The electroretinogram (ERG) and single-unit ganglion cell activity were recorded from the eyecup of the skate (Raja erinacea and R. oscellata), and the adaptation properties of both types of response compared with in situ rhodopsin measurements obtained by fundus reflectometry. Under all conditions tested, the b-wave of the ERG and the ganglion cell discharge showed identical adaptation properties. For example, after flash adaptation that bleached 80% of the rhodopsin, neither ganglion cell nor b-wave activity could be elicited for 10–15 min. Following this unresponsive period, thresholds fell rapidly; by 20 min after the flash, sensitivity was within 3 log units of the dark-adapted level. Further recovery of threshold was slow, requiring an additional 70–90 min to reach absolute threshold. Measurements of rhodopsin levels showed a close correlation with the slow recovery of threshold that occurred between 20 and 120 min of dark adaptation; there is a linear relation between rhodopsin concentration and log threshold. Other experiments dealt with the initial unresponsive period induced by light adaptation. The duration of this unresponsive period depended on the brightness of the adapting field; with bright backgrounds, suppression of retinal activity lasted 20–25 min, but sensitivity subsequently returned and thresholds fell to a steady-state value. At all background levels tested, increment thresholds were linearly related to background luminance.

https://rupress.org/jgp/article-pdf/56/4/491/1241311/491.pdf

Visual Adaptation in the Retina of the Skate

1. Rhodopsin has been measured by Rushton's method of reflexion densitometry in a retinal region 18 degrees temporal to the fovea, using a wavelength of measuring light (555 nm) so far into the long wave part of the spectrum that possible blue absorbing intermediates (e.g. transient orange) do not interfere.2. Rhodopsin was bleached by a strong light for 10 sec and then held steady by a weaker light. During a 10 sec bleach, no regeneration occurs and the rate of bleaching is proportional to the quantum catch. The proportionality constant is about 10(-7) (td sec)(-1).3. From 2, the rate of photolysis at equilibrium produced by the steady light was calculated. Since conditions were at equilibrium, photolysis matched regeneration. It was found that the rate of generation was proportional to the amount of pigment still bleached. The proportionality constant was about 0.0025 sec(-1).4. It was found by several different methods that the constant in 3 is the same in the light or dark and hence regeneration occurs independently of bleaching.5. Therefore, the results from bleaching and regeneration experiments can be combined to give the general equation [Formula: see text], where p is the fraction of rhodopsin, t is time in sec and I is the retinal illuminance.6. This equation describes the results of partial bleaching and regeneration experiments under a variety of different exposure intensities of moderately long (at least 10 min) exposure durations.7. The dark adaptation curve in a peripheral region of the rod monochromat's retina where there are few cones follows a simple exponential course over nearly 7 log(10) units. Rhodopsin regeneration and log threshold for this region are described by the same curve with a time constant of about 400 sec. Each log unit fal in threshold is accompanied by 0.835% increase in rhodopsin. This time constant is in agreement with Rushton's (1961) finding, but appreciably longer than that reported by Ripps & Weale (1969a).8. The Ripps & Weale result was, however, obtained by bleaching with a very short bright xenon flash (as they did). Under these conditions, blue absorbing intermediate(s) is (are) formed, the time constant of regeneration of rhodopsin is much faster than after long tungsten bleaches, and the kinetic equation is not valid.9. The general equation, together with the relation found in 7, successfully accounts for results previously published by others of the effect of duration and intensity of bleaching on the recovery of rod threshold in the dark, provided only that more than 5% of the rhodopsin was bleached at the beginning of dark adaptation.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1971.sp009580

Rhodopsin kinetics in the human eye.

The importance of being able to study the visual pigments in man lies in this; that visual performance depends upon the nature, amount and state of bleaching of the pigments in the rods and cones. There are two distinct ways in which visual pigments are related to vision. The first, (a), is easy to comprehend; roughly it is that receptors can only respond to the quanta they catch, and hence the spectral sensitivity of the receptor will correspond to the spectral absorption of the pigment it contains. The second, (b), is far more difficult to comprehend though the relation can easily be stated. Roughly it is that in recovery after bleaching, the logarithm of the visual threshold is raised by a quantity proportional to the fraction of pigment still in the bleached state.

Visual Pigments in Man

The inability of night-blind persons to see at low levels of illumination is usually attributed to a defect involving the biosynthesis of the visual pigment, rhodopsin. Accordingly, it has been assumed that the retinal rods of the congenital nyctalope are deficient in rhodopsin or its precursors. To test this hypothesis, fundus refiectometry was performed on two subjects, each having a different form of congenital night blindness (dominant and recessive, respectively). In both subjects, the concentration of rhodopsin and the rate at which it regenerated after bleaching were normal. Clearly, a photochemical basis for this anomaly is untenable. However, the electrophysiological responses of these subjects showed distinct abnormalities: (a) In the case of recessive inheritance, there was a normal light rise in the eye's standing potential, normal a-wave potentials in the ERG, but a very much reduced scotopic b-wave. (b) In the dominant form, the light rise in the standing potential was abnormal, and all ERG responses loere markedly depressed. These findings implicate the neural transmission pathways as the locus of disturbance in this anomaly. In view of a normal photochemistry, analysis of the restdts suggests that both the a-toave of the ERG and the light rise in the standing potential originate proximal to the outer segments of the receptors.

Rhodopsin and the electrical activity of the retina in congenital night blindness.

We investigated foveal cone photopigment kinetics by retinal densitometry in 34 eyes of 29 healthy subjects with clear optical media and good visual acuity, ranging in age from 39 to 79 years. Our aim was to assess possible senile disturbances of foveal cones. To assess the effects of ocular straylight, we measured not only in subjects with a clear crystalline lens, but also in pseudophakia and aphakia. In a limited number of subjects color vision was assessed with a Nagel anomaloscope; no systematic changes with age were found. A significant decrease in two-way density and in time constant of regeneration was found to occur only after age 60, with large individual variations. There was no indication that results for subjects with their natural crystalline lens, in aphakia, or in pseudophakia were different. We argue that a reduction in the number of cones with age, rather than an increase in ocular stray light is the most likely explanation of our findings.

R.A. Weale

Papers

Further studies of photo-chemical reactions in living human eyes