Abstract
Westheimer (1965) has shown that the rod threshold at the center of a 45′ disk of light can be lowered by adding light to the surrounding annular region of the retina. Hence, if the central disk is illuminated steadily, and the illumination on the annular surround is turned on (off) abruptly, the threshold at the center of the disk must fall (rise), with some time course. The present experiments show that there is a minimum in the threshold near the instant of onset of the annulus and a (highly variable) maximum in the threshold near the instant of offset of the annulus. The data do not support the notion of a relative delay in the neural signals generated by light falling on the annular region, nor do they support the notion that the time courses of these threshold changes are similar to those occurring in classical “early” dark and light adaptation.
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