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Adam Reeves, Rebecca Grayhem; Early scotopic dark adaptation; the square-root law. Journal of Vision 2006;6(13):61. doi: 10.1167/6.13.61.
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Just after turning off an steady adaptation field, the log threshold for cone vision abuptly drops half-way to absolute threshold, before the leisurely process of dark adaptation begins. The abrupt drop is due, we have argued, to the removal of photon-driven (square-root) noise consequent on shuttering the adaptation field (Krauskopf & Reeves,1980; Reeves,Wu, Schirillo, 1997). This theory predicts that the same half-way drop will occur for rod-mediated thresholds. We now document that it does so, over a range of 3 log units of field intensity, using stimulus parameters which isolate rod vision. This result extends the range of the square-root law from low scotopic levels to higher levels at which rods light adapt.
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