Detecting unnaturalness in biological motion with altered playback speeds.

Prior research showed that observers can tolerate large speed alterations in real-world videos. The present study examined how sensitive the human visual system is to the change of kinematic information of human actions with altered playback speeds. We recorded four persons walking at various speeds and produced point-light walker stimuli (standard stimuli), from which we also created test stimuli either by speeding up or slowing down the playback speed. In the experiments, two point-light walkers were presented sequentially: one standard and the other test stimuli. Importantly, in each trial, the expected speed of translation was kept constant (e.g., a pair of one walking at 5.40 km/h and the other walking at 2.70 km/h but played with double speed), differing only in gait kinematic information. Participants reported which stimulus was played at a normal speed. We also included the manipulations of orientation (upright vs. inverted) and spatial scrambling of the point-light dots. The results showed that the unnaturalness detection was performed at above chance levels, confirming that kinematic inconsistencies provided a discernible cue. However, detection was only reliable when the speed alteration in a test stimulus was fairly large. Interestingly, we found little differences in performance among upright-intact, inverted, and scrambled conditions. The lack of the large detriments from inversion or scrambling suggests that the participants did not rely strongly on global form or orientation cues to perform the unnaturalness detection and points to greater contributions of local motion signals.