Perception of contours and shapes: low and intermediate stage mechanisms.

This review focuses on low and intermediate stages of contour shape processing. It is split into two main sections, 'Contour Detection' and 'Shape Discrimination and Representation'. The first section examines contrast detection of elements within a contour ("collinear facilitation") and the detection of contours in noise ("contour integration"). The second section deals with the discrimination and representation of simple and complex shapes. Perceptual effects on contour detection have been linked to low-level, long-range lateral interactions between neighbouring neurons in V1. Experimental results suggest a complex network of interactions that are context dependent, with collinearity being the dominant factor. While lateral connections are an obvious candidate for linking contour elements into spatially extended contours, the long-range interactions are insufficient to account for human performance in a variety of tasks. Data suggest the existence of global mechanisms that integrate information beyond that of neighbouring cells and are influenced by the overall features of a stimulus. Evidence from psychophysics and physiology is converging towards the identification of an intermediate level of shape processing, where sensitivity to such global attributes emerge.