Abstract
Extrafoveal decremental and incremental visual thresholds were measured with a circular test-object of 57[image] diameter, an exposure time of 0.1 sec and background luminance ranging from 2.5 to 7.3 log. quanta (equivalent to 507 nm) per sec. per square degree at the cornea. The decremental threshold is lower than the incremental threshold by factors up to 0.4 log. unit when the background luminance is low; and the 2 thresholds are virtually the same when the background luminance is high.