On the depth reversibility of point-light actions

Abstract

We investigate the occurrence of perspective reversals for a depth-ambiguous point-light figure. In addition, we exploit the phenomenon of reversibility to search for stimulus features relevant in the process of depth assignment. Experiment 1 shows that perceptual switches indeed occur during prolonged viewing, although the switches occur infrequently. The reversibility is confirmed in Experiment 2, in which the perceptual ambiguity of the point-light action is manipulated as well as observers’ intention to perceive a particular alternative. In addition, the pattern of eye movements reveals local stimulus features specifically associated with the perception of the different alternatives. In Experiment 3, the importance of these features as determining factors of the initial interpretation is investigated by manipulating the location of the first fixation on the stimulus. Implications for a better understanding of biological motion perception are discussed.

Keywords:

• Biological motion
• Action perception
• Bistability
• Reversible figures
• Depth perception

Acknowledgements

Jan Vanrie is now at the Department of Architecture, PHL University College, Belgium. This research was supported by Concerted Research Effort Convention GOA/05/03/TBA and the Fund for Scientific Research of Flanders (grant to JV and grant FWO G.0621.07).

Notes

¹We added the Necker cube condition to explore whether or not the point-light walker elicits similar perspective reversals as more classic ambiguous stimuli. However, this condition should not be considered as an absolute baseline for the biological motion condition, because a point-light walker and a Necker cube differ on many, possibly confounding variables. For instance, the number of reversals can be affected by simple, low-level features of the Necker cube (e.g., the size of the stimulus, Washburn, Reagan, & Thurston, 1934, or luminance, Cipywnyk, 1959). Indeed, if a direct comparison with other bistable stimuli would have been the primary focus of the present study, a control condition with another SFM-stimulus (such as a rotating sphere or cylinder) would have been more appropriate. However, this was not our main aim. Because we were primarily interested in getting unbiased results for the condition with a point-light walker, we also did not counterbalance the order of presentation (the point-light walker was always presented first).

²Note that the reliability of this method is rather weak. Indeed, it relies on the assumption that observers correctly remembered their initial percept and never failed to report a perceptual switch. This problem will be avoided in the Experiment 2.