Biomedical subjects

W Verdon

Publications and source records attributed to W Verdon.

7 recordsLinked to original sources

PubMed1996

Visual performance after photorefractive keratectomy. A prospective study.

OBJECTIVE: To prospectively examine the effect of excimer laser photorefractive keratectomy (PRK) on best-corrected visual performance using psychophysical tests that were likely to be more sensitive to image degradation than high-contrast Snellen visual acuity. DESIGN: Prospective cases series. PATIENTS: A cohort of 18 subjects with an average of -5.08 diopters (D) of myopia (SD = +/- 1.63 D) was tested before PRK and at 3, 6, and 12 months after PRK. INTERVENTION: Photorefractive keratectomy was performed using a laser (Excimed UV200, Summit Technology, Waltham, Mass) and a polymethylmethacrylate mask; a 5-mm ablation zone was used. MAIN OUTCOME MEASURES: Best-corrected high-contrast visual acuity, best-corrected low-contrast visual acuity (18% Weber contrast), and best-corrected letter-contrast sensitivity. Measurements were repeated with dilated pupils and in the presence of a glare source. RESULTS: One year after PRK, the mean best-corrected high-contrast visual acuity was reduced by half a line (P = .01), and the mean best-corrected low-contrast visual acuity was reduced by 1 1/2 lines (P = .002). The losses were somewhat greater when the subject's pupils were dilated and a glare source was used. The reduction in dilated low-contrast visual acuity was positively correlated with the decentration of the ablation zone (r = 0.47), providing evidence of an association between corneal topography and the functional outcome of PRK. CONCLUSION: Low-contrast visual acuity losses after PRK are notably greater than high-contrast visual acuity losses for best-corrected vision. Low-contrast visual acuity is a sensitive measure for gauging the outcome success and safety of refractive surgery.

Adult↗

PubMed1996

Mechanisms underlying the detection of increments in parafoveal retina.

It is well established that the spectral sensitivity under photopic conditions varies across the human retina. We investigate the mechanisms underlying these spectral changes. Through the use of color appearance, flicker sensitivity, additivity, discrimination at threshold and modeling, we show that the changes in spectral sensitivity on a photopic white background across parafoveal retina are consistent with shifts in cone weightings to (L-M) and (M-L) chromatic channels. This two channel model, developed to account for foveal spectral sensitivity curves (Sperling & Harwerth, 1971 Science, 172, 180-184), provides a better description of parafoveal data than both a single color channel upper envelope model (comprised of a single red-green opponent channel and an achromatic mechanism) and a vector model (combining a red-green opponent channel with an achromatic component). Thus while the two channel model ([L-M] and [M-L]) of foveal color vision is generalizable to the parafovea, simple models with a unitary red/green process are not. Although the two channel model can accurately fit parafoveal spectral sensitivity curves without it, a small contribution from a luminance mechanism might improve the ability of the two channel model to account for threshold discrimination and additivity data.

Color Perception↗

PubMed1995

Ocular toxicity of iontophoretic foscarnet in rabbits.

Transscleral iontophoresis of foscarnet is a noninvasive drug delivery system for the local treatment of cytomegalovirus (CMV) retinopathy. We determined the retinotoxic effects of transscleral iontophoresis of foscarnet. Slit-lamp biomicroscopy revealed no toxic effects for any of the treated eyes. Indirect ophthalmoscopy showed retinal and choroidal burns 1-3 mm in diameter at the site of iontophoresis in both foscarnet-treated eyes and saline-treated control eyes. Light and electron microscopy revealed focal retinal, retinal pigment epithelial, and choroidal necrosis at the site of iontophoresis but no abnormalities elsewhere. Ganzfeld electroretinographic studies revealed no response differences between foscarnet-treated eyes vs. controls.

Animals↗

PubMed1990

Spatial sensitization in the short wavelength sensitive pathways under dichoptic viewing conditions.

Spatial sensitization (Westheimer) functions were measured under conditions that isolated the short wavelength sensitive pathways. The variable diameter pedestal and the test probe were either presented to the same eye (monoptic presentation) or to different eyes (dichoptic presentation). The most significant new finding was that a dichoptically presented, small, blue pedestal caused threshold elevations of about 1-2 long units for an S cone detected probe. However, a large pedestal caused little or no change in threshold. This result contrasts with previous results using white light stimuli, which showed that steadily presented dichoptic pedestals caused little or no threshold change. Furthermore, we show there is little masking when the probe is detected by the isolated M or L cone pathways. These data thus demonstrate a binocular, size dependent interaction revealed only when S cones detect the probe.

Contrast Sensitivity↗

PubMed1988

The pupil's response to short wavelength cone stimulation.

Using i.r. pupillometry, we measured the response of the pupil to tritanopic metamers alternating at 0.94 Hz. These are lights that differentially stimulate only the short wavelength (S) sensitive cones. We find a response at the alternation frequency for 5 of 7 observers. This shows, for the 5 observers, that S cone signals can influence pupil size, probably via the traditional retinotectal light reflex pathway. Changing the radiance of just one of the alternating pair of lights causes the two lights to differ in their total M + L cone stimulation. The pupil's response to this imbalance can antagonize its response to S cone stimulation. By this procedure we find that imbalances in M + L cone stimulation of less than 0.3 log10 unit offset the pupil's response to S cone stimulation of more than 0.8 log10 unit. This suggests that afferent pupillary signals from S cones are weak relative to those from M + L cones.

Color Perception↗

PubMed1988

Cone interaction occurs in the parafovea under pi 4 stimulus conditions.

This study shows that stimulus parameters that isolate Stiles' pi 4 mechanism at the fovea do not isolate pi 4 parafoveally. Instead, the parafoveal test sensitivity peaks at 520 nm and is relatively depressed at longer wavelengths. This narrowed spectral sensitivity is not due to rod intrusion or interaction but rather suggests cone interaction for a relatively wide range of spatial and temporal parameters. The anomalous shape of the parafoveal spectral sensitivity is not found in a protanope, further supporting the view that the effect seen in normal trichromats is the result of interaction between L and M cones. The study provides a unifying explanation for discrepant results by investigators studying the chromatic properties of spatial sensitization and the contrast after-flash effect. Foveal studies found independence of pi mechanisms while parafoveal studies found interaction. The present study emphasizes the importance of retinal eccentricity on color threshold experiments.

Color Perception↗

PubMed1987

Short-wavelength-sensitive cones do not contribute to mesopic luminosity.

It has been suggested that the short-wavelength-sensitive cones (S cones) play a significant role in the transition from scotopic to photopic vision (the Purkinje shift). We address this issue directly over a 5-log10-unit range of light levels covering scotopic, mesopic, and photopic vision. At each light level we make flicker matches to two reference stimuli by 2-Hz flicker photometry. The two reference lights (441 and 481 nm) differ only in their stimulation of S This novel technique utilizes the different magnitudes of the rod and cone Stiles--Crawford effects. Despite the large difference in S-cone stimulation by the two reference lights (more than 1 log10 unit), the pairs of luminosity functions are indistinguishable at each light level tested. The results indicate that S cones do not contribute to either photopic or mesopic luminosity.

Humans↗