Refractive state, contrast sensitivity, and resolution in the freshwater turtle, Pseudemys scripta elegans, determined by tectal visual-evoked potentials

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Abstract
Visual-evoked potentials (VEPs) were recorded from the surface of the optic tectum of the freshwater turtle, Pseudemys scripta elegans, in response to phase reversal of square-wave gratings of different spatial frequency and contrast. The refractive state of a group of 12 turtles in air was assessed from VEPs by placing trial lenses in front of the eye. The group mean refraction did not differ significantly from emmetropia, as compared to 4.8 diopters of hyperopia when refracted retinoscopically. The difference was explained by the retinoscopic reflex originating from the interface between vitreous humor and retina. Peak VEP amplitude was approximately linear with log grating contrast; extrapolation to zero VEP amplitude yielded contrast thresholds as low as 1%. High spatial-frequency cutoffs ranged from 4.4–9.9 cycle/deg in different animals, the highest values corresponding to the intercone spacing in the area centralis and to behavioral measures of acuity in a related species.