Rod‐cone interaction in light adaptation

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Abstract
The increment-threshold for a small test spot in the peripheral visual field [human] was measured against backgrounds that were red or blue. When the background was a large uniform field, threshold over most of the scotopic range depended exactly upon the background''s effect upon rods. This confirmed Flamant and Stiles. But when the background was small, threshold was elevated more by a long wavelength than a short wavelength background equated for its effect on rods. The influence of cones was explored in a further experiment. The scotopic increment-threshold was established for a short wavelength test spot on a large, short wavelength background. Then a steady red circular patch, conspicuous to cones, but below the increment-threshold for rod vision, was added to the background. When it was small, but not when it was large, this patch substantially raised the threshold for the test. When a similar experiment was made using, instead of a red patch, a short wavelength one that was conspicuous in rod vision, threshold varied similarly with patch size. These results support the notion that the influence of small backgrounds arises in some size-selective mechanism that is indifferent to the receptor system in which visual signals originate. Two corollaries of this hypothesis were tested in further experiments. A small patch was chosen so as to lift scotopic threshold substantially above its level on a uniform field. This threshold elevation persisted for minutes after extinction of the patch, but only when the patch was small. A large patch made bright enough to elevate threshold by as much as the small one gave rise to no corresponding after-effect. Increment-thresholds for a small red test spot, detected through cones, followed the same course whether a large uniform background was long or short wavelength. When the background was small, threshold upon the short wavelength one began to rise for much lower levels of background illumination, suggesting the influence of rods. This was confirmed by repeating the experiment after a strong bleach when the cones, but not rods, had rully recovered their sensitivity. Increment-thresholds upon small backgrounds of long or short wave-lengths then followed the same course.

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