Flicker studies generally employ a train of light pulses consisting of a single period of stimulation repeated serially. In a recent study, fusion points were measured using trains of light pulses in which alternate periods were of different duration. Fusion points obtained with this type of stimulation are best described by a fusion contour which identifies those combinations of two periods which, when alternated serially, result in a transition point between flicker and fusion. In the present study, data have been obtained on three flicker contours each of which identifies those combinations of two periods which, when alternated serially, have the same apparent rate of flicker. A matching technique was used to measure the subjective rate of trains of light pulses. Data points were obtained from observers who matched the apparent rate of a variable stimulus (composed of two periods of alternating duration) to the apparent rate of a standard stimulus (composed of a single period repeated serially). Standard rates of 33.3, 28.6, and 25 cps were used. The fusion contour for each of the two observers in the experiment is also shown. The fusion and flicker contours are not dichotomous: as the frequency of the standard stimulus is increased, the flicker contours tend to resemble more closely the shape of the fusion contour. The contours illustrate the limiting values within which various combinations of two alternating periods may have the same apparent rate: (1) The sum of the durations of the two alternating periods must be greater than the duration of the period of the standard stimulus. (2) The sum of the durations of the two periods must be less than twice the duration of the period of the standard stimulus.
D. M. FORSYTH and CHARLES R. BROWN, "Flicker Contours for Intermittent Photic Stimuli of Alternating Duration," J. Opt. Soc. Am. 49, 760-763 (1959)