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Adam Reeves, Rebecca Grayhem; Early scotopic dark adaptation; change in noise alone?. Journal of Vision 2008;8(6):955. doi: 10.1167/8.6.955.
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Just after turning off a steady adaptation field, the log threshold for a large, 1.3 deg, rod-mediated test flashes abruptly drops from the Weberian increment threshold curve half-way down to absolute threshold, before the leisurely process of dark adaptation begins (Reeves & Grayhem, VSS 2007). The abrupt drop parallels that seen in photopic vision and is due, we argued, to the removal of photon-driven (square-root) noise consequent on shutting off the adaptation field (Krauskopf & Reeves, Vision Research 20, 193–196). Method We now test the same idea with a tiny, 5 min arc, 200 ms rod-detected test seen in Maxwellian view, for which the increment threshold curve follows the Rose-DeVries square-root law. Result: Thresholds dropped abruptly from the square-root law right to the absolute threshold, as predicted by the removal of photon-driven noise from the field. Conclusions: photon noise from the field always drives up threshold following the square-root law. Light adaptation additionally reduces sensitivity by saturating responses to large scotopic tests in a manner also proportional to the square-root of field intensity, the combined effect giving rise to Weberian behavior. Tiny rod tests escape saturation and reveal the photon-driven noise effect alone.
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