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.