CFF and flash fusion task comparison
We also found no correlations between the thresholds obtained with the two different tasks. Though this may initially seem surprising, the flash fusion task fundamentally differs from the CFF task in several ways. For example, the former is essentially a two-alternative-forced-choice task, in which participants need to choose between two possible responses in each trial. This introduces a guess rate which may skew “true” flash-fusion thresholds. The responses may also be influenced by a pre-existing bias for one of the two options (Buergers & Noppeney; 2022). The flash fusion task also requires 600 trials for the proper fitting of a psychometric curve. This causes fatigue to become a significant factor and, though computationally corrected for during psychometric function fitting, introduces a certain lapse rate. The two tasks also differ in the number of different stimuli: the flash fusion task has five different fixed gap lengths, whereas no practical upper bound for flicker-fusion threshold exists in the CFF task, making the latter perhaps more suitable to assess inter-individual differences in the absolute maximum threshold, but less so for employing standardized tests to investigate brain activity correlates. Thresholds obtained with the CFF task may also not be directly comparable to those obtained with the flash fusion task due to a steady increase of mean luminance over time as flicker frequency increases in the former, which effectively increases flicker fusion thresholds up to a certain point. It is also important to note that Samaha & Postle’s flash fusion task was designed to assess possible correlates with alpha oscillations; which are a specific measure for visual attention and focus (Clayton et al., 2018, Pfurtscheller et al., 1996). The gap detection style task is very suitable to investigate the level of attention a participant pays to each stimulus. Any moment of inattention may mean that the gap in the stimulus is missed and a trial is considered incorrect. This is not the case for the critical flicker fusion task, in which the stimulus is always present, giving participants the opportunity to refocus and reconsider what they are perceiving throughout the whole task. The latter reason would not only explain the lack of correlation between the two behavioural task responses, but also between the CFF task and peak alpha frequency.
In conclusion, we have replicated some, but not all of the results from a previous study investigating the possible relationship between peak alpha frequency and visual temporal resolution. Our work adds to the ever-expanding body of research data aiming to tease apart the link between brainwave oscillations and visual perception. We have additionally shown that the choice of behavioural task is key in exploring these possible relationships.