THE EFFECTS OF RANDOM ELEMENT LOSS ON LETTER IDENTIFICATION - IMPLICATIONS FOR VISUAL-ACUITY LOSS IN PATIENTS WITH RETINITIS-PIGMENTOSA

The hypothesis that reductions in Snellen acuities in patients with re tinitis pigmentosa are due solely to losses of photoreceptors was test ed by measuring the effects of random losses of sampling elements on l etter identification. Sampling element losses were mimicked by setting the luminance of randomly selected pixels equal to the luminance of t he surround. The amount of pixel blanking ranged from 0 to 90%. Letter s varying in retinal subtense from 5 to 17 min are were presented for 500 msec. Although letter identification accuracy decreased with incre asing pixel blanking for all letter sizes, performance remained relati vely high even when a majority of the pixels was blanked. The data sug gest that unless the loss of cone photoreceptors in greater than 80%, loss of sampling elements alone can not account for letter acuities po orer than 20/40. In addition to loss of cone photoreceptors in patient s with RP, there are histological reports of photoreceptor abnormaliti es and psychophysical studies of visual sensory deficits. It is concei vable that these alone, or in combination with losses of photoreceptor s, could account for decreased visual acuity. In a series of experimen ts, stimulus parameters were manipulated in order to mimic the effects of some of these abnormalities and deficits and the effects on letter identification were examined. The results of these experiments demons trated that sampling element loss interacts with sensory factors (e.g. luminance and contrast sensitivity) and perceptual factors (e.g. set size and letter orientation) to reduce letter identification accuracy. The implication of these results is that decreases in letter acuity o bserved in patients with retinitis pigmentosa cannot be attributed sol ely to a random loss of sampling elements in the underlying retina, bu t may be due to the combination of photoreceptor degeneration and othe r sensory and perceptual factors.