The effect of bleaching and backgrounds on pupil size.

THE SURHUSING possibility that the widening of the human pupil in the dark accurately reflects regeneration of rhodopsin in the retinal rods was first suggested by the experiments of ALPERN and CAMPBELL (1962a). They found that recovery of the pupil size in the dark after a full rhodopsin bleach was very slow. After an initial transient widening and narrowing in the first minute, the pupil returned to the fully dark-adapted size “along a curve very similar to that of rod dark adaptation”. The action spectrum of this effect showed it to be dominated by rods. Transient pressure blindness of the bleached (but never the unbleached) eye transiently widened the pupil to its fully dark value. The time course of the recovery of pupil size in the dark after a full bleach was accurately described by an exponential curve with a time constant of about 6 min. The inference that signals generated by the amount of unregenerated rhodopsin sets the size of the pupil in the dark, rested on the observation (RUSHTON, 1961) that rhodopsin regeneration after a full bleach follows this same sort of recovery time course in the dark. If the agreement were exact, then pupil size could be used effectively as a retinal densitometer and it would follow that