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Michael Eugene Rudd, Fred Rieke; Brightness in human rod vision is regulated by neural adaptation to photon statistics. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3001.
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© ARVO (1962-2015); The Authors (2016-present)
For adapting luminances producing 0.01-1.0 photoisomerizations per rod-sec (R*/rod/s), visual threshold for small, brief flashes is proportional to the square root of adapting luminance. This law was classically attributed to masking of dim flashes by irreducible quantum statistical fluctuations in the photon absorption rate from the adapting field. An alternative hypothesis attributes the law to a neural light adaptation mechanism whose gain varies inversely with the standard deviation of the fluctuations in absorption rate within a rod pool. The second, but not the first, theory predicts square root gain effects for superthreshold stimuli.
Two subjects participated in each experiment. Adapting fields were 7.4 deg dia., centered 11.4 deg temporal. Stimuli were 491.5 nm. Experiment 1. Superthreshold brightness matches were made between 0.55 deg, 10 ms flashes presented simultaneously to the two eyes in a haploscope. The control eye (CE) was adapted to 0.1 R*/rod/s. A staircase procedure found the flash intensity in the CE matching a 20 R*/rod/s flash in the test eye (TE) on 0.01, 0.1, and 1.0 R*/rod/s fields. Experiment 2. Both eyes were adapted to 0.1 R*/rod/s, then TE field was either increased or decreased by a factor of 4. 2AFC judgments were made every sec for 6 min to compare brightness of 10 ms, 20 R*/rod/s flashes on the changed field to simultaneous flashes of various intensities in CE. This method was used to plot the time-course of light and dark adaptation. Experiment 3. Both eyes were adapted to fields of mean intensity 0.1 R*/rod/s. Full-field random flicker was added to the field in TE only. After 6 min, the flicker ceased while both fields remained on. 2AFC brightness judgments were made every sec for 6 min between identical 20 R*/rod/s flashes in the two eyes to measure the time course of recovery from flicker adaptation.
Brightness was determined by the monocular ratio of flash intensity to the square root of adapting intensity. It took ~3 min for an eye to adapt to a new adapting level. Brightness was reduced for ~90 s post-exposure to random flicker.
All results support the hypothesis that the square root law results from a neural light adaptation mechanism whose gain is set by the level of statistical fluctuations within the rod pool. The adaptation time of the mechanism is ~90-180 s, but is somewhat faster for light adaptation than for dark adaptation.
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