Rhodopsin bleaching signals in essential night blindness.

Abstract

1. The dark-adaptation curves of two subjects with essential night blindness revealed no evidence for functioning rod vision. Cone vision was normal.2. The photopupillomotor dark adaptation, and flash intensity response amplitude curves on one of these subjects confirmed the absence of rod function.3. However, there is the normal amount of rhodopsin in their rods with normal kinetics.4. Cone pigment kinetics are also nearly normal. After a full bleach, log threshold elevation of the foveal cones is linearly related to pigment regeneration. The constant of proportionality is about 3.0 as it is in the normal retina.5. After a full rhodopsin bleach, the contralateral pupil size recovered its full dark value along a curve which followed the regeneration of rhodopsin.6. The results in (5) are identical to those previously found on normal subjects.7. With the exception of a very small response attributed to the contribution of cones, no significant changes in pupil size were evoked by uniform ganzfeld steady backgrounds until the intensity of retinal illuminance was so high that appreciable rhodopsin was bleached. This contrast to the changes evoked by weak steady backgrounds in the normal eye.8. Therefore, rod bleaching signals are normal in such retinas but rod signals evoked by real lights are not functional. This supports Rushton's concept as to how bleaching signals influence retinal sensitivity as opposed to the view of Barlow.9. The defect in essential night blindness very probably involves the rod automatic gain control, but because of (4) the cone gain control must be normal.10. Therefore, rod and cone gain control mechanisms must be independent in these night blind retinas and, by analogy, in the normal retina as well.