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Abstract

High-level visual aftereffects (AEs) arise when adaptation to stimuli such as shapes, faces, or spatial configurations affect the subsequent perception of comparable figures. Like classical AEs, high-level AEs are characterized by a percept that is distorted in feature space in the opposite direction of the vector between adaptation and test stimulus. Shape-contrast AEs have been reported for aspect ratio, convexity and taper, but the combined effect of such parameters remained unexplored. In the present experiment the adapting stimulus consisted of two arcs. While keeping the flexion of the arcs constant we varied the distance between them in a range of ±8.2°, allowing us to examine the effects of (1) aspect ratio and (2) convexity/concavity. The test stimulus was a closed ellipsoid of about 7.8° in diameter. Perceptual distortions were assessed with repetitive adaptation and testing in the context of a staircase procedure converging to the point of subjective circularity. We found significant main effects for both factors and the interaction, i.e. smaller aspect ratios lead to stronger effects and convex stimuli result in larger effects than concave ones. The direct spatial correspondence between the location of the adapting and test stimulus was not critical for inducing an AE. We repeated the experiment with a set of smaller stimuli of roughly 1° and the distance between the curves scaled down proportionately. Results in both experiments were comparable, although the interaction was not significant in experiment 2. The results suggest that simple stimulus metrics, such as the absolute size of the curves and distance between them, are less important in creating this AE than the overall shape created by the pair of arcs together.