Abstract
Introduction: Critical fusion frequency (CFF) indicates a temporal frequency above which a flickering stimulus appears continuous and ranges from 50 to 90 Hz. However, when spatially abutted black and white patches flicker (black and white to white and black), the edge between patches is detectable at 500 Hz (Davis et al, 2015). They suggested that the elevation in CFFs of flicker-defined edge is likely due to saccades based on existing studies using flickering blobs (Burr & Morrone, 1996; Bedell et al., 2008), however, it has not been empirically tested whether fixational eye movements can account for CFFs in the flicker-defined edge stimuli. Methods: We used a two-interval-forced choice task (2IFC) where participants were asked to report the interval including a flickering stimulus. The target stimulus (4°x4°) was either a spatially uniform stimulus, which is commonly used to measure CFFs, or flicker-defined edge stimulus. In Experiment 1, participants fixated the center of the target. Participants completed 4950 and 990 trials for edge and uniform stimuli. The stimulus duration was 1000 ms. In Experiment 2, we manipulated the stimulus duration (100, 316, 600, and 1000 ms). Results: We replicated that CFFs of edge stimuli were significantly higher than CFFs of uniform stimuli (127 vs 60 Hz, p=0.0226). CFFs of edge stimuli increased as fixation stability decreased (p=0.0275), or the number of microsaccades increased (p=0.0343) while CFFs of uniform stimuli remained the same. As the stimulus duration increased, CFFs of edge stimuli increased (p=0.003) while CFFs of uniform stimuli remained consistent (p=0.148). Notably, when the stimulus duration was 100 ms, there was no significant difference between CFFs of edge and uniform stimuli (62.8 vs 60.1 Hz, p=0.580). Conclusion: Consistent with Davis et al.’s hypothesis, our results showed that the elevation in CFFs of flicker-defined edge stimuli is partly due to fixational eye movements.