds for identifying Sloan letters (0.7 log minimum angle of resolution; 20/100 Snellen equivalent) were measured over the same range of durations in the same subjects with RP. Compared with results from a control group of subjects with normal vision, the subjects with RP showed losses in visual acuity at all stimulus durations, with a slightly though significantly greater reduction in visual acuity at short durations. The letter-contrast thresholds of the subjects with RP were elevated above the normal range to the same degree at all durations. Analysis of the results in the format of letter contrast sensitivity functions indicates that temporal summation for letter identification was normal for these subjects with RP and that their relatively greater acuity loss at short exposure durations was most likely related to their elevated letter contrast thresholds.