Faculty Bibliography

The basic rule, when using the flow charts for detecting and localizing visual abnormalities is to be mindful of the level of recorded activity. Lesions affecting cellular behavior at or close to the source (i.e. at the photoreceptors) necessarily impair the flow of visual information downstream from that level. A disturbance in the macular region may produce activity loss indistinguishable from a disease affecting the optic nerve. However, separate recordings of the focal ERG, pattern ERG, and VEP provide information from three major levels of current flow which greatly assists identification of the lesion site as well as providing a certain amount of quantitative data about the extent of damage and a clue to the nature of the disease itself

It is possible to record a pattern electroretinogram (PERG) of near normal amplitude in a situation when the eye containing the active electrode is occluded. Because PERG recording requires high amplification and sensitive signal retrieval techniques, the electrode in the occluded eye records a distant potential from the unoccluded eye. Referencing the active electrode to an ipsilateral ear diminishes, but does not eliminate the referred PERG potential. Such unlooked for interaction may provide misleading data in situations where binocular viewing is used because of poor vision in one eye; therefore, occlusion of the eye not being tested should be undertaken whenever possible

Visual evoked potentials (VEPs) and contrast sensitivity measurements were determined in normally sighted patients with Parkinson's disease (PD). Prolonged latencies of the first major positive (P100) wave of the VEP elicited by an alternating vertical grating were noted. The delay in latency was positively correlated with the severity of the movement disability. Subjective testing of contrast thresholds demonstrated significant elevation of thresholds in PD. Electroretinographic oscillatory potentials were elicited to determine if the delay in P100 latency was of retinal origin. No specific lesion could be identified to account for the abnormality in VEP or in contrast testing results

Contrast thresholds were determined for patients with compressive lesions of the chiasm or optic nerve but whose visual acuity was 20/20 on the Snellen scale. A generalized loss of contrast sensitivity for spatial frequencies ranging from 0.2 to 6.4 cycles per degree was found even in patients with normal color vision and full visual fields. After surgery was performed, improvements in the fields and color vision were seen in those patients who also showed a significant increase in their postoperative contrast sensitivity scores. Contrast sensitivity is shown to be a useful procedure for evaluating very mild visual complaints as well as for establishing early diagnosis and evaluation of surgical therapy in patients with compressive lesions of the anterior visual pathway

A broadband red filter placed over one eye will have the effect of improving the ability of certain color vision defectives to name colors correctly. A red-tinted contact lens will have the same effect; such a device is marketed under the name, the X-Chrom lens. In this article, the author examines the basic properties of color vision defects, explains the optical effect of the red filter, and reviews several studies which evaluate the effect of the X-Chrom lens. He concludes that while the lens may allow the wearer to achieve a better score on certain color vision tests, it does not actually correct color vision in the natural environment. Deleterious effects of wearing a red filter are also discussed. It is advised that patients wearing the X-Chrom lens be fully apprised of its limitations and of dangers that may be created by the associated visual distortions.

The bleaching and regeneration of rhodopsin in the living cat retina was studied by means of fundus reflectometry. Bleaching was effected by continuous light exposures of 1 min or 20 min, and the changes in retinal absorbance were measured at 29 wavelengths. For all of the conditions studied (fractional bleaches of from 65 to 100%), the regeneration of rhodopsin to its prebleach levels required greater than 60 min in darkness. After the 1-min exposures, the difference spectra recorded during the first 10 min of dark adaptation were dominated by photoproduct absorption, and rhodopsin regeneration kinetics were obscured by these intermediate processes. Extending the bleaching duration to 20 min gave the products of photolysis an opportunity to dissipate, and it was possible to follow the regenerative process over its full time-course. It was not possible, however, to fit these data with the simple exponential function predicted by first-order reaction kinetics. Other possible mechanisms were considered and are presented in the text. Nevertheless, the kinetics of regeneration compared favorably with the temporal changes in log sensitivity determined electrophysiologically by other investigators. Based on the bleaching curve for cat rhodopsin, the photosensitivity was determined and found to approximate closely the value obtained for human rhodopsin; i.e., the energy Ec required to bleach 1-e-1 of the available rhodopsin was 7.09 log scotopic troland-seconds (corrected for the optics of the cat eye), as compared with approximately 7.0 in man.

In a survey of 546 rhesus monkeys of various ages, 6.1% of the animals showed ophthalmoscopically visible hypopigmented spots in their maculas. There was a statistically significant correlation between the age of the animal and the degree of hypopigmentation. Electroretinographic responses and visually evoked potentials were evaluated in a selected group of monkeys with and without hypopigmented macular spots. No significant change in retinal function as a result of the macular abnormalities could be detected.

The bleaching of rhodopsin by short-duration flashes of a xenon discharge lamp was studied in vivo in the cat retina with the aid of a rapid, spectral-scan fundus reflectometer. Difference spectra recorded over a broad range of intensities showed that the bleaching efficacy of high-intensity flashes was less than that of longer duration, steady lights delivering the same amount of energy. Both the empirical results and those derived from a theoretical analysis of flash photolysis indicate that, under the conditions of these experiments, the upper limit of the flash bleaching of rhodopsin in cat is approximately 90%. Although the fact that a full bleach could not be attained is attributable to photoreversal, i.e., the photic regeneration of rhodopsin from its light-sensitive intermediates, the 90% limit is considerably higher than the 50% (or lower) value obtained under other experimental circumstances. Thus, it appears that the duration (approximately 1 ms) and spectral composition of the flash, coupled with the kinetic parameters of the thermal and photic reactions in the cat retina, reduce the light-induced regeneration of rhodopsin to approximately 10%.