Walking into a dark room leaves you blind for minutes, and the recovery is slow enough to be felt. Two very different ways of measuring it exist: asking a person when they can just see a light, and recording the electrical response of the retina itself. If both track the same underlying event they should recover together. So the two were compared after bleaches ranging from a few percent to nearly total.
Both track the same chemical clearance. Over much of its range the recovery followed an S2 component in both measures, though with different slopes, 0.36 log units per minute psychophysically against 0.22 for the retinal recording. The psychophysical sat about one log unit lower. Despite those differences, the two methods extract a common time course for the decline of in the rods.
The familiar experience of walking into a dark cinema and seeing nothing for minutes is dark adaptation, and it is slow enough that something physical must be limiting it. Two independent methods measure it: psychophysical threshold, where an observer reports the faintest light they can see, and the b-wave of the , which records the retina directly.
This study compared the time course of both, for bleaches ranging from a few percent of the pigment up to near total. The authors also recorded light adaptation, so a Crawford transformation could turn the raw figures into equivalent background intensities, putting the two instruments on a common scale.
For both the psychophysical threshold equivalent and the electroretinogram sensitivity equivalent, the equivalent background declined over much of its range with an S2 component. The slopes differed, at 0.36 log units per minute psychophysically against 0.22 for the retinal recording, and the magnitude of the equivalent background was roughly one log unit lower in the psychophysical experiments.
Despite those differences the two approaches extract a common time course for the decline in the level of free opsin following moderately large bleaches. The authors conclude that the recovery of psychophysical threshold over the S2 region reflects events already present by the first synapse of rod vision, stemming from unregenerated opsin in the rod outer segments. So the wait in a dark room is a chemical one, happening before the signal has left the retina.
wat. Dark adaptation is the slow clearance of unregenerated opsin from the rods, and two independent measures of it agree on the timetable.
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It identifies what the delay actually is. You are not waiting for your pupils or your attention, you are waiting for a molecule to clear, and the clearance is measurable from two independent directions.
How do we know?
Ruseckaite and colleagues compared the time course of human scotopic dark adaptation, measured psychophysically and from the electroretinogram b-wave, across bleaches from a few percent to near total. A Crawford transformation converted the measurements into equivalent background intensities.
What is still unknown
A technical psychophysics and electrophysiology study with a small number of observers, and the conclusion about the first synapse is an inference from where the two measures agree rather than a direct recording there.
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