Abstract
Adapting to a rebounding apparent motion sequence (e.g. LEFT-RIGHT-LEFT-RIGHT) leads to illusory apparent motion (IAM) upon subsequent presentation of random frames that rebounds along the adapting axis (Davidenko et al., 2017; Davidenko & Heller, 2018). However, this phenomenon has only been examined in the horizontal-vertical plane. The present study investigated whether IAM extends to three-dimensional space by adapting participants to rebounding motion along the three cardinal axes using virtual reality. Participants (N=20) viewed dense arrays of small white cubes in an otherwise dark virtual environment through a head-mounted display. Each participant completed four “catch” trials and twelve “real” trials. In the catch trials, 70% of the cubes rebounded back and forth along a cardinal axis at 1.5Hz for six frame transitions, while the remaining 30% of cubes moved randomly. In the real trials, the first four frame transitions showed 70% of the cubes rebounding along a cardinal axis, but in the final two frames 100% of the cubes moved randomly. Following each trial, participants verbally indicated which axis the cubes appeared to move along during the final two frame transitions, or indicated “random” if their percept was inconsistent with motion along a single axis. A 2-way repeated measures ANOVA revealed a significant interaction between the adapted and reported axes (F(4,179)=3.97, p=0.006). Excluding “random” responses (with proportions 0.55, 0.26, and 0.29 following depth, horizontal, and vertical adapters, respectively), IAM reports were more frequent along the adapted axis compared to the non-adapted axes (see Figure 1). However, depth IAM was reported less frequently than horizontal or vertical IAM, which may be due to depth adapters being harder to detect, as suggested by catch trial performance (Figure 2). Overall, our results provide novel evidence that rebounding IAM can be experienced beyond the horizontal-vertical plane.