Dark adaptation threshold

Abstract: Luminance thresholds for the visual resolution of various widths of alternating light and dark lines were determined at various times during dark adaptation. The finest gratings, representing high degrees of visual acuity, show only a single cone curve that drops from a high luminance threshold during the first moments of dark adaptation to a final steady level that is reached after about 7 to 10 minutes in the dark. Coarse gratings produce a duplex curve that shows an initial cone portion and a delayed rod portion. Visual acuity is a parameter that sets the position of a given curve on the log threshold axis. The higher the degree of resolution required, the higher the dark adaptation threshold. At a constant grating luminance, visual acuity rises rapidly to a maximum during dark adaptation; the higher the luminance, the earlier and more rapid the rise and the higher the maximum. Visual acuity increases at all dark adaptation times with increase in luminance.

Abstract: The effects of 16 d of sustained hypoxia (4300-m equivalent) on the dark adaptation threshold function were studied in 12 male subjects measured periodically (days 2,4,6,9,11,13,16 of exposure) over a 20-min test period for both red and green stimuli using a new computerized dark adaptometer. Comparison with sea level performance showed negligible elevations of thresholds for red response, but highly significant impairment of green response (p less than 0.00001) over almost the entire dark adaptation function. These losses peaked between the sixth and ninth day followed by little recovery, except at the eleventh day when the subjects descended briefly to 3200 m elevation. Impairments developed rapidly again upon return to the original higher altitude. The results differ from previous findings after shorter exposure periods, which showed only slight impairments of the early segment of dark adaptation. Implications of the results are discussed.

Abstract: The course of foveal dark adaptation was studied as a function of the intensity and duration of preexposure. Four intensities (11,300, 5,650, 1,130, and 565 mL.) and four durations (300, 150, 30, and 15 seconds) were used in all combinations of intensity and duration. The threshold-measuring instrument was a monocular Hecht-Shlaer adaptometer and the threshold measurements were recorded in log micromicrolamberts. There were two subjects and each went through the complete series of intensities and durations five times. The five logarithmic values obtained for each threshold were converted into a geometric mean and these means were the data used in the analysis of the results. The chief results were as follows:— 1. For each subject the final steady threshold value was in the region of 7.0 log µµL. 2. As the intensity, or duration, or both, were increased the initial foveal dark adaptation threshold rose, the slope of the curve decreased, and the time to reach a final steady threshold value increased. 3. For those values of preexposure intensity and time for which the product, I x t, is a constant it was found that for the two higher intensities and two longer durations and also for the two lower intensities and two shorter durations, the dark adaptation curves were the same. For other values of I x t = C the curves were generally not the same.

Abstract: 1. Sensitivity to white light was quantified in euthymic bipolar male and female patients maintained on long term lithium therapy and age and sex matched unmedicated controls. 2. The Dark Adaptation Threshold procedure was used to assess sensitivity of both the cone and rod photoreceptors to short pulses of light. 3. Male and female controls did not differ in sensitivity to light. 4. Male patients in comparison to both controls and female patients evidenced reduced sensitivity to light during the cone and rod portion of the dark adaptation procedure. 5. Female patients did not differ from controls on sensitivity to light. 6. Using these and other published data the results were interpreted as suggesting that lithium reduces sensitivity to light during adaptation to dark.