Biological Motion Perception: From Inversion to Upright Display Orientation

How does biological motion perception change with display orientation? As previously shown, display inversion (180°) completely prevents veridical perception of biological motion. However, with upright orientation (0°), observers are able to recover the invariant structure through biological motion despite reverse transformation (showing the film backwards) or changing the presentation rate (Pavlova, 1995 Perception24 Supplement, 112). In the present experiments, observers saw the biological motion pattern at various display deviations, from inverted to upright orientation (180°, 150°, 120°, 90°, 60°, 30°, 0°), in the right or left hemifield, on a circular screen monitor. The display consisted of an array of 11 dots on the main joints of an invisible walker moving as if on a treadmill. While viewing (60 s), observers pressed a key each time their perception changed from one stable percept to another (eg when the direction of apparent rotation of the pattern reversed). The perceived multistability (the number of key-presses) increased as orientation was varied from inverted to 90°, and then decreased between 90° and upright. The recognition of walking figure improved abruptly with changing orientation: at deviations of 60° and 30° most observer reported seeing the walking figure spontaneously, yet the pattern was seen as multistable. The findings imply the relative power of constraints (such as orientation) in perception of biological motion that is discussed in relation to the KSD principle in event perception [Runeson, 1994, in Perceiving Events and Objects Eds Jansson, Epstein, Bergström (Hillsdale, NJ: Erlbaum) pp 383 – 405].