Rod–cone interactions in congenital stationary night blindness

Abstract

In normal observers the rod system influences both cone-mediated flicker thresholds and thresholds for color (Lie specific threshold). Both types of thresholds become elevated as rods recover sensitivity after a bleach and both are reduced by light adapting the rod system. We have compared these two types of rod–cone interaction in two patients with congenital stationary night blindness (CSNB). Evidence suggests that in this condition the rod outer segments function normally (e.g., the a-wave of the scotopic ERG was normal) but there is a defect in the transmission of rod signals within the retina (rod absolute thresholds were elevated at least three log units and the scotopic ERG showed a markedly reduced b-wave). Thresholds for detecting flicker (white, 25 Hz, 1.7°, 3-s test) or color (525 nm, 1.7°, 500-ms test) were measured with a Tubinger perimeter in dark-adapted conditions, following a bleach and against a rod-desensitizing background. The rod–cone flicker interaction was normal in the two CSNB patients, but the rod influence on color thresholds was absent. These results indicate that the two types of rod–cone interaction are mediated by different neural pathways and provide additional constraints about the locus of the visual defect in these two CSNB patients.