Stockman, A; Henning, B; Michaelides, M; Moore, AT; Webster, AR; Cammack, J; Ripamonti, C; (2014) Cone dystrophy with "supernormal" rod ERG: psychophysical testing reveals comparable rod and cone temporal sensitivity losses with no gain in rod function. Investigative Ophthalmology & Visual Science , 55 (2) pp. 832-840. 10.1167/iovs.13-12919.

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Abstract

Purpose: We report a psychophysical investigation of five observers with the retinal disorder "cone dystrophy with supernormal rod ERG", caused by mutations in the gene KCNV2 that encodes a voltage-gated potassium channel found in rod and cone photoreceptors. We compare losses for rod- and for cone-mediated vision to further investigate the disorder and to assess whether the supernormal ERG is associated with any visual benefit. Methods: L-cone, S-cone and rod temporal acuity (critical flicker fusion frequency-cff) was measured as a function of target irradiance; L-cone temporal contrast-sensitivity was measured as a function of temporal frequency. Results: Temporal acuity measures reveal that losses for vision mediated by rods, S-cones and L-cones are roughly equivalent. Further, the gain in rod function implied by the supernormal ERG provides no apparent benefit to near-threshold rod-mediated visual performance. The L-cone temporal contrast-sensitivity function in affected observers is similar in shape to the mean normal function but only after the mean function has been compressed by halving the logarithmic sensitivities. Conclusions: The name of this disorder is potentially misleading because the comparable losses found across rod and cone vision suggest that the disorder is a generalized cone-rod dystrophy. Temporal acuity and temporal contrast-sensitivity measures are broadly consistent with the defect in the voltage-gated potassium channel producing a nonlinear distortion of the photoreceptor response but after otherwise normal transduction processes.

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