The directionality of photoreceptors in the human retina.

The directional sensitivity of photoreceptors is a result of their structure that make them act as optical fibers. Therefore measurement of photoreceptor directionality is a tool for testing the physical properties of photoreceptors in vivo. Clinical studies of photoreceptor directionality are limited by the fact that psycho-physical methods for measuring the Stiles-Crawford effect are time consuming and require excellent co-operation from the subject. Thus different reflectometric techniques have been developed recently. This paper describes such methods, that allow to characterize the optical properties of photoreceptors, i.e. their orientation and directionality. Mechanisms likely to explain the discrepancy between the directionality factor values given by these techniques are discussed. Finally the functional advantages of photoreceptor optics are considered.

Scleral expansion bands for presbyopia.

OBJECTIVE: To evaluate the safety and effectiveness of surgery using scleral expansion bands. DESIGN: Prospective, noncomparative, small case series. PARTICIPANTS: Six subjects were enrolled. Four subjects received implants in one eye and two subjects received implants in both eyes. METHODS: Implantation of the scleral expansion bands (SEB) was performed using Schachar's standard technique. Subjects were observed at six postoperative examination intervals through 1 year. MAIN OUTCOME MEASURES: The key efficacy measures were distance-corrected near visual acuity (at 40 cm and 60 cm) and subjective amplitude of accommodation (push-up, push-down, and minus lens procedures). RESULTS: Distance visual acuity was similar before and after surgery. The near visual acuity and the subjective amplitude of accommodation were temporarily improved in three eyes. However, in the other five eyes, we did not note any improvement of accommodation or near vision after the scleral expansion surgery. CONCLUSIONS: In this study, the outcome of the SEB intervention was characterized by inconsistent and unpredictable results with a low level of patient satisfaction.

Reflectance and curvature of the inner limiting membrane at the foveola.

Light reflected specularly by the inner limiting membrane (ILM) provides information on the topography of the retinal surface. The ILM in the central part of the foveal pit acts as a concave mirror. Light reflected specularly by this mirror forms an image of the entrance pupil in front of the retina. In 15 normal subjects (ages 16-56 years) we have measured photometric and geometrical-properties of this image to derive two characteristics of the ILM: its reflectance rho at the foveola and its radius of curvature r in the central part of the fovea. rho and r are found to decrease significantly with age (p = 0.0073 and p = 0.01, respectively). The equations of the regression lines are log10 rho = -4.234 - 0.0118 age and radius r = 1484 - 13.6 age, respectively (age in years, r in micrometers).

Functional protection of photoreceptors from light-induced damage by dimethylthiourea and Ginkgo biloba extract.

PURPOSE: To investigate the functional protective effect of a synthetic (dimethylthiourea, DMTU) and a natural antioxidant (Ginkgo biloba extract, EGb 761) against light-induced retinal degeneration. METHODS: Wistar rats were exposed for 24 hours to 1700-lux light after treatment with DMTU or EGb 761. Electroretinograms were recorded before and on day (D)1, D3, D8, D15, D22, and D29 after light exposure. The b-wave amplitude was plotted against log L (ganzfeld luminance), providing the b-wave sensitivity curve. The Naka-Rushton function fitted to the sensitivity curve enabled derivation of the parameters Bmax (saturated amplitude) and K (luminance-inducing Bmax/2). In addition, rats from each group were killed for retinal morphometric analyses. RESULTS: In the untreated group, light exposure caused collapse of the b-wave sensitivity curves. Bmax was reduced by 51% at D1 without subsequent recovery. K increased temporarily, reverting to normal values 8 days later. The outer nuclear layer thicknesses decreased markedly in the superior retina. In the treated groups, light exposure had a weaker effect on sensitivity curves. The values of Bmax were not significantly different from those in the unexposed-untreated group, although K increased temporarily. Retinal morphometry was preserved. CONCLUSIONS: Dimethylthiourea and EGb 761 afford functional protection against light-induced retinal damage.

Light-induced variations of retinal sensitivity in rats.

PURPOSE: ERG responses were measured as a function of Ganzfeld luminance to evaluate functional damage induced by light on rat retinas. METHODS: Wistar rats were exposed to a fluorescent light of 1700 lux for 12 h, 24 h, 48 h and 72 h. We recorded ERGs before and one night after exposure, then 3, 8, 15, 22 and 29 days later. The b- and PIII-wave amplitudes were plotted against luminance for each group at each recovery time. RESULTS: The retinal damage induced by a pupillary illuminance of 1700 lux ranged from low to severe as exposure duration increased from 12 h to 72 h, respectively. We observed an effect immediately after light exposure but no improvement during the recovery period. The b-wave amplitude was reduced by 40, 60, 80 and 90 percent after 12, 24, 48 and 72 h of light exposure, respectively; the PIII-wave amplitude was reduced by 30, 40, 70 and 90 percent after these respective exposures. The Ganzfeld luminance eliciting a 50 microV b-wave amplitude increased significantly with exposure duration, but the luminance eliciting the maximal b-wave amplitude was not dependent on this duration. Hence we suggest that the ERG decrease is due to a reduction in photoreceptor number. CONCLUSIONS: We present a full analysis of the electrophysiological parameters recorded from light-exposed or non-exposed rats. This model is a useful tool to study in vivo retinal degeneration.

A reflectometric technique for assessing photoreceptor alignment.

Clinical studies of photoreceptor orientation are limited by the fact that psychophysical methods for measuring the Stiles-Crawford effect are time consuming and require excellent co-operation from the subject. We have developed a novel instrument, the photoreceptor alignment reflectometer (PAR), that determines photoreceptor alignment by measuring the distribution in the pupil of light reflected by one retinal location. This determination is accomplished in a measurement time of 4 sec and requires minimal co-operation from the subject. The technique is not significantly affected by reflections at the limiting membrane, or by changes in entrance and exit pupil configuration, or by location of bleaching light entry. The PAR was used to measure the orientation of foveal photoreceptors, their directionality, and the ratio of directional to diffuse flux in 20 normal subjects ranging in age from 20 to 60 yr.

Use of reflecto-modulometry to study the optical quality of the inner retina.

The modulation of interference fringes reflected by the peripheral fundus is always lowered by passage through the outer segments of the photoreceptors. In addition, it also may be degraded by passage through a diseased inner retina or through the choroid (in cases where there is only light pigmentation of the fundus). The contributions of the inner retina and choroid can be differentiated by varying the spatial frequency of fringes, or by studying the directionality of the fundus reflection. Reflecto-modulometry seems to be an appropriate method for analysing pathological changes in the optical quality of the inner retina.

Reflection characteristics of the human fovea assessed by reflecto-modulometry.

The optical quality of the human fovea was assessed by measuring the modulation of aerial fringes conjugate with the retina. The different possible causes of the decrease in modulation of this pattern were successively investigated, the main one being the passage through outer segments in unguided modes. Modulation diagrams measured on the fovea were compared with those of the perifovea.

Separation of the reflection by the inner limiting membrane.

A ray incident upon the inner limiting membrane is specularly reflected, whereas the other retinal structures reflect light diffusely into large solid angles. Thus by moving a small diaphragm inside the exit pupil, it is possible to separate the component of the reflected light which originates at the inner limiting membrane. To check that this light added to the aerial image is not caused by the specular reflection from an anterior surface (cornea or lens), interference fringes were projected onto the retina, and their modulation depth was measured in the plane conjugate with the retina: since the inner limiting membrane and the anterior media were the sources of modulated and unmodulated aerial images respectively, their contributions could be separately measured.

Diffusion of the retinal layers of the living human eye.

We measured the modulation depth of an interference pattern after double passage through the eye with different entry positions of the linearly polarized incident beam in the pupil plane. This shifting of the incident beam greatly modifies the mean irradiance and the modulation depth of the aerial image, whatever the direction of the polarizer. These measurements give a better understanding of the respective contributions of the retinal layers to the formation of the aerial image. Consequently, the modulation transfer function of the optics of the eye deduced from measurements with the aerial image must be considered cautiously.

[Use of contact lenses during argon laser iridectomies (author's transl)]. / Intérêt du verre de contact pour l'iridectomie par photocoagulation au laser à l'argon.

Advantages appear to be gained by the use of a specific contact lens during argon laser iridectomy. On the one hand, the energy level is increased on the iris and diminished on the retina. On the other hand, coagulation is directed near the iris base and far from the posterior pole. Specifications of a new contact lens are presented.