Decremental and incremental visual thresholds

Extrafoveal decremental and incremental visual thresholds have been measured with a circular test-object of 57' 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 second per square degree at the cornea.2. 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 two thresholds are virtually the same when the background luminance is high.