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 the size of the individual perturbations or whether they computed global shape. By manipulating the phase, the number of cycles of modulation and the spatial arrangement of the perturbations we argue that although either aspect can be detected, performance is ultimately limited by a global shape detecting mechanism. We show that this global mechanism receives input from spatially coarse, crossed orientationally tuned filters whose peak position in orientation depends on the overall shape to be detected.