Are judgements of circularity local or global?

We assessed, in a task where subjects had to detect smooth deviations from circularity, whether the underlying mechanisms were localised in space to t he size of the individual perturbations or whether they computed global sha pe. By manipulating the phase, the number of cycles of modulation and the s patial arrangement of the perturbations we argue that although either aspec t can be detected, performance is ultimately limited by a global shape dete cting mechanism. We show that this global mechanism receives input from spa tially coarse, crossed orientationally tuned filters whose peak position in orientation depends on the overall shape to be detected. (C) 1999 Elsevier Science Ltd. All rights reserved.