Parallel versus serial processing: new vistas on the distributed organization of the visual system.

Recent functional studies question the validity of the hierarchical model of organization for processing visual information in cortical areas. The results of these studies suggest that beyond the primary visual cortex (V1), information is not serially processed through successive cortical areas, but that it is simultaneously processed in several areas. The idea that visual information is functionally segregated into different, parallel channels as it circulates through V1 and V2 towards V4 and the middle temporal visual area is also challenged by recent studies that report a smaller degree of functional specialization within the visual areas than previously thought.