The increment threshold ΔI as a function of the background field I is independent of the amount of bleaching in rods and cones; the value of ΔI for rods depends upon the activity of their neighbors. The absorption of one quantum always elicits a rod signal, but its effectiveness depends upon the threshold for signals in the summation pool.
In dark adaptation, log threshold for rods is proportional to the amount of rhodopsin bleached. It also depends upon the size of the test flash and upon the bleaching of neighboring rods. Each rhodopsin molecule while in the bleached state sends a continued signal of “dark light” to the summation pool. This total background of dark light adds simply to the bright background field (if any) and the dark adapted threshold is the increment threshold against this total background. Quantitative experimental relations are given and a speculative hypothesis advanced which could account for them.
© 1963 Optical Society of AmericaFull Article | PDF Article
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