Faculty Bibliography

The purpose of this study was to compare the effects of glaucoma, at different stages of the disease process, on the two color-opponent system and on the luminance system. Discrimination thresholds were measured along the two equiluminant cardinal color axes (RG and YV) and along an achromatic luminance axis (LD) in 27 patients with open-angle glaucoma (OAG) and in 13 glaucoma suspects. Patients with OAG showed increased thresholds along all three axes. The threshold increases correlated significantly with the level of visual field loss. For glaucoma suspects, thresholds were also increased along all three axes. A subgroup of patients with OAG, those with pigmentary glaucoma, showed minimal increases in threshold along the RG axis. To further investigate this finding an additional 15 patients, seven with primary OAG and eight with pigmentary glaucoma were run in a two-alternative forced-choice experiment. For patients with pigmentary glaucoma, thresholds were increased less along the RG axis. The results of the study for OAG patients and glaucoma suspects are consistent with deficits in the two color-opponent systems, and in the luminance system

The hypothesis that reductions in Snellen acuities in patients with retinitis pigmentosa are due solely to losses of photoreceptors was tested by measuring the effects of random losses of sampling elements on letter identification. Sampling element losses were mimicked by setting the luminance of randomly selected pixels equal to the luminance of the surround. The amount of pixel blanking ranged from 0 to 90%. Letters varying in retinal subtense from 5 to 17 min arc were presented for 500 msec. Although letter identification accuracy decreased with increasing pixel blanking for all letter sizes, performance remained relatively high even when a majority of the pixels was blanked. The data suggest that unless the loss of cone photoreceptors in greater than 80%, loss of sampling elements alone can not account for letter acuities poorer than 20/40. In addition to loss of cone photoreceptors in patients with RP, there are histological reports of photoreceptor abnormalities and psychophysical studies of visual sensory deficits. It is conceivable that these alone, or in combination with losses of photoreceptors, could account for decreased visual acuity. In a series of experiments, 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 demonstrated 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 observed in patients with retinitis pigmentosa cannot be attributed solely to a random loss of sampling elements in the underlying retina, but may be due to the combination of photoreceptor degeneration and other sensory and perceptual factors

The effects of glaucoma and diabetes on the sensitivities of the opponent and achromatic systems were investigated by measuring thresholds along theoretically defined axes in a three-dimensional color space. Thresholds were measured along two equiluminant chromatic axes and one achromatic axis in patients with diabetes or glaucoma and in glaucoma suspects. The results were compared with measures of sensitivities of short- and middle-wavelength-sensitive-cone pathways [S (Stiles pi 1) and M (Stiles pi 4), respectively] and with measures of hue discrimination by use of the Farnsworth-Munsell 100-hue test. The glaucoma suspects and diabetic patients showed preferential S-cone-pathway sensitivity losses. For glaucoma patients, however, these losses were associated with significant decreases in the sensitivity of the L-M opponent system and with decreased sensitivity to achromatic contrast