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Receptor and Middle Retina ERG Components in Achromatopsia

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Abstract

The ganzfeld ERG of autosomal recessive achromats is characterized by the lack of cone responses, and frequently borderline or mildly subnormal dark adapted (rod) responses. The purpose of this research is to determine why rod responses of achromats often appear subnormal. One possibility is that the achromats' rod responses are compared to norms that have significant cone components, at least at some flash intensities. If so, we might expect that achromats' dark adapted responses to appear progressively more subnormal at higher flash intensities where the normals' cones make a larger contribution. In fact this has been documented (Krill, 1977). However, it is also possible that this accounts for only part of the discrepancy, and that the rods themselves or the inner retinal responses of achromats are in some way abnormal. To help understand the cause of the achromats' borderline dark adapted ERG we isolated components that correspond to rod receptor activity (Granit's P3, derived from the leading edge of the a-wave) and middle/inner retina activity (Granit's P2, which sums with P3 to give the b-wave). Fig 1 shows the relationship between P2, P3 and the ERG recorded at the cornea (data show the response to a standard Grass 18, for a dark adapted achromat). P3 components were fit using a computational model based on Lamb and Pugh's (1992) description of individual rod responses (Hood and Birch, 1993; Breton et al. 1994). By subtracting the P3 element from the corneal ERG, the electro-positive P2 component remains. We fit P2 responses with the Naka-Rushton equation, over approximately a 6 log unit range of flash intensities.

© 1996 Optical Society of America

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