Orientation tuning of curvature adaptation reveals both curvature-polarity-selective and non-selective mechanisms

We have used a curvature after-effect, or CAE, to explore whether curvature detectors are tuned for the overall orientation of a curve. CAEs were measured for half-cycle cosine-shaped contours as a function of adaptor contour orientation for a fixed test contour orientation. CAEs (i) were greatest when the adaptor and test contours had the same orientation, (ii) decreased rapidly as the orientation of the adapting contours rotated away from the test, the data being well fit by a Gaussian function with a standard deviation of 16 degrees , (iii) increased again to a secondary peak when the adapting contours were rotated 180 degrees relative to the test. Control experiments showed that the shape of the curvature-orientation tuning function could not be explained by local orientation adaptation, and that instead curvature encoding mechanisms are tuned for orientation. The secondary peak in the CAE at 180 degrees is argued to be inconsistent with curvature opponency and instead a result of the combination of polarity-selective and polarity-non-selective curvature mechanisms. The results are discussed in relation to recent psychophysical and physiological models of form processing and the possible significance of the findings with regard to symmetry processing.