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W.R. Elliott, R.D. Glickman, H. Rentmeister-Bryant, H. Zwick; Functional Assessment of Snake Retina using Pattern ERG . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2706.
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Purpose: The small size, high numerical aperture, all cone retina, and spectacle of the snake eye combine to form a unique suite of ocular properties. These features facilitate the prolonged imaging through the natural pupil of cellular detail in distinct layers of retinal tissue using a confocal Scanning Laser Ophthalmoscope (cSLO). This animal model has been used to examine, in-vivo, laser induced retinal photoreceptor damage and its temporal development (Zwick, et al., SPIE, CIS,V.2732). Now we report the use of the pattern ERG (PERG) to provide data for the assessment of local retinal function. Methods: Three Great Plains Rat snakes (Elaphe guttata emoryi) were imaged using a Rodenstock cSLO. Subjects were anesthetized with a cocktail of ketamine and xylazine. Visual stimuli, consisting of square wave gratings, were projected on to the retina using the cSLO’s HeNe laser (632.8 nm). The cSLO’s acousto-optical modulator was driven with an external, computer-controlled stimulus generator (VisionProbe). PERG’s were recorded with a Grass needle electrode inserted through a small incision made in the snake’s spectacle, near the edge of the clear cornea. PERG’s were obtained with signal averaging, as well as with synchronous demodulation using a lock-in amplifier referenced to the grating counterphase rate. Results: Gratings of ~0.25 cpd counterphased at 1 to 2 Hz elicited the largest amplitude PERG’s, ca. 0.75 – 5 microvolt. The PERG amplitude declined between 2 and 6 Hz, and was unrecordable at higher counterphase frequencies. Defocusing the pattern stimulus, by introducing additional dioptric corrections into the cSLO, degraded the PERG response. There did not appear to be any retinal specificity to the PERG response, at least for the central 40 deg of visual field. Conclusions: Previous work characterized the spectral sensitivity of the flash ERG (Jacobs et al., J. Comp. Physiol. A 170:701, 1992), but this is the first report of a pattern-evoked response recorded from the snake’s eye. The PERG provides an objective means to characterize the effect of laser exposures and other external stressors on retinal function, including the ability to relate decrements in visual function with local disruption of retinal photoreceptors imaged in vivo.
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