Increment Thresholds for Two Nonidentical Flashes

Two flashes of 1° visual angle and equal luminance were superimposed upon a steady background of the same size, seen foveally. The second flash followed the first after intervals ranging from 0 to 60 ms. The duration of the first flash was varied from 0 to 4.8 ms, and, for each duration of the first flash, the duration of the second flash required for detection of the display was determined. (When seen, the two flashes appeared as one.) At all intervals less than 25 ms, the threshold energy of the two flashes combined was constant, regardless of the duration of the first flash. At each interval between 25 and 50 ms, at short durations of the first flash, the duration of the second flash was the same as that required when no first flash was presented. However, at longer durations of the first flash, the threshold average luminance provided by the two flashes during the total display time was constant. At the interval that required the maximum energy for threshold, 55 ins, for every duration of the first flash, the duration of the second flash was greater than that required when no first flash was presented. These findings, and others, are incorporated in a simple model.