Neurophysiology of shape processing

Recent physiological findings are reviewed and synthesized into a model of shape processing and object recognition. Gestalt laws (e.g. good continuation, closure) and ‘non-accidental’ image properties (e.g. colinear terminating lines) are resolved in prestriate visual cortex, (areas V2 and V3) to support the extraction of 2D shape boundaries. Processing of shape continues along a ventral route through inferior temporal (IT) cortex where a vast catalogue of 2D shape primitives is established. Each catalogue entry is size-specific (±0.5 log scale unit) and orientation-specific (±45°), but can generalize over position (±150 degree 2). Several shape components are used to activate representations of the approximate appearance of one object type at one view, orientation and size. Subsequent generalization, first over orientation and size, then over view, and finally over object sub-component, is achieved in the anterior temporal cortex by combining descriptions of the same object from different orientations and views, through associative learning. This scheme provides a route to 3D object recognition through 2D shape description and reduces the problem of perceptual invariance to a series of independent analyses with an associative link established between the outputs. The system relies on parallel processing with computations performed in a series of hierarchical steps with relatively simple operations at each stage.