Spatial properties of curvature-encoding mechanisms revealed through the shape-frequency and shape-amplitude after-effects

The shape-frequency and shape-amplitude after-effects, or SFAE and SAAE, are phenomena in which adaptation to a sinusoidal-shaped contour results in a shift in, respectively, the perceived shape-frequency and perceived shape-amplitude of a test contour in a direction away from that of the adapting stimulus. Recent evidence shows that the SFAE and SAAE are mediated by mechanisms sensitive to curvature [Gheorghiu, E., & Kingdom, F. A. A. (2007a). The spatial feature underlying the shape-frequency and shape-amplitude after-effects. Vision Research, 47(6), 834-844]. Therefore we have used the SFAE and SAAE as a tool to study curvature processing. We examined whether curvature-encoding mechanisms are selective for (i) shape-phase, (ii) curvature polarity (or sign) and (iii) local orientation. We also investigated whether (iv) the two orthogonal dimensions of a curve, the sag and the cord, are encoded independently, and (v) whether curvature encoders are organized in an opponent manner. SFAEs/SAAEs were measured for adapting and test contours that differed or not in a given spatial property, the rationale being that if the after-effects were smaller when adaptor and test differed in a particular spatial property then curvature-encoding mechanisms must be selective for that spatial property. Our results reveal that SFAEs and SAAEs show (i) a degree of selectivity to curves that are mirror symmetric (in our stimuli half-cycle sine-wave contours in cosine (0/180deg) shape-phase); (ii) a degree of selectivity to the sign or polarity of curvature; (iii) a degree of selectivity to local orientation; (iv) independent coding of the sag and the cord of the curve, and (v) no evidence for opponent-curvature coding. The results agree with neurophysiological studies showing that simple shape dimensions are encoded independently.