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Masking a point-light walker


Bülthoff,  I
Department Human Perception, Cognition and Action, Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Pavlova, M., Sokolov, A., & Bülthoff, I. (1998). Masking a point-light walker. Poster presented at 1. Tübinger Wahrnehmungskonferenz (TWK 98), Tübingen, Germany.

In spite of potential perceptual ambiguity of a point-light walking figure, with upright display orientation observers can readily recover the invariant structure from biological motion. However, regardless of the same low-level relations between moving dots within upright and inverted orientation, perception of a point-light walker is dramatically impeded with 180º-display inversion. Spontaneous recognition was found to improve abruptly with changing display orientation from inverted to upright (Pavlova, 1996, Perception 25, Suppl.). This evidence implies that the visual system implements additional processing constraints for the unambiguous interpretation of biological motion. We used a masking paradigm to study the processing constraints in biological motion perception. At each of randomly presented five orientations (0°, 45°, 90°, 135°, and 180°), viewers saw a sequence of 210 displays. Half of them comprised a canonical 11 pointlight walker, and half a partly distorted walker, in which rigid pair-wise connections between moving dots were perturbed. A 66-dot “scrambled-walker” mask camouflaged both figures. Prior each experimental sequence, a sample of a canonical walker in respective orientation was demonstrated. Observers judged whether a canonical figure was present. A jackknife estimating of the ROC parameters indicated that detectability leveled off with changing orientation from upright to 135°, and then slightly increased to display inversion. However, even with 135° and 180° it was above chance. For orientations 0°, 45° and 90°, perceptual learning to detect a canonical walker proceeded rather rapidly in the course of the experiment. Comparison with the data on spontaneous recognition of biological motion suggests that display orientation affects bottom-up processing of biological motion more strongly than top-down. We suppose that some processing constraints (such as axis-of-symmetry, dynamic constraints) in perception of biological motion be hierarchically nested. Dynamic constraints appear to be the most powerful: the highest detectability was found with upright orientation. While with changing orientation these constraints lose their strength, others processing constraints are getting more influential. For instance, the lower sensitivity for 135° as compared to 180° might be accounted for by the axis-of-symmetry constraint that is implemented by the visual system at 180°. Likewise, due to the inefficiency of this constraint, biological motion pattern is perceived as more multistable with 90°-150°, as compared to 180° display orientation.