Dark adaptation
= Dark adaptation
\b[Dark adaptation] is recovery of visual sensitivity after exposure changes from brighter surroundings to darkness or much dimmer conditions. Operationally, a fixed test stimulus becomes detectable at a lower intensity as adaptation proceeds. This is more than the increase in light capture caused by dilation of the <pupil>: changes in retinal sensitivity and <visual cycle> chemistry also matter.
The direction of a threshold plot can be misleading. In an invented example, detection threshold falls from 1,000 to 1 arbitrary intensity unit under otherwise matched testing conditions. Sensitivity defined as reciprocal threshold has increased 1,000-fold; the threshold has fallen by three logarithmic units. A descending curve therefore represents improving sensitivity.
<Selig Hecht> and collaborators measured how preceding light exposure changed subsequent human <dark adaptation>. Stronger pre-exposure produced an initial cone-dominated phase followed by a delayed rod phase in their experiments, while lower pre-exposures did not show the same two-phase pattern. The exposure history is thus part of the measurement, not an irrelevant preparation detail.<The influence of light adaptation on subsequent dark adaptation of the eye by Selig Hecht and others>[1937 abstract: preadaptation intensity, conditional cone/rod phases and delayed rod recovery.]{ref}
The <visual cycle> provides a biochemical connection. <RPE65>-deficient mice lacked normal rod pigment and rod responses despite retained <opsin>, showing that producing the receptor chain alone cannot replace the chemical supply needed for functional <rhodopsin>.<Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle by T. M. Redmond and others>[Abstract: retinoid composition, opsin and rod electroretinographic phenotype.]{ref}
The knockout is not a normal human recovery curve. Psychophysical thresholds, electrical <electroretinography> responses and pigment chemistry measure different levels of the system. Comparisons of adaptation curves should specify stimulus properties and preceding exposure rather than assuming a universal recovery time.