Two Visual Brains in Action

ABSTRACT: Milner and Goodale review a wealth of evidence, much of it from their own research, showing that visually guided behavior and perception are controlled by two separate and quasi-independent 'visual brains'. Early evidence showed that motor ability was sometimes preserved despite simultaneous perceptual illusions, and work with patients has differentiated the two systems neurologically. Of the two visual systems, the motor system is less well known: it has a body-based frame of reference, but no memory and limited pattern-recognition capacity. Though vision seems to its users to be a single sense, we now know that visually guided behavior and perception are controlled by two separate and quasi-independent 'visual brains'. Perception depends upon a pathway from the primary visual cortex through several intermediates to the inferotemporal (IT) cortex, identified by Pribram (1971) as a seat of pattern recognition and necessary to solve problems involving visual choice. If one asks people what they see, they report the contents of this system. The motor system is more difficult to uncover, because its representations of visual space are not perceived; in Milner and Goodale's definition, this means that they also are no