Abstract
The critical fusion frequency (CFF) is a temporal frequency threshold of a flickering stimulus, above which humans cannot perceive a flicker. Interestingly, humans can detect the transition of frequencies between two sequentially presented flickering stimuli even when the two frequencies are above CFF, which is called the transient twinkle perception (TTP). To further characterize TTP we ran two experiments. In Experiment 1, we asked participants to report whether the flashing percept of TTP is lighter or darker than the background to test whether participants could discriminate the luminance of the first frame after the transition. The frequency before the transition was 1440 Hz and the frequency after the transition was one of six frequencies: 120, 144, 240, 288, 360, and 480 Hz. The stimulus diameter was 6˚. Results showed that participants could robustly discriminate the luminance of the first frame after the transition although the contrast threshold was constantly higher than that of the detection task. As expected, the detection and discrimination threshold increased as the frequency after the transition increased. In Experiment 2, we tested the effect of stimulus size on TTP. The frequency before the transition was 1440 Hz and the frequency after the transition was 144 Hz. The stimulus diameter was one of four sizes: 2, 6, 10, and 20˚. Results showed that the contrast threshold decreased as the stimulus size increased in the detection task (F-value is 24.89, df = 3, 18, effect size is 0.81, p-value is 1.26e^(-6)). However, in the discrimination task, the contrast threshold increased as the stimulus size increased (F-value is 4.32, df = 3, 18 effect size is 0.42, p-value is 0.018). Our findings suggest that the spatiotemporal interaction involved in processing large size brief stimulus facilitates the detection of luminance change but disturbs the discrimination of luminance change direction.