Rapid dark adaptation in the frog rod

The aspartate-isolated late receptor potential of the perfused frog retina was studied. Evidence was obtained for the existence of a rapid dark adaptation process within the rods themselves. Possible mechanisms for the phenomenon are discussed and it is suggested that the adaptation may involve an interaction between an inhibitory particle and either an excitatory particle or an active site on the plasma membrane of the rod.     

Prolonged rod dark adaptation in retinitis pigmentosa.

The time course of rod dark adaptation was measured in 12 patients and carriers of retinitis pigmentosa (RP). In contrast to previous studies the rod absolute threshold was determined prior to any exposure to the bleaching light. For seven of the patients and carriers the recovery of rod sensitivity to the prebleach level was prolonged, with the delay ranging from mild to severe. The prolongation appeared to be limited to the late phase of bleaching recovery; the early portion of rod dark adaptation, measured following a weak bleach, was normal. These results suggest a selective impairment in the processes underlying rod dark adaptation in some patients with RP.     

Delayed rod dark adaptation in patients with Stargardt's disease

Twelve patients with Stargardt's dystrophy were each found to have a prolongation in rod dark adaptation. All had a normal rate of recovery during the early portion of rod dark adaptation but a selective prolongation of the later segment of rod recovery. This observation was apparent in patients with limited fundus flecks and those with extensive fundus flecks, whether or not a dark choroid was observed and independent of the presence or absence of an atrophic-appearing macular lesion. A defect within retinal pigment epithelial cells of an enzyme or intracellular transport mechanism involved in the visual pigment regeneration cycle could account for these findings.     

Pupillary changes during dark adaptation in human infants

Pupillary diameter of 10 infants (age 10 weeks) and four adult subjects was measured during 30 min of dark adaptation following exposure to a full-field adapting light. Adult results confirm that, under these conditions, the course of pupillary recovery was reasonably well described by an exponential time course (t0 = 408 sec; SD = 42 sec), as is rhodopsin regeneration. Pupillary recovery of infants also could be described by an exponential course (average t0 = 399 sec; SD = 31 sec). These results, demonstrating similarities between adaptive functions of infants and adults, suggest that pupillographic techniques can be used to investigate dynamic processes accompanying dark adaptation in preverbal children.     

The human rod ERG: Correlation with psychophysical responses in light and dark adaptation

The a- and b-waves over a large range of human scotopic ERG responses in various states of adaptation may be represented compactly by “H₂” curves. This representation allows correlation of b-wave and psychophysical thresholds obtained in the same conditions of adaptation and shows sensitivity and scaling of response size are two separable features of both the a- and b-waves in light and dark adaptation. The relation of these responses to the underlying retinal processes is discussed.     

Aging and dark adaptation

Older adults have serious difficulty seeing under low illumination and at night, even in the absence of ocular disease. Optical changes in the aged eye, such as pupillary miosis and increased lens density, cannot account for the severity of this problem, and little is known about its neural basis. Dark adaptation functions were measured on 94 adults ranging in age from the 20s to the 80s to assess the rate of rod-mediated sensitivity recovery after exposure to a 98% bleach. Fundus photography and a grading scale were used to characterize macular health in subjects over age 49 in order to control for macular disease. Thresholds for each subject were corrected for lens density based on individual estimates, and pupil diameter was controlled. Results indicated that during human aging there is a dramatic slowing in rod-mediated dark adaptation that can be attributed to delayed rhodopsin regeneration. During the second component of the rod-mediated phase of dark adaptation, the rate of sensitivity recovery decreased 0.02 log unit/min per decade, and the time constant of rhodopsin regeneration increased 8.4 s/decade. The amount of time to reach within 0.3 log units of baseline scotopic sensitivity increased 2.76 min/decade. These aging-related changes in rod-mediated dark adaptation may contribute to night vision problems commonly experienced by the elderly.     

Rhodopsin content and rod outer segment length in albino rat eyes: Modification by dark adaptation

Whole eye rhodopsin content and rod outer segment length have been determined in albino rat eyes after different periods of dark adaptation, dark-rearing or cyclic light maintenance. The rhodopsin content in the eyes of dark-reared albino rats is approximately 50% higher than that in the eyes of littermates reared in cyclic light (in-cage illumination less than 15 ft-cd). The same increase in rhodopsin can be obtained in albino rats after only a 10-day dark adaptation period. Furthermore, the increased level of rhodopsin is the same as that in the eyes of pigmented rats reared in cyclic light. The increase in rhodopsin content in the albino rat eyes is due at least in part to an increase in rod outer segment length. The increase in length with dark adaptation and dark-rearing was somewhat variable, with an average increase of approximately 25%. No apparent increase was found in rod outer segment diameter, rod outer segment disc packing density or eye size. Rod outer segment lengths were consistently longer in the superior hemisphere of the eye than in the inferior hemisphere in both dark-adapted albino rats and pigmented rats maintained in cyclic light, but not in albino rats maintained in cyclic light.     

The repeatability of the Goldmann-Weekers adaptometer for measuring cone adaptation

To assess the inter-session repeatability of the Goldmann-Weekers adaptometer for the measurement of cone dark adaptation in a population of healthy subjects. Data were obtained from 31 healthy adults (mean age 21.5 ± 2.5) on 2 days. At each visit, pupils were dilated and a 96% bleach of cone photopigment was administered to the test eye before threshold was monitored continuously for 5 min in the dark using the Goldmann-Weekers adaptometer. A single exponential function was fitted to the threshold recovery data on a least squares basis. The coefficient of repeatability (CoR) was calculated to assess the repeatability of the time constant of recovery (τ), initial threshold and final threshold. Cone dark adaptation functions were successfully recorded from all subjects on both visits. The CoR was 79.48 s for τ, 0.71 log cdm⁻² for the initial threshold, and 0.58 log cdm⁻² for the final threshold. Paired samples t-tests showed that there were no significant differences between visits for any of the parameters assesed. Although the Goldmann-Weekers adaptometer was capable of monitoring the rapid changes in threshold that occur during cone dark adaptation, the CoR for τ was relatively large compared to the mean recovery time constants (126.48 ± 40.33 and 119.94 ± 33.25 s at the first and second visits, respectively). This indicates that the Goldmann-Weekers adaptometer is unlikely to be a useful instrument to chart changes in an individual’s vision over time.     

Abnormal rod dark adaptation in autosomal dominant retinitis pigmentosa with proline-23-histidine rhodopsin mutation.

We studied rod and cone function in 13 patients from four families with autosomal dominant retinitis pigmentosa and the proline-23-histidine rhodopsin mutation. In patients with early stages of this disease, rod sensitivity was mildly abnormal throughout the retina and cone sensitivity was normal. In more severely affected patients, sensitivity loss varied with retinal region, some regions showing mild rod loss only and other regions having pronounced rod and cone dysfunction. Rhodopsin levels were decreased below normal by amounts that indicated the rod sensitivity loss was determined by the reduced ability to absorb light. The most characteristic abnormality of this genotype was a slowed rod branch of dark adaptation, which was present regardless of the extent or severity of disease. The time required for recovery of rod sensitivity was more than twice the normal time. These findings with dark-adapted perimetry, fundus reflectometry, and dark adaptometry showed intrafamilial and interfamilial consistency.     

Abnormal dark adaptation kinetics in autosomal dominant sector retinitis pigmentosa due to rod opsin mutation.

The time course of dark adaptation was measured in 10 subjects from three families with autosomal dominant sector retinitis pigmentosa (RP) due to mutations in the first exon of the rod opsin gene. In each subject cone adaptation and the early part of the recovery of rod sensitivity followed the normal time course, but the later phase of rod adaptation was markedly prolonged. The recovery of rod sensitivity is much slower than that reported in any other outer retinal dystrophy. Using a model based upon primate data of rod outer segment length and turnover, we have calculated that the delayed phase of the recovery of rod sensitivity in the RP patients tested following strong light adaptation could be due in part to formation of new disc membrane with its normal concentration of rhodopsin rather than in situ regeneration of photopigment.     

Surround configuration and cone dark adaptation

Parafoveal cone dark adaptation curves were recorded following bleaches of various spatial configurations. Bleaching patterns having a contour in the neighborhood of the test stimulus gave dark adaptation curves similar to those after uniform bleaching patterns. This is evidence against an explanation of the sensitization observed in cone dark adaptation in terms of central perceptual interference or masking of the test spot by nearby contours, and suggests that the total amount of surround light, and not its spatial distribution, is what is important.