Two Visual Systems

Beyond primary visual cortex (VI), visual information is conveyed to a complex array of higherorder visual areas in the cerebral cortex (for review, see Zeki 1993). Nevertheless, as long ago as 1982, Ungerleider and Mishkin were able to identify two broad "streams" of projections arising from V1 in the macaque monkey: a ventral stream projecting eventually to the infero-temporal cortex and a dorsal stream projecting to the posterior parietal cortex (figure 12.1). These regions also receive inputs from a number of other subcortical visual structures, such as the superior colliculus, which sends prominent projections to the dorsal stream (via connections in the thalamus). Evidence from neuropsycho-logical studies and more recently from neuro-imaging suggests that the visual projections from primary visual cortex to the temporal and parietal lobes in the human brain also involve a separation into ventral and dorsal streams (for review, see Goodale and Humphrey 1998; Milner and Goodale 1995).

Ungerleider and Mishkin (1982) originally proposed that the two streams of visual processing play different but complementary roles in the processing of incoming visual information. According to their account, the ventral stream plays a critical role in the identification and recognition of objects, while the dorsal stream mediates the localization of the same objects. Some have referred to this distinction in visual processing as one between object vision and spatial vision—"what" versus "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, in which the relative position of a landmark object with respect to two food wells determined which food well contained a food reward (Pohl 1973; Ungerleider and Brody 1977). Conversely, lesions of the posterior parietal cortex produced deficits in performance on the landmark task but did not affect object discrimination learning (for critiques of these studies, see Goodale 1995; Milner and Goodale 1995). 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 have necessitated a reinterpretation of the division of labor between the two streams.

Some of the most telling evidence against the "what" versus "where" distinction has come from studies with neurological 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 for objects placed in different positions in the visual field contralateral to their lesion (even though they have no difficulty reaching out and touching parts of their body touched by the

Figure 12.1

The major routes of visual input into the dorsal and ventral streams. The diagram of the macaque brain on the right of the figure shows the approximate routes of the cortico-cortical projections from the primary visual cortex to the posterior parietal and the inferotemporal cortex, respectively. LGNd: lateral geniculate nucleus, pars dorsalis; Pulv: pulvinar; SC: superior colliculus. (Adapted with permission from Goodale et al. 1994).

Figure 12.1

The major routes of visual input into the dorsal and ventral streams. The diagram of the macaque brain on the right of the figure shows the approximate routes of the cortico-cortical projections from the primary visual cortex to the posterior parietal and the inferotemporal cortex, respectively. LGNd: lateral geniculate nucleus, pars dorsalis; Pulv: pulvinar; SC: superior colliculus. (Adapted with permission from Goodale et al. 1994).

experimenter). Although this deficit in visually guided behavior, which is known clinically as optic ataxia (Balint 1909), 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 (Goodale et al. 1994; Goodale et al. 1993; Jakobson et al. 1991; Perenin and Vighetto 1988). 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 (Jean-nerod 1988; Perenin and Vighetto 1988, Goodale et al. 1994). Clearly, this pattern of deficits and spared abilities cannot be explained by appealing to a general deficit in spatial vision.

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, but have no problem grasping objects placed in front of them or moving through the world without bumping into things. Consider, for example, the patient DF, a young woman who suffered damage to her ventral stream pathways from anoxia that was the result of carbon monoxide poisoning. As a result of the brain damage, DF shows visual form agnosia and is unable to recognize objects visually on the basis of their shape. Even though DF cannot 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; Goodale et al. 1994). Appealing to a general deficit in "object vision" does not help us to understand her problem. 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 has no conscious perception of those object features.

On the basis of these neuropsychological findings and a number of related behavioral and electrophysiological observations in the monkey, Goodale and Milner (1992) set out a new account of the division of labor between the dorsal and ventral streams of processing. In contrast to Ungerleider and Mishkin (1982), they argued that both streams process information about object features and about their spatial relations —but each stream uses this visual information in different ways. According to Goodale and Mil-ner, the ventral stream is primarily concerned with the enduring characteristics of objects and their relations, permitting the formation of long-term perceptual representations. Such representations play an essential role in the identification of objects and enable us to classify objects and events, attach meaning and significance to them, and establish their causal relations. These are operations that are essential to the accumulation of a knowledge base about the world. The dorsal stream, according to Goodale and Milner, is not involved in this kind of long-term perceptual representation of objects in the world. Instead, the visual networks in the dorsal stream are more concerned with the moment-to-moment control of skilled actions, such as reaching and grasping movements, directed at a particular object in the world.

Thus, the two streams play complementary roles in the production of adaptive behavior. The perceptual representations constructed by the ventral stream interact with various high-level cognitive mechanisms and enable an organism to select a particular course of action with respect to objects in the world, while the visuomotor networks in the dorsal stream (and associated cortical and subcortical pathways) are responsible for the programming and on-line control of the particular movements that action entails (for a detailed discussion of these ideas, see Milner and Goodale 1995). In short, the ventral stream mechanisms deliver conscious visual percepts; the dorsal stream provides the unconscious visual control of action.

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