Stimuli and
procedure
We used PsychoPy \cite{Peirce2019} to manage stimuli presentation and to collect data. Participants were seated approximately 60 cm from the screen in a soundproof, dimly lit cabin. The stimuli were presented on a 24-inch DELL monitor (refresh rate 60 Hz) against a light grey background (39.3 cd/m2).
As outlined in Figure 1, each trial began
with a fixation dot for half a second (0.5° in diameter with a brightness of 85.7
cd/m2), which cued the start of the
trial and drew participants' attention. Next came the encoding
phase, wherein a random dot kinematogram (RDK) featuring 15 white dots (each dot diameter of 0.4°; the luminance of 85.7 cd/m2) against a dark disc (17.8°, 16.5 cd/m2) appeared at the center of the screen.
Initially, the dots within the RDK moved randomly for 400 to 600 ms, without any pattern (at a speed of 1 °/s and a coherence level of 0%). Subsequently, these dots turned
green (45.8 cd/m2) and began moving together
(at 100% coherence) at a speed of 6°/s in a predetermined direction (randomly selected from
11.25° to 348.75°, in steps of 22.5°) for a randomly chosen length of [0.4, 0.6, 0.8, 1.2, 1.4, 1.6] s. When a dot exited
the dark disc boundary, another dot appeared randomly inside to maintain a constant count of fifteen. These green, coherently moving dots served as the target, which participants were asked to memorize regarding their movement direction and duration. After this, the dots returned to their initial random motion
for another 400 to 600 ms. The alternating white dot displays served as visual masks to present any residual visual effects from the
previous trial.
Following the encoding phase, a post cue - either the letter ‘D’ (0.8° × 1.0°, 85.7 cd/m2) for the direction task
or ‘T’ for the time task - appeared at the center of the display for half a second, prompting participants to report either the direction or duration. Participants could respond at their own pace. For the duration discrimination task, a
display showing the left and the right arrows ("< or >") prompted participants to assess if the duration of the coherent motion was shorter or longer than one second. They made this two-alternative forced choice (2AFC) judgment by pressing the left arrow for "shorter than one second" or the right arrow for "longer than one second".
In the direction task, a
line segment started from the center with an overlaid 'D', pointing to a random direction. Participants rotated this line to match the observed motion's direction using the left (counterclockwise) and right (clockwise) arrow keys. A continuous readjustment updated the pointer's direction, and they finalized their choice by pressing the spacebar. If their estimated direction deviated by more
than 60°, a warning message "Direction deviated
a lot!" would flash on-screen for half a second. The next
trial began after a one-second intertrial interval.
To prepare participants for the main experiment, a practice session with 24 practice trials exposed them to a standard
one-second stimulus, represented by yellow dots moving horizontally (at a speed of 6 °/s; coherence of 100%). Following a 500 ms blank interval, a comparison stimulus with a duration randomly chosen of [0.4, 0.6, 0.8, 1.2, 1.4, 1.6] s was presented. The comparison stimulus was the same RDK display used in the main experiment. Participants had to judge which one was longer. After the response, they received feedback on their accuracy. The formal
experiment consisted of 480 trials, randomly shuffled, and split evenly between duration and direction tasks. The inter-trial
transitional probability (from trial n-1 to trial n )
between the duration and direction trials ensured an equal probability
of all inter-trial combinations. Participants
could take a short break after each block of 30 trials.