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Dorita H. F. Chang, Nikolaus F. Troje; Animacy and direction from point-light displays: Is there a life detector?. Journal of Vision 2007;7(9):481. doi: 10.1167/7.9.481.
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Directional information can be extracted from scrambled point-light displays that are devoid of all structural cues prompting the suggestion of a distinct local mechanism in biological motion perception that may serve as a general “life detector” (Troje & Westhoff, 2006). We investigated this hypothesis by testing the perception of both animacy and direction from point-light stimuli. Coherent and scrambled point-light displays of humans, cats, and pigeons that were upright or inverted were embedded in a random dot mask and presented in saggital view to two groups of naïve observers (n = 12/grp). The first group assessed the animacy of the walker on a six-point Likert scale and the second group discriminated the direction of walking. Across blocks, stimulus duration varied from 200 – 1000 ms. Coherent stimuli appeared more animate than scrambled stimuli (p [[lt]] 0.001) and inversion decreased animacy ratings (p [[lt]] 0.001), although more substantially for coherent than for scrambled walkers (p = 0.007). Similarly, discrimination accuracies were higher for coherent versus scrambled stimuli (p [[lt]] 0.001) and inversion decreased performance (p [[lt]] 0.001), but more substantially for coherent than for scrambled walkers (p = 0.004). Both animacy ratings and discrimination accuracies did not differ for animal type (ps [[gt]] 0.200) nor stimulus duration (ps [[gt]] 0.300). The results indicate that like the ability to discriminate direction, the perception of animacy from scrambled displays is orientation-specific. We suggest that the responsible mechanism uses a dynamic, gravity-dependent framework to assess the presence of life in the environment and is remarkably robust, operating efficiently at limited exposure times.
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