Visual Processing Streams

Vision, more than any other sense, provides us with information about the world beyond our bodies. The importance of vision in our daily lives, and the lives of our primate cousins, is reflected in the fact that we have large and highly mobile eyes. But our reliance on vision is also evident in the large amount of brain devoted to visual processing. It has been estimated, for example, that more than half of the CEREBRAL CORTEX in the macaque monkey is involved in processing visual signals.

Although the RETINA projects to a number of different nuclei in the primate brain, one of the most prominent projections is to the dorsal part of the lateral geniculate nucleus (LGNd), a multilayered structure in the THALAMUS. New projections arise from the LGNd and project in turn to an area in the occipital lobe of the cerebral cortex known variously as striate cortex, area 17, primary visual cortex, or V1. Beyond V1, visual information is conveyed to a bewildering number of extrastriate areas (for review, see Zeki 1993). Despite the complexity of the interconnections between these different areas, two broad "streams" of projections from V1 have been identified in the macaque monkey brain: a ventral stream projecting eventually to the inferotemporal cortex and a dorsal stream projecting to the posterior parietal cortex (Ungerleider and Mishkin 1982). Of course, these regions also receive differential inputs from a number of other subcortical visual structures, such as the superior colliculus (via the thalamus). Although some caution must be exercised in generalizing from monkey to human, it seems likely that the visual projections from primary VISUAL CORTEX to the temporal and parietal lobes in the human brain may involve a separation into ventral and dorsal streams similar to that seen in the monkey.

In 1982, Ungerleider and Mishkin argued that the two streams of visual processing play different but complementary roles in the processing of incoming visual information. According to their original account, the ventral stream plays a critical role in the identification and recognition of objects, while the dorsal stream mediates the localization of those same objects. Some have referred to this distinction in visual processing as one between object vision and spatial vision -- "what" vs. "where." Support for this idea came from work with monkeys. Lesions of inferotemporal cortex in monkeys produced deficits in their ability to discriminate between objects on the basis of their visual features, but did not affect their performance on a spatially demanding "landmark" task. Conversely, lesions of the posterior parietal cortex produced deficits in performance on the landmark task but did not affect object discrimination learning. Although the evidence for the original Ungerleider and Mishkin proposal initially seemed quite compelling, recent findings from a broad range of studies in both humans and monkeys has forced a reinterpretation of the division of labor between the two streams (for review, see Jeannerod 1997; Milner and Goodale 1995).

Some of the most telling evidence against a simple "what" vs. "where" distinction has come from studies with neurologically damaged patients. It has been known for a long time that patients with damage to the human homologue of the dorsal stream have difficulty reaching in the correct direction to objects placed in different positions in the visual field contralateral to their lesion (even though they have no difficulty reaching out and grasping different parts of their own body indicated by the experimenter). Although this deficit in visually guided behavior, know clinically as optic ataxia, has often been interpreted as a failure of spatial vision, two other sets of observations in these patients suggest a rather different interpretation. First, patients with damage to this region of cortex often show an inability to rotate their hand or open their fingers properly to grasp an object placed in front of them, even when it is always placed in the same location. Second, these same patients are able to describe the orientation, size, shape, and even the relative spatial location of the very objects they are unable to grasp correctly (Perenin and Vighetto 1988). Clearly, this pattern of deficits and spared abilities cannot be explained by appealing to a general deficit in SPATIAL PERCEPTION.

Other patients, in whom the brain damage appears to involve ventral rather than dorsal stream structures, show the complementary pattern of deficits and spared visual abilities. Such patients have great difficulty recognizing common objects on the basis of their visual appearance (visual agnosia), but have no problem grasping objects placed in front of them or moving through the world without bumping into things. Consider, for example, patient D.F., a young woman who suffered damage to her ventral stream pathways as a result of anoxia from carbon monoxide poisoning. Even though D.F. is unable to indicate the size, shape, and orientation of an object, either verbally or manually, she shows normal preshaping and rotation of her hand when reaching out to grasp it (Goodale et al. 1991). Appealing to a general deficit in "object vision" does not help us to understand her problem. In her case, she is able to use visual information about the location, size, shape, and orientation of objects to control her grasping movements (and other visually guided movements) despite the fact that she is unable to perceive those same object features.

Goodale and Milner (1992) have suggested that one way to understand what is happening in these patients is to think about the dorsal stream not as a system for spatial vision per se, but rather as a system for the visual control of skilled action. To pick up a coffee cup, for example, not only must we have information about the spatial location of the cup with respect to our hand, but we must also have information about its size, shape, and orientation so that we can pick it up efficiently. The evidence from patients, and from studies with subjects with normal vision, suggests that the visual processing involved in the control of this kind of skilled behavior may take place quite independently of the visual processing mediating what we normally think of as visual perception. Indeed, Goodale and Milner have suggested that our visual experience of the world and the objects within it depends on visual processing in the ventral stream. In short, both streams process information about the orientation, size, and shape of objects, and about their spatial relations; both streams are also subject to modulation by ATTENTION. Each stream, however, deals with the incoming visual information in different ways. The ventral stream transforms visual information into perceptual representations that embody the enduring characteristics of objects and their spatial relations with each other. 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 goal-directed acts. Such a division of labor not only accounts for the behavioral dissociations observed in neurological 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 (for review, see Milner and Goodale 1995).

Adaptive goal-directed behavior in humans and other primates depends on the integrated function of both these streams of visual processing. The execution of a goal-directed action might depend on dedicated control systems in the dorsal stream, but the selection of appropriate goal objects and the action to be performed depends on the perceptual machinery of the ventral stream. One of the important questions that remains to be answered is how the two streams interact both with each other and with other brain regions in the production of purposive behavior.

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-- Melvyn A. Goodale

References

Goodale, M. A., and A. D. Milner. (1992). Separate visual pathways for perception and action. Trends in Neurosciences 15:20-25.

Goodale, M. A., A. D. Milner, L. S. Jakobson, and D. P. Carey. (1991). A neurological dissociation between perceiving objects and grasping them. Nature 349:154-156.

Jeannerod, M. (1997). The Cognitive Neuroscience of Action. Oxford: Blackwell.

Milner, A. D., and M. A. Goodale. (1995). The Visual Brain in Action. Oxford: Oxford University Press.

Perenin, M. -T., and A. Vighetto. (1988). Optic ataxia: A specific disruption in visuomotor mechanisms. 1. Different aspects of the deficit in reaching for objects. Brain 111:643-674.

Ungerleider, L. G., and M. Mishkin. (1982). Two cortical visual systems. In D. J. Ingle, M. A. Goodale, and R. J. W. Mansfield, Eds., Analysis of Visual Behavior. Cambridge, MA: MIT Press, pp. 549-586.

Zeki, S. (1993). A Vision of the Brain. Oxford: Blackwell.

Visual Processing Streams

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