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Marouane Ouhnana, Frederick Kingdom; Common fate versus cast shadows as influences on perceived motion direction and depth. Journal of Vision 2013;13(9):820. doi: 10.1167/13.9.820.
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Aim: Manipulation of an object's cast shadow has been shown to alter the object's perceived depth and motion direction in both static and dynamic displays (Mamassian et al., 1998 Trends Cogn. Sci. 2:288–95). However the effect holds even if the cast shadow looks unrealistic, for example by deforming its shape. Our aim is to test the possibility that something other than ‘shadowness’ is driving the percept. Method: We employed a variant of the ball-in-box stimulus used by Kersten et al. (1994 Max-Planck-Institut fuer biologische Kybernetik, Technical Report no 6) in which a floating test sphere recedes diagonally into the background over a checkered surface. Instead of a shadow, we introduced a second sphere below the first and moved it either diagonally or horizontally in temporal synchrony with the test sphere. Result: The test sphere appeared to move either diagonally or to rise up in the frontal plane depending on the movement of the second sphere, similarly to the effect of a shadow. Conclusion: We suggest that the different percepts implicate a form of common fate. As both spheres move in synchrony, the visual system groups them as a unitary entity. The second sphere serves to disambiguate the motion of the floating sphere by locking its motion into one of two planes, a diagonal plane in which the two motion directions are the same, and a frontal plane in which they are different. This suggestion expands on the common fate principle and may be an example of the broader range of common fate phenomena that Wertheimer had in mind (Wagemans et al., 2012 Psychology Bulletin, 1172:1217).
Meeting abstract presented at VSS 2013
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