Vitrectomy surgery increases oxygen exposure to the lens: a possible mechanism for nuclear cataract formation.

Colour vision allows animals to reliably distinguish differences in the distributions of spectral energies reaching the eye. Although not universal, a capacity for colour vision is sufficiently widespread across the animal kingdom to provide prima facie evidence of its importance as a tool for analysing and interpreting the visual environment. The basic biological mechanisms on which vertebrate colour vision ultimately rests, the cone opsin genes and the photopigments they specify, are highly conserved. Within that constraint, however, the utilization of these basic elements varies in striking ways in that they appear, disappear and emerge in altered form during the course of evolution. These changes, along with other alterations in the visual system, have led to profound variations in the nature and salience of colour vision among the vertebrates. This article concerns the evolution of colour vision among the mammals, viewing that process in the context of relevant biological mechanisms, of variations in mammalian colour vision, and of the utility of colour vision.

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Evolution of colour vision in mammals

Time as an ecological niche variable or “temporal niche” can be defined in the context of the most prominent environmental cycles, including the tidal cycle, the lunar day and month, the solar day, and the earth year. For the current review, we focus on the 24-h domain generated through the earth's rotation around its axis (solar day). The daily environmental cycles of light and temperature are a dominant ecological factor generating a variety of adaptations among animals. In this review, we describe these adaptations with a special focus on the visual system and on the adaptive plasticity of activity patterns. Our goals are: (1) Underscore the importance of the 24-h time axis as critical variable in the ecological niche. (2) Highlight cases of temporal niche switches at the evolutionary timescale (phylogenetic level). (3) Review temporal niche switching within an individual's lifespan. (4) Evaluate possible underlying mechanisms for temporal niche switching. (5) Describe a new hypothesis of circadian thermoenergetics which may explain several cases of temporal niche switching in mammals. With this, we hope to inspire experiments under natural conditions or more complex laboratory environments, aimed to reveal environmental factors and mechanisms underlying specific temporal programs.

In search of a temporal niche: environmental factors.

The spectral information available to an animal’s visual system depends both on the wavelengths reaching its outer segments and the visual pigments contained within them. The former is governed not only by the chromatic stimuli present in the environment, but also by the degree to which these are modified through intraocular filters, before being absorbed by the visual pigments. Although the ocular media of the majority of animals are transparent to light above about 310 nm, their primary function being either refractive or nutritive, in some, pigments are present that filter the spectral content of the light reaching the retina. Light impinging on the visual pigments may have its spectrum further modified by both filters within the retina itself and by reflective structures behind the retina.

A review of vertebrate and invertebrate ocular filters

PURPOSE. Vitrectomy is associated with the rapid progression of nuclear cataracts. This study was conducted to test the hypothesis that age-related liquefaction of the vitreous gel may also be associated with lens opacification. METHODS. Lenses from eye bank eyes were graded for nuclear, cortical, and posterior subcapsular opacities, and the amount of liquid vitreous was measured. RESULTS. Nuclear sclerosis (NS) grade, percent liquefaction, and age all correlated highly (P 0.0001). After adjustment was made for age, the correlation between NS and percent liquefaction was 0.37 (P 0.0001), and the correlation between age and NS after adjustment for liquefaction was 0.57 (P 0.0001). After the eyes were stratified into age groups, the correlation between NS and percent liquefaction was found to be highest in eyes from donors aged 51 to 70. Cortical and posterior subcapsular opacities increased with age, but scores for these cataract types did not significantly correlate with vitreous liquefaction. CONCLUSIONS. Loss of vitreous gel during aging increases the risk of nuclear cataracts. Preservation or replacement of the vitreous gel may protect patients from nuclear cataract. (Invest Ophthalmol Vis Sci. 2004;45:77‐ 85) DOI:10.1167/iovs.030820

Importance of vitreous liquefaction in age-related cataract.

Visual sensitivity and color vision in ground squirrels

Center-surround balance in receptive fields of cells in the lateral geniculate nucleus

Spectrophotometric measurements of preretinal absorbance were made on eyes from prairie dogs, western gray squirrels, four species of ground squirrels, and a southern flying squirrel. Results indicate that lenses from all species except the flying squirrel absorb significantly in the spectral region from 520 to 440 nanometers. At 440 nanometers, relative preretinal absorbance was greatest for the prairie dogs (1.31) followed in order by the ground squirrels (1.12 to 1.04), the gray squirrels (0.54), and the flying squirrel, which had an essentially transparent lens and cornea (0.08).

https://www.jstor.org/stable/info/1379253

Preretinal absorbance in sciurid eyes.

Background. Hyperbaric oxygen therapy (HBOT) involves exposing patients to elevated oxygen pressures and concentrations for the treatment of soft tissue degenerative diseases. As side effects, some patients experience cataract development and/or a significant increase in myopia. Subjects. Eight subjects participated in this project. All were receiving HBOT for health problems. Methods. For all subjects, acuities and refractive error corrections were measured before and after HBOT sessions at the beginning, midpoint, and end of therapy. For four subjects, A-scan ultrasound and corneal curvature measurements were also made before and after selected HBOT sessions. In addition, susceptibility to free radical damage and indicators of actual damage were measured by blood analyses. During the first half of HBOT therapy, four subjects took placebo tablets; during the second half, antioxidant supplements were given. Results. No subject showed significant acuity or refractive error changes produced by single HBOT sessions. Two of the 8 subjects showed significant increases in myopia over the 20 session course of HBOT. No corresponding changes were found in the ultrasound, corneal curvature, or blood analysis data. Discussion. Theoretically, the increases in myopia could have been caused by lenticular refractive index changes resulting from oxidative damage to lens proteins. However, the biochemical assays used in this project did not predict which subjects were susceptible to increased myopia, nor did they demonstrate elevated levels of free radical activity in a subject who did experience an increase in myopia.

Robert L. Yolton

Papers

Visual sensitivity and color vision in ground squirrels

Center-surround balance in receptive fields of cells in the lateral geniculate nucleus

Preretinal absorbance in sciurid eyes.

Myopia associated with hyperbaric oxygen therapy

Some characteristics of the eye and the electroretinogram of the prairie dog