Shape contrast: a global mechanism?

We investigated whether a shape contrast bias is caused by local contrast enhancement or by a global mechanism. In a baseline condition, observers performed a shape discrimination task on an isolated hinged plane. But in the experimental conditions, five dihedral surfaces, of which we varied the dihedral angle distribution, were added on each side. Shape perception was influenced not only by the adjacent surface but also by the mean of the shape distribution in the extended surround. Thus, shape contrast is not locally determined and has to be understood from a global mechanism. We propose divisive normalization of shape signals as such a mechanism.