All data were analyzed using MATLAB (MathWorks, Natick, MA, USA). Twenty key response data were recorded for each of the four VR scenes. To ensure that the participants maintained their gaze at the fixation point, eye positions were recorded during the experiment using Tobii eye cameras with an HMD (VIVE Pro Eye with Tobii eye tracking) at a sampling rate of 120 Hz.
Figures 2A and
2B illustrate the horizontal and vertical eye positions during target motion (670 ms) for two typical participants in all 20 trials of the Lower-Ascend scene. As shown in
Figure 2A, Participant 1 succeeded in looking close to (within the ± 3° acceptance window) the fixation point in every trial (blue dots). However, Participant 2 in
Figure 2B looked at the goal in some trials (gray dots). Based on the eye position results, we excluded the key response data in which the participant's eye position once moved more than 3° horizontally or vertically from the fixation point (area) during the target motion in a trial. For some participants, all 20 trials of key response data from a VR scene were excluded when they violated the fixation area in more than half the trials in one VR scene (
n = 1 from the Upper- and Lower-Descend scenes, and
n = 2 from the Upper- and Lower-Ascend scenes). The mean excluded the trials of key response data for each participant, except the participant whose data were excluded from more than half of all 20 trials in one VR scene, as follows: 1.27 trials (
SD = ± 2.17) in the Upper-Ascend scene, 0.82 trials (
SD = ± 1.84) in the Upper-Descend scene, 1.0 trial (
SD = ± 1.66) in the Lower-Ascend scene, and 1.0 trial (
SD = ± 1.76) in the Lower-Descend scene. The excluded key response data were not used for further analysis.
Timing differences (TDs) were calculated for each key response to assess the accuracy of arrival time estimation for each VR scene. TD was defined as the difference between the actual timing of target arrival at the goal (670 ms) and the keypress time (
Hirata et al., 2024). A positive TD value indicates that the key is pressed before the target arrives at the goal, whereas a negative TD value indicates that the key is pressed after the target reaches the goal. After calculating the TDs for all the trials, the mean TDs for each participant were calculated for each VR scene. Finally, we averaged the mean TDs of all the participants in each VR scene. A two-way analysis of variance (ANOVA) was performed to evaluate the differences in mean TDs with the target direction (ascending and descending) and goal location (upper and lower). Tukey's honest significant difference (HSD) test was performed as a post hoc test, and the level of statistical significance was set at
p < 0.05 for all analyses.