Increment Thresholds for Two Identical Flashes*

Abstract

Two identical 1° flashes were superimposed upon a steady background of the same size seen foveally. The second flash followed the first after delays ranging from 0 to 200 ms. As the delays increased from 0, the energy required for threshold remained constant up to a critical duration, TC2. (The magnitude of TC2 decreased as background luminance increased.) As the delays continued to increase beyond TC2, the threshold energy increased to a maximum, following the rule that the average luminance remained constant. After the maximum, the threshold energy decreased following another simple rule. At the longer delays, threshold energy was again constant. At all background luminances, the single-flash critical duration TC1 was about 40% higher than TC2.