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K. Fujimoto, A. Yagi, T. Sato; Incompatible body-translation delays visual perception of human gait. Journal of Vision 2001;1(3):352. https://doi.org/10.1167/1.3.352.
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The moonwalk, in which dancers' gaits pretend forward walking but their bodies actually move backwards, gives us a strange impression. This indicates interactions of articulatory motions of limbs and global body motion in perceiving biological motion. Specifically, it suggests perceptual differences between conditions where gait and global motion are compatible and not (e.g. moonwalk). To exemplify such interactions, in this study, we examined reaction time for discriminating body views or articulatory motions in compatible and incompatible conditions. Stimuli were the four types of walker defined by the combination of the body facing orientation (left or right) and the direction of articulatory movements of limbs (forward or backward). Each walker was presented as random dot kinematograms, in which a stick-figure walker consisted of thin moving areas was embedded in a static random-dot field. Global body-translation was produced by horizontal movement of the walker at a natural walking velocity. Trials without translation were also presented as control. Ten observers discriminated body orientations or directions of articulatory motions. As a result, under the incompatible condition, the discrimination of articulations was heavily slowed down (1270 ms vs. 840 ms) and error rate also increased. In addition, articulations were discriminated significantly slower than orientations under all conditions (compatible: 948 ms vs. 720 ms, no translation: 910 ms vs. 735 ms). These results suggested existence of two separate mechanisms to interpret articulatory motions. One is a low-level, local process to evaluate limb motions themselves, and the other to interpret limb motions from top-down factors such as body orientations or global motion directions. Therefore, the interference of these two mechanisms can explain the longer reaction time to articulatory motions in incompatible condition.
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