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Adam Reeves, Rebecca Grayhem; Scotopic sensitivity regulation: Noise or gain?. Journal of Vision 2011;11(11):362. doi: 10.1167/11.11.362.
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Thresholds for 200 ms test spots in rod isolation conditions were obtained after dark adaptation was complete (‘absolute thresholds’), on dim fields (‘increment thresholds’), and 200 to 600 ms after the field was turned off (‘extinction’ thresholds). Fields were steady or flashed. Field intensity was below 3 scotopic td to avoid rod light adaptation. For tiny (5′ arc) spots, increment thresholds followed the square-root (Rose-DeVries) law, and extinction thresholds fell to the absolute threshold. For large (1.3 deg) spots, increment thresholds followed Weber's law, while extinction thresholds followed the square-root law after turning off steady fields but remained at absolute threshold after the fields were flashed. These results can be explained by a combination of dark light, photon-driven noise (which is eliminated when the field is turned off), and light adaptation of the neural elements mediating detection of the large (but not the tiny) spot.
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