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Frederick A. A. Kingdom, Elena Gheorghiu; On the mechanisms for contour-shape after-effects. Journal of Vision 2006;6(6):339. doi: 10.1167/6.6.339.
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Adaptation to a sinusoidally modulated contour produces a shift in the apparent shape frequency of a subsequently presented test contour, in a direction away from that of the adaptation stimulus. The phenomenon has been termed the ‘shape-frequency after-effect’ or SFAE (Kingdom & Prins, 2005, JOV, 5, 464). We describe an even larger after-effect of contour-shape amplitude, which we term the ‘shape-amplitude after-effect’ or SAAE. What underlies these after-effects? They occur even when the shape-phase of the contour is randomly changed every half second during the adaptation period, which would tend to lead one to reject the idea that local tilt (or orientation) after-effects (TAEs) are the underlying cause. However we show that even with adaptation contour phase-randomization, the geometrical relationships between adaptor and test are such that the TAE is difficult to rule out. We provide evidence against the TAE: sizeable after-effects are obtained for adaptors that are sine-wave-shaped and tests that are square-wave-shaped. In addition we test, and reject, three other candidates besides local orientation: global average curvature, local signed curvature and global spatial frequency. We suggest that contour shape after-effects result from adaptation to the sizes of local, partly-bounded regions defined by the contour's shape. This in turn implies that contour shape is processed by mechanisms that encode the sizes of partly-bounded regions of the stimulus.
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