Fig. 9. Threshold to detect a flash changes rapidly in these data from Crawford.5 A conditioning flash is delivered at time 0 and turns off at 0.5 seconds. Threshold is altered to a log unit within 0.1 after the start of the conditioning flash. The paradoxical rise in threshold prior to the test flash is discussed in the text. (Crawford B. Visual adaptation in relation to brief conditioning stimuli. Proc R Soc Lond [Biol] 134:238, 1947.)