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Gene R. Stoner; Contextual influences of shadows on motion interpretation. Journal of Vision 2002;2(10):59. doi: 10.1167/2.10.59.
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A horizontally moving grating viewed through a diamond-shaped aperture can be made to appear to move either upwards or downwards if binocular depth cues (disparity and monocular half-occlusions) are used to simulate partial occlusion of that grating (Duncan et al, 2000). For these “barber-diamond” stimuli, the grating is perceived to move towards (and slide under) the occluding near surfaces and to move parallel to the far surfaces. We asked whether depth from shadows could similarly affect perceived direction of motion. Ray-tracing software was used to render photorealistic shadowed barber-diamonds. Stimuli had two directions of horizontal motion and two depth configurations. In addition to these two main independent variables, we examined the effect of placing the light source above (light-above) versus below (light-below). We also created anti-shadows that were brighter, not darker, than non-shadowed regions. Subjects (n=7) indicated the perceived direction of the moving grating by adjusting the orientation of an arrow placed in the center of the display. For light-above conditions, directional reports were highly biased in favor of the direction of the occluding surfaces (650:81). This bias was also seen for the light-below conditions but the effect was reduced (513:204). This reduction is consistent with previous demonstrations that the human visual system favors the assumption that light comes from above. For anti-shadow conditions, the bias seen for standard shadows was reversed. This result argues against low-level explanations based on local variation in image intensity. Taken together, our results demonstrate that the visual system can use depth from shadows to interpret the direction of motion of ambiguously moving features. We discuss the implications of these results to contextual interactions within visual cortex.
Duncan, R.O., T.D. Albright, and G.R. Stoner (2000). Occlusion and the interpretation of visual motion: perceptual and neuronal effects of context. J Neurosci, 20(15), 5885–97.
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