Abstract
Circadian rhythms of physiological processes in the retina such as disk shedding, dopamine levels, and melatonin levels have been a topic of recent research interest. These physiological rhythms suggest the possibility of corresponding rhythms in visual sensitivity. To examine this possibility, we obtained psychophysical dark adaptation curves using a digitally controlled two-channel Maxwellian view adaptometer at 2-h intervals around the clock. One curve was obtained at a randomly selected time per day, and at least 24 h elapsed between test sessions. Cone and rod terminal thresholds as well as time constants for recovery of sensitivity were compared for different times of day. Preliminary results indicate an increase in threshold for both cones and rods in the middle of the night (2-4 a.m.) compared with all other times, with no apparent change in time constant of recovery. These results confirm a recent report1 but contradict an earlier report2 which showed a decrease in threshold in the middle of the night. Methodological differences which may contribute to the discrepant results will be discussed. Further testing includes the use of a two-alternative forced choice procedure intended to give a measure of observer criterion changes over time of day.
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