Computerized flash-fusion task
We modelled the flash-fusion task after the one used in Samaha & Postle
and followed a similar protocol (Fig 1). Participants were seated
~60 cm away from a computer monitor with a 100 Hz
refresh rate. The task consisted of white disk stimuli appearing against
a black background. Each trial started with a white fixation cross in
the middle of the screen, at 0.98 degrees visual angle. The fixation
cross reduced in luminance before stimulus onset to prepare
participants. The duration of this period was drawn from a uniform
distribution between 1000 and 1500 ms. The fixation cross remained
present for the duration of the trial. Disk stimuli, at 1.23 degrees
visual angle, were presented either to the left or to the right side, at
2.45 degrees visual angle (0.20 cd/m2) of the fixation
cross, with equal probability. Fifty percent of the trials were
two-flash trials, in which a disk appeared for 40 ms, then disappeared
for either 10, 20, 30, 40 or 50 ms, and then reappeared for 40 ms. The
other fifty percent were one-flash trials, in which a disk appeared for
durations that varied to match the stimulus onset to stimulus offset
lengths for each stimulus type of the two-flash trials. 800 ms after
stimulus offset the fixation cross increased in luminance to prompt
participants to make a response. Participants held a mouse in both hands
during the task and responded by clicking the left button with their
left thumb to indicate when they perceived a single flash trial, or by
clicking the right button with their right thumb to indicate when they
perceived a two-flash trial. The next trial did not start until a
response had been made and participants were instructed to focus on
accuracy over speed. Participants completed 20 practice trials, followed
by 600 test trials divided into 6 blocks. Participants took self-paced
breaks in between each block.
Our task design deviated from Samaha & Postle’s design in three minor
ways: due to the physical set-up of equipment in our lab, participants
were seated at ~60 cm away from the monitor, whereas in
the original design this was ~70 cm. Because the
original task design did not specify any specific colour values for the
stimulus disks, or the amount with which fixation luminance decreased
before stimulus onset, we were unable to replicate these variables
exactly. We therefore opted to use white stimulus disks to optimize
contrast, and we decreased the luminance of the fixation cross by 40%
before stimulus onset.
Figure 1: Task set-up and stimulus conditions . Reproduced from
Samaha & Postle (2015). The image presented here has been altered from
the original by removing a “confidence” step at the end of each trial,
which was not analysed in the original study and therefore is not
included here.
A: Depiction of a trial. The start of a trial was indicated with a white
fixation cross, which reduced in luminance to prompt for an upcoming
stimulus. Stimulus consisted of a single flash event in half of the
trials and a double flash event in the other half. Flash events were
presented either to the left or to the right of the fixation cross with
equal probability.
B: Double flash events consisted of a white stimulus disk appearing for
40 ms, followed by a gap of 10 – 50 ms, after which the stimulus disk
reappeared for another 40 ms. Lengths of single flash events were varied
to match the duration of the double flash events.