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Pietronilla Penna, Baingio Pinna; Inverse Zoellner illusiondue to implicit orientations. Journal of Vision 2007;7(9):612. doi: 10.1167/7.9.612.
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Two types of orientation cues can instill “polarity” to a basic square element made up of two contiguous white and two black edges on a grey background. (i) Cues of explicitly oriented forms (explicit orientation), i.e. the orientation of the checks themselves. (ii) Cues rendered solely by the internal organization of luminance - such as the diagonal organization of the light and dark sides in the square element (implicit orientation), i.e. the orientation of the invisible base of the vertex. Under these conditions, the dominant orientation is along its internal diagonals of asymmetry. The role of implicit polarity is basic to elicit apparent motion (Pinna, 1990; see also Pinna & Brelstaff, 2000; Gurnsey et al., 2002; Morgan 2002), providing the directional cues for biasing the motion vectors that affect the perceived motion direction. Given parallel columns made up of square elements with opposite implicit orientations but with the same width, length, and orientation, because of the reversal of the vertex orientation, the implicit diagonals at the base of the vertices along each column have opposite inclinations. Under these conditions, the parallel columns of checks appear to be tilted in the same direction as the intersecting oblique implicit orientations. This result is opposite to what happens in the Zoellner illusion, where parallel lines are perceived as being tilted in a direction perpendicular to the intersecting oblique segments. The roles of explicit and implicit orientations were studied through psychophysical experiments. The results suggest the opposite role played by implicit and explicit orientations in affecting the direction of the tilt. The most invoked explanation of the Zoellner illusion, based on the principle of perceptual enlargement of acute angles of intersection between the oblique segments and parallel lines, cannot be applied in our conditions. A unitary explanation for both illusions is suggested.
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