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Robert Follett Rosencrans, Keith Perkins, William C Gordon, Corinne Richards-Zawacki, Nicolas G Bazan, Hamilton E. Farris; Validating Optical Predictions of Sensitivity in Vertebrate Eyes. Invest. Ophthalmol. Vis. Sci. 2016;57(12):6256.
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© ARVO (1962-2015); The Authors (2016-present)
The Land sensitivity equation (1981) has the potential to describe the optical sensitivity of any eye. In practice, however, this simple and useful tool has primarily been used to examine invertebrate optics. In the current study, using a comparative approach we empirically demonstrate that the Land sensitivity equation can predict physiologically measured sensitivity in vertebrate eyes.
Electroretinograms (ERGs) were conducted in two species of diurnal frogs and two species of nocturnal frogs using the Espion Ganzfeld Dome (Diagnosys LLC). Following ERGs, optical measures were obtained. Maximal pupillary diameter (aperture) was measured using infrared photography on dark-adapted, atropine-dilated animals. Following euthanasia, photoreceptor outer segment dimensions (diameter and length) were obtained from both plastic embedded semithin sections and formalin-fixed frozen sections. Focal lengths were obtained from fresh, flash-frozen retinal sections.
As predicted by visual ecology, ERGs in diurnal frogs required significantly higher light intensities to achieve 10% response thresholds, as compared to nocturnal frogs. Diurnal and nocturnal frogs were significantly different with respect to all optical variables with the exception of photoreceptor outer segment diameter. Nocturnal frogs exhibit 3- to 4-fold larger aperture diameters as compared to diurnal frogs. Similarly, focal lengths are between 2 and 3 times larger in nocturnal frogs. Finally, nocturnal frogs have between 1- and 2-fold longer photoreceptor outer segments. Assuming constant densities of rhodopsin across the four species, solutions to the Land sensitivity equation match sensitivity differences measured using ERGs: the equation predicts 1 order of magnitude greater sensitivity in nocturnal frogs, similar to the 1-2 orders of magnitude differences observed in ERG thresholds.
Using the comparative method, four species from different light environments were selected. As predicted, these species differed with respect to absolute sensitivity to light. Because optical measurements predicting sensitivity are consistent with independent physiological measures of sensitivity using ERGs, the data represent a validation of the Land sensitivity equation in vertebrates and provide insight into how visual systems may evolve in different visual ecologies.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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