Transient twinkle perception is induced by sequential presentation of stimuli that flicker at frequencies above the critical fusion frequency

The critical fusion frequency (CFF) is a threshold that represents the temporal limits of the human visual system. If two flickering stimuli with equal subjective luminances are presented simultaneously at different locations, the CFF is the temporal frequency above which they cannot be distinguished. However, when the stimuli are presented sequentially at the same position, a transient twinkle can be perceived around the moment of the changeover. To investigate the mechanism underlying this transient twinkle perception (TTP), we independently manipulated the luminance contrast and temporal frequency of the flicker, as well as the interstimulus interval (ISI). We found that TTP was greater as the luminance step was larger, was stably perceived for flicker frequencies up to 200 Hz, and was robust for all ISIs if flicker frequencies were below 250 Hz. For 250- and 300-Hz flickers, TTP was attenuated in conditions in which one-frame and two-frame ISIs were inserted. These results can be explained by a simple filtering model: TTP occurs if the temporal change in the weighted moving average of stimulus luminance exceeds a certain threshold. TTP gives additional evidence that the human visual system can detect the transient change of flicker stimuli at much higher temporal frequencies than the CFF, by an averaging mechanism of luminance.