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Biomedical subjects

V C Greenstein

Publications and source records attributed to V C Greenstein.

17 recordsLinked to original sources
PubMed1995

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 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.

Adult↗
PubMed1993

Effects of early diabetic retinopathy on rod system sensitivity.

Previous studies have shown that S-cone pathway sensitivity is selectively decreased in the early stages of diabetic retinopathy. In the present study, rod system sensitivity was evaluated in a group of diabetic patients using psychophysical techniques. The course of dark adaptation was first determined, then absolute thresholds were measured in the horizontal and vertical meridians. For all patients, although the recovery of the initial portions of rod dark adaptation were normal, absolute thresholds were increased in both the horizontal and vertical meridians. The findings provide evidence that patients with early diabetic retinopathy show a generalized dysfunction of the rod system.

Adult↗
PubMed1993

Chromatic and luminance sensitivity in diabetes and glaucoma.

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.

Adolescent↗
PubMed1993

Sites of cone system sensitivity loss in retinitis pigmentosa.

PURPOSE: To examine the sites of cone sensitivity loss in patients with retinitis pigmentosa by comparing focal electroretinographic and psychophysical modulation thresholds. METHODS: Both psychophysical and electrophysiologic increment threshold curves were obtained in retinitis pigmentosa patients and a group of age-matched, normally-sighted adults. RESULTS: The majority of the retinitis pigmentosa data could be accounted for by a vertical displacement of the normal curve. The retinitis pigmentosa patients showed similar patterns of cone sensitivity losses using both techniques. CONCLUSIONS: The combined electrophysiologic and psychophysical results provide support for an outer retina locus for these cone sensitivity losses. The data suggest that these deficits may be caused by a spatially independent loss of cone photoreceptors with normal adaptation properties in the remaining photoreceptors.

Adaptation, Ocular↗
PubMed1993

The effects of acetazolamide on visual function in retinitis pigmentosa.

PURPOSE: To study the effects of acetazolamide on central and peripheral visual function in patients with retinitis pigmentosa (RP) who showed no evidence of macular edema. METHODS: Thirteen patients with retinitis pigmentosa participated in a preliminary study. Measures of central and peripheral visual function were obtained before and after an 8 wk period on acetazolamide. An additional 10 patients participated in a cross-over study. They were placed on a placebo for an 8 wk period, then on acetazolamide for a second 8 wk period. RESULTS: None of the patients in the preliminary study showed significant changes in visual acuity, color vision, foveal cone pathway sensitivities, focal electroretinogram (ERG) amplitudes, or in any ERG parameter. Three patients, however, showed significant changes in visual field area and in dark-adapted thresholds. None of the patients in the cross-over study showed significant increases in visual field area. CONCLUSIONS: Given the results and the reports of side-effects, it is difficult to justify using acetazolamide to improve retinal function in RP patients who show no evidence of cystoid macular edema.

Acetazolamide↗
PubMed1992

Psychophysical evidence for post-receptoral sensitivity loss in diabetics.

Although numerous reports show that the sensitivity of the S cone system is decreased in diabetic patients, few studies have been directed toward identifying the possible sites of the sensitivity loss. In this study, a psychophysical technique was used to test hypotheses about sites of S cone system sensitivity loss in a group of patients with early diabetic retinopathy. A model of the S cone system was assumed and the experimental conditions were chosen to distinguish between explanations for S cone sensitivity loss at the receptor level from explanations for loss at a post-receptoral level. Within the context of the model, the data were consistent with S cone system sensitivity loss occurring at a post-receptoral level.

Adult↗
PubMed1989

S (blue) cone pathway vulnerability in retinitis pigmentosa, diabetes and glaucoma.

A variety of retinal disease lead to a decrease in the sensitivity of the S (blue) cone pathways. To determine the possible sites and mechanisms of this loss we compared the sensitivities of an S (blue/pi-1) and an M (green/pi-4) cone pathway in patients with retinal diseases that differ as to their primary locus of sensitivity loss. The sensitivities of an S and an M cone pathway were assessed in patients with retinitis pigmentosa, insulin-dependent diabetes mellitus and open-angle glaucoma using Stiles two-color increment threshold technique. A greater loss in sensitivity of an S than an M cone pathway was found for all three disease groups; however, the diabetic patients showed a more selective loss. The results suggest that multiple sites are involved and that the combined effects of metabolic abnormalities and hypoxia contribute to the selective loss.

Adaptation, Ocular↗
PubMed1987

Cone function in congenital nyctalopia.

A patient with congenital stationary night blindness (CSNB) (Schubert-Bornschein type) transmitted as an autosomal recessive trait was studied with several tests of electrical function as well as a variety of psychophysical procedures. Comparison of the patient's present findings with those obtained 23 years earlier showed that while rod thresholds have remained the same, cone sensitivity has decreased. Subjective flicker thresholds obtained following a bleach were unchanged during the course of dark adaptation. The absence of rod-cone interaction, together with an absent scotopic b-wave, implies that the defect is in the mid-retinal layers. Further, the absence of oscillatory potentials in the photopic electroretinogram (ERG) suggests that the interplexiform cell may be implicated in some manner. The focal ERG of the CSNB patient showed normal amplitude and normal phase delays, supporting the idea that the focal ERG samples primarily cone photoreceptor activity.

Adult↗
PubMed1987

Progressive cone dystrophy.

Psychophysical, reflectometric, and electrophysiologic studies were done on four members of a dominant pedigree with progressive cone dystrophy. The two youngest individuals were asymptomatic at the initial examination, and none of the subjects complained of problems associated with night vision. Nevertheless, absent or grossly reduced cone-mediated electroretinographic (ERG) responses showed the widespread loss of cone function, and moderate elevations (less than 1 log unit) in absolute threshold together with reductions in rhodopsin levels in the mid-peripheral retina provided evidence of impairment of the rod system. The progressive nature of the disease was apparent from the case histories and the changes in visual performance that occurred on re-test after a 5-year interval. Moreover, the results of increment threshold measurements at several retinal loci suggested that peripheral cones may be affected earlier and more severely than those in the central retina.

Adolescent↗
PubMed1987

Progressive cone dystrophy.

Psychophysical, reflectometric, and electrophysiological studies were performed on four members of a dominant pedigree with progressive cone dystrophy. The two youngest individuals were asymptomatic at the initial examination, and none of the subjects complained of problems associated with night vision. Absent or grossly reduced cone-mediated ERG responses revealed the widespread loss of cone function. Moderate elevations (1 log unit) in absolute threshold together with reductions in rhodopsin levels in the midperipheral retina provided evidence of a mild impairment of the rod system also, although not to the degree seen in a cone-rod dystrophy. The progressive nature of the disease was apparent from the case histories and the changes in visual performance that occurred on re-test after a 5-year interval. Likewise, the results of incremental threshold measurements at several retinal loci suggested that peripheral cones may be affected earlier and more severely than those in the central retina.

Adolescent↗
PubMed1986

Test of the decreased responsiveness hypothesis in retinitis pigmentosa.

Retinitis pigmentosa (RP) frequently leads to a decrease in cone system sensitivity. A number of alternative explanations have been proposed for this decrease. Based on the results of a psychophysical technique, the probe-flash paradigm, the authors suggest that a decrease in responsiveness of retinal elements can account for much of this loss. In this paper the decreased responsiveness hypothesis is tested by obtaining data at two levels of steady adaptation. The results of the study indicate that sensitivity loss is greater for the dark adapted than the light adapted state. The data rule out the decreased responsiveness hypothesis coupled with a simple model of adaptation. More complicated adaptation models cannot be excluded. The importance of considering models of adaptation when testing models of disease-related sensitivity loss is underscored.

Adult↗
PubMed1984

Functional abnormalities in vincristine-induced night blindness.

Various noninvasive test procedures were used to evaluate retinal function in a patient who had become night blind following vincristine chemotherapy. The results obtained were strikingly similar to those reported previously in subjects with recessively inherited stationary night blindness; the dark-adaptation curve was monophasic (ie, no evidence of a scotopic branch), rhodopsin kinetics were entirely normal, and spectral threshold data revealed the presence of residual rod-mediated vision. Also like the heritable condition, the b-wave of the ERG was depressed grossly despite normal a-wave potentials. These findings, and the fact that vincristine is known to disrupt the structural integrity of neuronal microtubules, suggest that the drug-induced defect involves the process of synaptic transmission between the photoreceptors and their second-order neurons.

Adaptation, Ocular↗
PubMed1984

The response range of the blue-cone pathways: a source of vulnerability to disease.

Retinal disease preferentially affects the sensitivity of the "blue"-cone pathways. This vulnerability to disease may be due, in part, to a more limited response range. A psychophysical technique, the probe-flash paradigm, was used to test this hypothesis. The data suggest that the S-cone pathways have a more limited response range than the L-cone pathways. Explanations for blue-cone vulnerability are discessed in the context of this finding.

Differential Threshold↗