At the start of each trial, observers had to place their right index finger at a designated start location on the desk in front of them and fixate their eyes on the stationary ball for a random period between 600 and 1000 ms. The target was always presented to the left of the screen center and moved from left to right toward the hit zone into the observer's ipsilateral reach space. After a brief presentation duration, and before it reached the hit zone, the ball was occluded from view. Observers had to extrapolate the ball trajectory and intercept it anywhere within the hit zone by hitting its estimated location on the screen with their right index finger. Upon interception, observers received visual feedback about their interception position and the position of the ball at the time of interception (
Figure 1B).
In separate parts of the experiment, we independently manipulated ball presentation or occlusion duration to investigate the effect of different sources of stimulus uncertainty (i.e., how long an object is visible or how long it is occluded) on interception performance. When manipulating presentation duration, the ball was visible for 100, 200, 300, 400, or 500 ms and occluded for 500 ms before reaching the hit zone. When manipulating occlusion duration, the ball disappeared for 100, 200, 300, 400, or 500 ms before reaching the hit zone while presentation duration was constant at 500 ms. Thus, total movement duration (presentation duration + occlusion duration) of the target until it reached the border of the hit zone was 600, 700, 800, 900, or 1000 ms in both experiment versions (
Figure 1C). The target displacement between motion onset and the time the target reached the border of the hit zone (which was at a fixed position on the screen) depended on target speed and total movement duration. Thus, we shifted the target start position along the
x-axis to account for the different target displacements. The manipulated occlusion duration (which was constant with varied presentation duration in experiment part A and was varied with constant presentation duration in experiment part B) was defined as the time from when the target disappeared until it entered the hit zone. However, because observers were free to choose when and where to intercept the target within the hit zone, the actual target occlusion duration (i.e., time between target disappearance and interception time) was slightly longer (mean = 149 ms;
SD = 100 ms) than the manipulated occlusion duration and varied from trial to trial.
Before starting the experiment, observers performed 12 practice trials where they were able to see the full trajectory of the target for the first six practice trials. This was to ensure that observers understood the task and the parabolic motion of the target, as well as to familiarize themselves with the setup. Both experiment parts used a 5 (presentation duration or occlusion duration) × 3 (target speed) within-subject design. Observers performed five blocks of 60 trials each, resulting in 300 trials for each experiment part. The experiment was completed in two separate sessions, one for each part, of 1 hour duration per session. The order of experimental versions was counterbalanced between observers.