Biases in the judgement of lengths in the plane abound; examples include the Muller-Lyer illusion and the vertical-horizontal illusion. Here we demonstrate a new form of bias induced by shape. There are two reasons to predict such a bias. First, some theories hold that a shape is neurally represented as a deformation process. Such a mechanism might induce artifactual distortions in the perceived metric structure of the image. Second, if the shape induces a 3D percept, this might bias 2D judgements. Methods. Observers are presented with a triplet of points, in proximity to the outline of an animal shape. Each triplet forms an isosceles right-angle triangle with a horizontal long side. Although the two oblique sides are equal, observers are asked to judge which appears shorter. Interpreting each judgement as a distortion of the perceived triangle, from a sequence of trials we derive a vector field of perceived distortion over the image. In a control condition, observers make the same judgements in the absence of the outline shape: the difference in the resulting vector fields provides an estimate of distortion independent of static inhomogeneities of the visual field. Results. For three of four observers, distortion was found to be significantly larger inside the shape (figure) than outside the shape (ground), and in these same three observers, the distortion grew larger nearer the contour. For two of the observers distortion was significantly biased to flow radially out from the centre of the shape in both figure and ground regions; for the other two observers this radial bias was significant only for the ground region. Conclusion. These results show that shape outlines can induce distortions in the image. These distortions may be artifacts induced by cortical mechanisms for shape representation, or illusions based on 3D perceptual interpretations of the image.
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