To determine threshold luminance as a function of the interval separating successive subliminal flashes, 2, 3, 6, and 100 5-ms flashes were added to a centrally fixated background luminance. The interval separating successive flashes was varied from 0 to 150 ms, and four background luminances were used. At any background luminance, for any number of flashes, four empirical laws, collectively termed the TEpee effect, describe the results. First, as the interval increases from 0 the threshold total energy required in the flashes remains constant up to a critical interval, <i>i <sub>c</sub></i>. (The critical interval <i>i <sub>c</sub></i> varies systematically with both the background luminance and the number of flashes.) Second, as the interval increases beyond <i>i <sub>c</sub></i> threshold energy increases to a maximum, at interval <i>i <sub>M</sub></i>, following the rule that the average luminance during the total display time remains constant. [At any given background luminance, for any number of flashes, both <i>i <sub>M</sub></i> and the threshold luminance increment (Δ<i>I</i>) at <i>i <sub>M</sub></i> are constant. Also, the increment Δ<i>I</i> at <i>i <sub>M</sub></i> for any number of 5-ms flashes is slightly greater than that required for a single 5-ms flash.] Third, as the interval increases beyond <i>i <sub>M</sub></i>, threshold energy decreases following the rule that the threshold energy times the total display time equals a constant. This decrease continues until a threshold-energy level predicted by the probability-summation hypothesis is reached. Fourth, with further increases in the interval, the threshold energy remains constant. The findings are related to results of variable-duration, single-flash experiments and to results of critical-flicker-frequency experiments.
Robert M. Herrick, "Foveal increment thresholds for multiple flashes," J. Opt. Soc. Am. 63, 870-878 (1973)