Visual stimuli consisting of two alternating trains of high-frequency square-wave pulses of the same time-average luminance were presented monocularly to human observers. Two stimulus sizes were used, which subtended visual angles of ½° and 4.3°. In many cases the observers perceived flicker even when each train of pulses by itself appeared fused. The results of the present experiments show that perception of flicker with this type of stimulus pattern depends critically on two variables: (1) the difference t1-t2, where t1 and t2 denote the periods of single pulses in the two trains; (2) the combinations of T1 and T2, where T1 and T2 denote the durations of the two trains. The results with the ½° field show some remarkable differences from the results with the 4.3° field, thus, confirming the findings of numerous earlier studies that in many visual tasks the central fovea behaves differently from larger areas around it. The model of de Lange was checked by Fourier analysis of the data. This analysis shows that de Lange’s model can be adequately fitted to the data for the 4.3° field, but not for the ½° field. In the case of the 4.3° field, the amplitudes of the second Fourier component are, for many of the data, at or near the threshold values of their corresponding fundamental components. This means that the possibility of a contribution from the higher harmonics to the threshold of flicker cannot be neglected.
TAPAS K. SEN, "Visual Responses to Two Alternating Trains of High-Frequency Intermittent Stimuli," J. Opt. Soc. Am. 54, 386-393 (1964)