Active versus passive processing of biological motion

Johansson's point-light walker figures remain one of the most powerful and convincing examples of the role that motion can play in the perception of form (Johansson, 1973 Perception & Psychophysics 14 201 ^ 211; 1975 Scientific American 232(6) 76 ^ 88). In the current work, we use a dual-task paradigm to explore the role of attention in the processing of such stimuli. In two experiments we find striking differences in the degree to which direction-discrimination performance in point-light walker displays appears to rely on attention. Specifically, we find that performance in displays thought to involve top ^ down processing, either in time (experiment 1) or space (experiment 2) is adversely affected by dividing attention. In contrast, dividing attention has little effect on performance in displays that allow low-level, bottom ^ up computations to be carried out. We interpret these results using the active/passive motion distinction introduced by Cavanagh (1991 Spatial Vision 5 303 ^ 309). DOI:10.1068/p3072 ôAuthor to whom all correspondence and requests for reprints should be sent at his present address: Max Planck Institute for Biological Cybernetics, Spemannstrasse 38, D 72076 TÏbingen, Germany; e-mail: ian.thornton@tuebingen.mpg. de 1⁄2 Present address: Department of Psychology, University of British Columbia,Vancouver, BC V6T 1Z4, Canada. was adjusted appropriately, all that could be seen were a dozen or so moving points of light. Despite the lack of form information, observers were still able to accurately identify the action being portrayed. These displays remain one of the most powerful and convincing demonstrations of the role that motion can play in the perception of form. Johansson's stimuli have inspired a great deal of research on biological-motion processing. We now know, for instance, that the ability to process such displays develops within the first few months of life (eg Fox and McDaniel 1982; Bertenthal et al 1985), and that it is a rapid (Johansson 1976), relatively spontaneous (Proffitt et al 1984) process that can be achieved in the presence of fairly complex masking elements (Cutting et al 1988; Bertenthal and Pinto 1994). With point-light walker displays, masking elements typically take the form of additional points of light that are placed around the walking figure and differ only by virtue of motion or grouping characteristics (see figure 1). Johansson began his work on biological-motion processing from within a particular theoretical frameworkövisual vector analysisöwhich emphasized the `̀ spontaneous'' and `̀ automatic'' extraction of ``mathematically lawful spatio-temporal relations'' in early visual patterns of stimulation (Johansson 1973, 1975). Given this theoretical perspective, it is not very surprising that much early work in this area favored a low-level or bottom^ up processing explanation for the perception of biological motion. The fact that na|« ve observers could quickly and spontaneously identify the events being portrayed, and the success of early bottom ^ up computational models also lent support to this perspective (Hoffman and Flinchbaugh 1982; Webb and Aggarwal 1982). While there can be little doubt that such low-level motion mechanisms play an important role in biological-motion processing (eg Mather et al 1992), this does not appear to be the whole story. Specifically, there is now a growing body of behavioral and neurophysiological work indicating that higher-level or top ^ down processes play an important role in the perception of biological motion. Dittrich (1993) noted that the speed and efficiency of biological-motion processing can be strongly influenced by the category of the depicted action. For example, locomotory actions, such as walking and climbing stairs, are generally recognized faster and more accurately than social actions, such as greeting and dancing. He suggested this difference might reflect the operation of `̀ selective movement filters'' that help enhance the recognition of familiar biological motions by Figure 1. Three static views illustrating the construction of a masked point-light walker display. The outline of the human body, shown in the first frame, is never shown in experimental stimuli. When presented statically, the final displays (third panel) are difficult to interpret. However, when set in motion, observers easily organize the complex patterns of point motion into a coherent perception of human locomotion. In the experimental stimuli, the walker points and mask points are identical, as shown in the third panel. 838 I M Thornton, R A Rensink, M Shiffrar