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Javier O. Garcia, Emily D. Grossman; Perception of point-light biological motion at isoluminance. Journal of Vision 2005;5(8):21. doi: 10.1167/5.8.21.
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Background. Individuals are remarkably adept at recognizing the human form in action solely from the motion of the joints (Johansson, 1973). Recognition of biological motion has been shown to depend on spatio-temporal integration of the dots, though recent evidence has called into question the extent to which image motion is critical (e.g. Beintema & Lappe, 2002). These experiments examine the extent to which viewing biological motion without luminance cues, a manipulation that degrades motion sensitivity, impairs perception of biological motion. Method. All subjects were screened for normal color vision as measured by the Ishihara Color Plates. Observers viewed animations depicting either a human action (biological motion) or non-biological motion-matched controls (‘scrambled motion’). Psychometric functions were measured for two-alternative forced choice discriminations on the animations embedded in four levels of dynamic noise. The dynamic noise consisted of the same motion trajectories as the biological or scrambled targets. The displays were rendered as green dots on a red background, or vice versa, and either differed in luminance or had the same perceived luminance as determined by a minimum flicker task using random dot patterns. Subjects also performed a direction discrimination task on red/green random-dot kinematograms with varying levels of direction coherence. Results. Direction discrimination was better with the luminance cue that at isoluminant light levels, verification that subjects were indeed able to determine their point of red/green isoluminance in the minimum flicker task. Discrimination performance on the biological motion task was also better in the luminance condition than in the isoluminance condition. Conclusions. Removing luminance cues from point-light animations reduced sensitivity to biological motion. These results will be discussed in the context of the relative contributions of form and motion in biological motion perception.
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