Defining the cortical visual systems: "What", "Where", and "How"

The visual system historically has been defined as consisting of at least t wo broad subsystems subserving object and spatial vision. These visual proc essing streams have been organized both structurally as two distinct pathwa ys in the brain, and functionally for the types of tasks that they mediate. The classic definition by Ungerleider and Mishkin labeled a ventral "what" stream to process object information and a dorsal "where" stream to proces s spatial information. More recently, Goodale and Milner redefined the two visual systems with a focus on the different ways in which visual informati on is transformed for different goals. They relabeled the dorsal stream as a "how" system for transforming visual information using an egocentric fram e of reference in preparation for direct action. This paper reviews recent research from psychophysics, neurophysiology, neuropsychology and neuroimag ing to define the roles of the ventral and dorsal visual processing streams . We discuss a possible solution that allows for both "where" and "how" sys tems that are functionally and structurally organized within the posterior parietal lobe. (C) 2001 Elsevier Science B.V. All rights reserved.