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FIG. 1. Visual processing pathways in monkeys. Areas in the dorsal stream, having primarily visuospatial functions, are shown in green, and areas in the ventral stream, having primary object recognition functions, are shown in red. Lines connecting the areas indicate known anatomical connections, with heavy arrowheads indicating feed-forward connections from lower-order areas to higher-order ones and open arrowheads indicating feedback connections from higher-order areas to lower-order ones. Solid lines indicate connections from both central and peripheral visual field representations, and dotted lines indicate connections restricted to peripheral field representations. Shaded region on lateral view of the brain indicates extent of cortex included in the diagram. (From ref. 85; for further details of the visual areas, see ref. 86.)

Areas within both processing pathways appear to be organized hierarchically, in the sense that low-level inputs are transformed into progressively more integrated representations through successive stages of processing. Within the ventral stream, for example, the processing of object features begins with simple spatial filtering by cells in V1, but by the time the inferior temporal cortex (area TE) is activated, the cells respond to global object features, such as shape, and some cells are even specialized for the analysis of faces (5). Likewise, within the dorsal stream, the processing of moving stimuli begins with simple direction-of-motion selectivity by V1 cells, but in the higher-order areas of the parietal cortex (such as the lateral intraparietal and medial superior temporal areas) the cells respond selectively to complex patterns of motion, such as rotation, and to the optic flow patterns produced when one moves through the environment (1214).

Thus, much of the neural mechanism for both object vision and spatial vision can be viewed as a “bottom-up” process subserved by feed-forward projections within a pathway. Each of these feed-forward projections, however, is reciprocated by a feedback, or reentrant, projection (6, 15). Projections from higher-order processing stations to lower-order ones could mediate some “top-down” aspects of vision, such as the effects of selective attention on perceptual processing.

Monkey Prefrontal Cortex and Working Memory

Both processing streams, dorsal and ventral, have reciprocal connections with regions beyond the modality-specific visual system. One such set of connections includes those with the prefrontal cortex. Anatomical studies have shown that projections from areas in the dorsal stream terminate mainly in and around the principal sulcus [Brodmann area (BA) 46] of dorsolateral prefrontal cortex (1618), whereas projections from areas in the ventral stream terminate mainly on the inferior convexity (BA 12 and 45) in ventrolateral prefrontal cortex (1921). Thus, from an anatomical point of view, the domain specificity of spatial vs. object vision that is present in

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