Probing intermediate stages of shape processing

The visual system provides a representation of what and where objects are. This entails parsing the visual scene into distinct objects. Initially, the visual system encodes information locally. While interactions between adjacent cells can explain how local fragments of an object's contour are extracted from a scene, such computations are ill suited to capture extended objects. This article reviews some of the evidence in favor of intermediate-level computations, tuned to the shape of an object, in the transformation from discrete local sampling to representation of complex objects. Two main paradigms, employed to study how information about the position and orientation of local signals are combined at intermediate levels, are considered here: a shape detection task (measuring the number of signal elements required to detect a shape in noise) and a shape discrimination task (requiring observers to discriminate between shapes). Results support the notion of global mechanisms that integrate information beyond neighboring cells and are optimally tuned to a range of different shapes. These intermediate processing stages appear vulnerable to damage. Diverse clinical conditions (amblyopia, macular disease, migraine, premature birth) show specific deficits for these tasks. Taken together, evidence is converging in favor of intermediate levels of processing, at which sensitivity to the global shape of objects emerges.