Neuroanatomy

Milner and Goodale proposed that the two prominent cortical visual pathways that have been identified in the primate brain (after Mishkin, Ungerleider, and Macko) are each involved in two very different processes. The underlying mechanisms in the ventral stream, which projects from primary visual cortex to the inferotemporal cortex (via many routes involving areas V2, the ventral portion of V3, V4, and TEO) are thought to be involved in visual perception, whereas the dorsal stream, which projects from primary visual cortex (and the superior colliculus via the pulvinar) to the posterior parietal cortex is thought to be involved in the visual control of action (Fig. 3). Both streams are thought to process information about object features and their spatial locations, but each stream uses this visual information in different ways. The transformations carried out by the dorsal stream deal with moment-to-moment information about the location and orientation of objects and thereby mediate the visual control of skilled actions, such as manual prehension, directed at those objects. In contrast, visual information is transformed in the ventral stream to deliver the enduring characteristics of objects and their relations, permitting the formation of long-term perceptual representations of the world. Such representations play an essential role in the recognition and identification of objects and enable us to classify objects and events, attach meaning and significance to them, and establish their causal relations. Such operations are essential for accumulating a knowledge base about the world.

Posterior Parietal Cortex Causality

Figure 3 Diagram of the major routes leading from the retina into the dorsal and ventral streams. LGNd, lateral geniculate nucleus, pars dorsalis; Pulv, pulvinar; SC, superior colliculus. Reprinted from Current Biology 4(7), Goodale, M. A., Meenan, J. P., Bulthoff, H. H., Nicolle, D. A., Murphy, K. J., and Racicot, C. L., Separate neural pathways for the visual analysis of object shape in perception and prehension, pp. 604-610, copyright 1994, with permission from Elsevier Science.

Figure 3 Diagram of the major routes leading from the retina into the dorsal and ventral streams. LGNd, lateral geniculate nucleus, pars dorsalis; Pulv, pulvinar; SC, superior colliculus. Reprinted from Current Biology 4(7), Goodale, M. A., Meenan, J. P., Bulthoff, H. H., Nicolle, D. A., Murphy, K. J., and Racicot, C. L., Separate neural pathways for the visual analysis of object shape in perception and prehension, pp. 604-610, copyright 1994, with permission from Elsevier Science.

Many of the cells in inferotemporal cortex, the terminus of the ventral stream, respond best to complex visual stimuli, such as hands and faces; in particular, the more anterior parts of the inferotemporal cortex are remarkably selective in their responses to object attributes. The receptive field of virtually every cell in the inferotemporal (IT) cortex, a complex of areas lying ventrally below the superior temporal sulcus, including various subdivisions of area TE along with area TEO, includes the foveal region, where fine discriminations are made. These cells also have large receptive fields that allow for generalization across the entire visual field and for coding the intrinsic features of an object independent of its location. The critical features that activate cells in the anterior IT cortex are moderately complex and can be thought of as partial features common to images of several different natural objects. There are also neurons in the IT cortex that demonstrate properties consistent with object constancy in that they remain selectively responsive to a visual stimulus despite changes in stimulus viewpoint, retinal image size, or even color. Thus, the ventral stream is uniquely set up to process visual information into perceptual representations to which meaning and significance can be attached and stored. Damage to the ventral stream is believed to cause the disturbances of object recognition that are characteristic of visual agnosia.

Evidence for this derives from nonhuman primate work, in which large bilateral resections of the temporal lobe result in a form of visual agnosia, Kluver-Bucy syndrome. Lesions of the IT cortex impaired the monkey's ability to identify objects when the discriminations required use of color, pattern, or shape. These monkeys had difficulty using vision to learn associations with objects and could no longer recognize objects or distinguish between objects on the basis of their visual dimensions. They were unable to distinguish food from nonfood objects using vision alone and were unable to learn new visual discriminations between patterns for food reward. Although they incessantly examined all objects in sight, these animals recognized very little and often picked up the same item repeatedly. Kluver-Bucy syndrome can also be achieved with just the removal of IT but, like human visual agnosia, the IT monkey's recognition deficits cannot be explained by "low-level" sensory impairments since large bilateral lesions of IT have been found to have no residual effect on visual acuity.

Recent functional neuroimaging studies of regional blood flow in normal human subjects have revealed many different visual areas beyond primary visual cortex that appear to correspond to those in the ventral stream of the monkey brain that are specialized for the processing of color, texture, and form differences of objects. These studies have shown that face-matching tasks involve the occipitotemporal regions, detection of shape activates regions along the superior temporal sulcus, and the ventral region of the temporal lobe, and the perception of color is associated with activation of the lingual gyrus (V4).

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