To better understand the cortical mechanisms mediating perception and memory, we have been examining the functional areas that comprise visual association cortex in nonhuman primates and explored their interconnections by the use of neuroanatomical tracing techniques in combination with physiological recording of neural activity. Our results indicate that a multiplicity of separate visual areas lie beyond the primary visual cortex in the stream of information processing. These areas form two hierarchically organized and functionally specialized cortical pathways: an occipitotemporal pathway, or 'ventral stream', for the identification of objects, and an occipitoparietal pathway, or 'dorsal stream', for the appreciation of the spatial relationships among objects and the visual guidance of movement towards them. Whereas neurons in the areas along the pathway into the temporal lobe process the visual features objects, such as color and form, those in areas along the pathway into the parietal lobe process the visuospatial attributes of objects, such as their direction of motion. Not only are the two processing pathways anatomically and functionally distinct, but they can also be differentiated by their neurochemical profiles. Whereas the vast majority of neurons within the occipitoparietal pathway contain neurofilament protein, this is true for less than half the neurons within the occipitotempoal pathway. In an attempt to identify homologies between the human and monkey brains, we have begun investigations of human subjects using functional brain imaging and of human brain tissue using the same immunohistologcial staining techniques that we have used on monkey brain tissue. Thus far in our brain imaging studies, we have localized the two visual processing pathways within the human brain, investigated functional specialization of visual attributes within them, and identified their further extensions for working memory in the prefrontal cortex. Additionally, we have demonstrated that the learning and retention of simple skills, both perceptual and motor, involves neural mechanisms located at very early stages of cortical processing, namely, the primary visual and motor cortex. In our work with human brain tissue, we have successfully identified several cortical areas in the human brain that have been described in the monkey brain, including visual areas V2, V3 and MT. Future studies will pursue functional brain imaging studies of perception and memory in humans and parallel single-cell recording studies in awake, behaving monkeys. A major effort will be to determine how """"""""top- down"""""""" influences of attention and memory affect perceptual processing.
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