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Alejo Freire, Daphne Maurer, Terri L. Lewis, Randolph Blake; The ups and downs of point-light displays: Sensitivity to upright and inverted biological motion. Journal of Vision 2005;5(8):20. doi: 10.1167/5.8.20.
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Inversion of point-light displays depicting biological motion impairs identification of the depicted action, actor, and emotion (e.g., Dittrich, 1993). This impairment arises from the discrepancy between the orientations of the observer and the display rather than from the inversion of the display itself (Troje, 2003). In the present study, we quantified the impairment by comparing the processing of upright and inverted biological motion embedded in noise in a 2-interval temporal forced-choice task. Each trial comprised a point-light biological motion stimulus depicting one of 12 possible actions, and a scrambled version of the same action. Scrambled stimuli were equivalent to biological motion stimuli in the movement of individual dots (black, 10 arc min, moving on a gray background), but the movements were disrupted in both temporal phase and spatial location. Each biological motion and scrambled display lasted 1 sec and was embedded in a mask composed of a variable number of dots. The number of masking dots was varied according to a staircase procedure to determine the maximum number of masking dots yielding 71% accuracy. Masking dots each underwent equivalent motion to one of the dots in the biological motion display. Participants (n=20) tolerated substantial amounts of noise in both orientations of the display, although thresholds for the inverted condition (63 noise dots) were significantly worse than thresholds for the upright condition (88 noise dots), p < .01. The results are consistent with fMRI findings that the area in the posterior STS that responds preferentially to upright biological motion also responds above the baseline rate to scrambled biological motion when the stimuli are inverted (Grossman & Blake, 2001). The findings substantiate the exquisite sensitivity of the human nervous system to patterns of human motion, even patterns that have never been experienced in the real world such as point-light displays of inverted climbing of stairs.
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