According to the two-visual-systems model put forward by Goodale and Milner, both streams of visual processing handle information about the orientation, size, and shape of objects, and about their spatial relations.  Each stream, however, deals with the incoming visual information in different ways. The ventral stream transforms visual information into perceptual representations, which embody the enduring characteristics of objects and their spatial relations.  These representations form the foundation for our cognitive life, allowing us to recognize objects and understand their causal relations, to communicate with others about the world beyond our bodies, and to identify goals and plan actions with respect to those goals.  The visual transformations carried out in the dorsal stream, which utilize moment-to-moment information about the disposition of objects within egocentric frames of reference, mediate the control of those goal-directed acts.  Such a division of labor not only accounts for the neurological dissociations observed in patients with damage to different regions of the cerebral cortex, but it is also supported by a wealth of anatomical, electrophysiological, and behavioral studies in the monkey.

There is also evidence for dissociations between ‘vision for perception’ and ‘vision for action’ in normal observers.  For example, the scaling of grasping movements are relatively unaffected by size-contrast illusions that have a profound affect on perceptual judgements of size.   Such paradoxes show that what we think we 'see' is not always what guides our actions and provide additional evidence for the parallel operation of two types of visual processing, each apparently designed to serve quite different purposes, and each characterized by quite different properties.

The results of recent functional neuroimaging studies converge nicely with the evidence from earlier neuropsychological studies in humans and neurophysiological studies in the macaque monkey.  Thus, areas in the human ventral stream appear to be specialized for the processing the identity of objects and materials, whereas regions in the human dorsal stream play a critical role in the control of visually guided movements, such as saccades, reaching, and grasping.

Although the ventral and dorsal streams are functionally distinct, the two evolved together and play complementary roles in the control of behavior.   In some ways, the limitations of one system are the strengths of the other.  Thus, although the ventral perception system delivers a rich and detailed representation of the world, the metrics of the world with respect to the organism are not well specified.  In contrast, the dorsal action system delivers accurate metrical information in the required egocentric coordinates but these computations are spare and evanescent.  Both systems are required for purposive behaviour – one system to select the goal object from the visual array, the other to carry out the required metrical computations for the goal-directed action. One of the most important questions yet to be addressed is how the two streams communicate with one another.