Dark Adaptation and Visual Pigment Regeneration in Human Cones

Abstract

Foveal threshold elevation and red-green cone pigment regeneration have been studied in the dark after a wide range of bleaches in normal man with a view to probing the limits of the application of the Dowling-Rushton relation: i.e., the direct proportionality between log threshold elevation and fraction of unregenerated pigment. Cone pigment regeneration (and threshold recovery) is much faster after short bleaches than expected from the kinetics of a simple monomolecular reaction. Recovery is faster after a fixed (short) duration bleach the weaker it is. Except for the first 30 s after relatively weak bleaches and the entire recovery after a very brief (<0.001 s) saturating bright flash which bleaches a little more than 50 %, the results are accurately fit by the Dowling-Rushton relation over the entire range tested with only one arbitrary constant (the proportionality factor). Theory predicts too low threshold in comparison with what is obtained, for both of these exceptions It is a familiar fact that the recovery of visual sensitivity in the dark after significant bleaching proceeds only very slowly, and it was long presumed that this was because the visual pigment regenerated at a corresponding rate. However, experimental attempts to provide the justification for this presumption for rods went unrewarded until Dowling (1960), working with the electroretinogram of the rat, and Rushton (1961), measuring psychophysical thresholds on a human subject deficient in cones, found a linear relationship between the log threshold and the fraction of unregenerated rhodopsin. Their equation can be written log (Et/E 0 ) = a(l -p).

The effect of bleach duration and age on the ERG photostress test

“…The literature suggests that the kinetics of cone photopigment recovery are different in these two situations. Following an equilibrium bleach the time constant of cone recovery is not dependent on the percentage of photopigment bleached, whilst the recovery following a photoflash is prolonged with an increasing percentage of photopigment bleached [3]. For exposures that bleach the same amount of photopigment, the time constant of cone recovery is also shorter after exposure to a brief flash than after an equilibrium bleach [3].…”

“…Following an equilibrium bleach the time constant of cone recovery is not dependent on the percentage of photopigment bleached, whilst the recovery following a photoflash is prolonged with an increasing percentage of photopigment bleached [3]. For exposures that bleach the same amount of photopigment, the time constant of cone recovery is also shorter after exposure to a brief flash than after an equilibrium bleach [3]. Given that clinic time is at a premium, the use of a photoflash is an attractive alternative to the longer duration equilibrium bleach currently used with the ERG photostress test [2].…”

“…The effective retinal illuminance was calculated as 5.2 log photopic td, when adjusted for the Stiles-Crawford effect, which bleached approximately 84% of the cone visual pigment [3].…”

“…The first method typically involves the use of a photoflash unit which produces a very brief but intense flash of light [3][4][5][6][7][8]. The second method is to expose the retina to a less intense but longer duration light until equilibrium is reached between photopigment bleaching and regeneration [1][2][9][10][11].…”

“…The eye was exposed to a bright white flash of 7.3 log td.s for a period of approximately 6.6 ms. The effective retinal illuminance, adjusted for the Stiles-Crawford effect, was calculated as 6.9 log photopic td.s, which bleached approximately 98% of the cone visual pigment [6][7].…”