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This chapter discusses two distinct and quasi-separate brain systems, the dorsal and ventral visual streams. First, it uses the metaphor of two people singing from the same hymn sheet, and argues that the two visual streams necessarily sing from different, though in large part mutually consistent ones. It then considers evidence derived from experimental strategies to inquire into the nature of the hymn sheets from which each visual processing stream is singing. Clearly the hymn sheets have to be different. In order to provide direct control of one's movements, the dorsal stream has to see the world egocentrically, that is to code visual information in direct relation to the observer's body coordinates. In contrast, the ventral stream specifically needs to encode the world in a way that will be useful not only in the short but also in the long term, for which purpose egocentric coding would be useless.

This chapter provides an overview of the syndrome of unilateral spatial neglect (USN) as a multicomponent deficit, focusing on the distinction between its ‘perceptual’ and ‘premotor’ impairments, as well as on related evidence from the processing of visual illusions. It compares the USN syndrome, and its neural correlates with the two disorders representing the neuropsychological counterparts of vision-for-perception (the ‘ventral’ stream), and of vision-for-action (the ‘dorsal’ stream). From the neuropsychological vantage-point of USN, the chapter takes the view that the two visual streams dichotomy — both in the original version of Ungerleider and Mishkin (1982), and in the development by Milner and Goodale — captures only partially the neural loops concerned with perception and action in the visual domain. The syndrome of USN suggests the existence of a neural system supporting perceptual awareness in spatial reference frames, for vision, and for other sensory modalities, as well as goal-directed, intentional action in the space surrounding us. A third, dorsal-ventral, stream, including the inferior parietal lobule, and the ventral premotor cortex, may constitute the neural underpinnings of spatial awareness for perception and action.

This chapter reviews experimental work in macaque monkeys concerning the processing of visual information for object recognition. Going from the primary visual cortex, area V1, to the inferior temporal cortex (IT), the end station of the ventral visual stream, neurons become selective for complex object features and display a greater tolerance for stimulus changes that preserve object identity. Single IT neurons code for object properties such as shape, texture, and color. Current data suggest that IT neurons do not represent whole visual objects or visual categories, but rather represent features less complex than a whole object (except perhaps in facial representation). The responses of IT neurons are affected by changes in the image that preserve object identity, but their object feature preference is largely invariant to such changes. The stimulus selectivity of IT neurons facilitates the read-out of visual categories and object identity in the regions to which IT projects.

The fronto-occipital fasciculus (FOF), also known as the occipitofrontal fasciculus, is one of the long association systems of the dorsal visual stream. The subcallosal fasciculus of Muratoff that links the cerebral cortex with the caudate nucleus was mistaken for the FOF, and this conceptual and terminological confusion continues to the present day. This chapter begins with historical accounts of the FOF and Muratoff bundles. It then presents the results of the investigation of the FOF of rhesus monkey brain. Observations confirm the existence of the FOF where Dejerine located it in the human, and provide compelling evidence that it is a true association fasciculus linking parieto-occipital regions with the dorsolateral premotor and prefrontal areas. The present study also adds detail to the understanding of its location and to the origin and termination of its fibers.

This chapter begins with a summary of the neuroarchitecture of the two primary visual streams. The summary focuses on the flow of visual information beginning, for both streams, in primary visual cortex and then extending to the temporal and parietal lobes for the ventral and dorsal streams, respectively. The second section discusses the effects of localized injury to the ventral and dorsal systems in adult patients. The third section provides an overview of the available literature on the typical profiles of development for visuospatial functions. The fourth and largest section of the chapter focuses on the effects of early brain injury on the development of visuospatial processing. It shows that the effects of early injury on visuospatial functions associated with both visual streams are more pronounced than those observed for language, and the relationship between site of injury and type of deficit is more similar to the profiles observed among adult patient populations.

This chapter examines some neural processes that a scene image undergoes as it moves through the visual system. It focuses on two opposite yet highly interactive neural systems, the frontoparietal network and the ventral visual stream. Visual recognition mechanisms in the ventral stream lean toward certain objects in visual scenes because they occupy a space that has already been allotted for a high priority by the lateral intraparietal area and the frontal eye fields. While the ventral visual system processes and determines the objects in that environment, the frontoparietal network allocates and points visual attention to important features of the environment.This division of labor by the two systems is supported by the view that spatial selection and target identification are separable parts of finding objects in visual scenes.

This chapter discusses the anatomical and functional continuation of the three parallel visual pathways in cortical areas beyond the striate cortex. These partially segregated visual streams are the magnocellular system, the parvocellular-interblob system, and the parvocellular-blob system. The findings indicate that visual attentional mechanisms allow these processing streams to interact and the attentional effects are implemented via top-down feedback parallel interactions.

Research into the functions of the perirhinal cortex has taken us beyond the concept of the putative medial temporal lobe memory system. This chapter describes how the perirhinal cortex can be thought of, not only as a region that can interact with the hippocampus, but also as part of the ventral visual stream. In addition, the perirhinal cortex almost certainly interacts directly with regions other than the hippocampus. One essential contribution of the perirhinal cortex to these systems, it is believed, is to provide complex conjunctive representations that aid in situations requiring object identification. The progress that has been made in this area cautions against emphasizing the role of the perirhinal cortex as a part of a single system.

Many aspects of visual development over the first months of life can be understood in terms of visual function becoming increasingly dominated by cortical processes. More recent formulations have developed this idea in terms of cortical processes modulating continuing visual functions of subcortical structures and of differential development of distinct visual cortical streams and areas. One important source of evidence for visual cortical development is the development of capabilities that require the specific kinds of stimulus selectivity found in cortical neurons. These include selectivity for orientation, for directional motion selectivity, and binocular disparity. This chapter concentrates on three other lines of evidence; from the control of shifts in visual attention, from optokinetic responses in children with localized brain damage, and from sensitivity to ‘second-order’ stimuli that are presumed to require relatively elaborate cortical processing. These new kinds of evidence are consistent with the broad idea of increasing cortical dominance in the early months but also make clear that the developmental relationships between cortical and subcortical processing, and between cortical areas, are complex ones.

This chapter reviews neuropsychological evidence, from patients with selective brain lesions, indicating that there can be several kinds of binding in vision. Damage to early processes within the ventral visual stream impairs the binding of contours into shapes. This impairment can leave unaffected a more elementary operation of binding form elements into contours. Thus the process of binding elements into a contour is distinct from the process of binding contours into more holistic shapes. In other patients with damage to the parietal lobe, there can be poor binding of shape to surface information in objects. This problem in turn can coexist with a relatively intact process of binding contours into shapes. These findings suggest that there are multiple stages of binding in vision, including binding to derive shape descriptions and binding shape and surface detail together. This chapter concludes that the unity of consciousness is derived from several separable neural processes of binding.

This chapter outlines the contribution of the parietal cortex to spatial cognition. It discusses the structure and function in spatial attention and spatial representation of the parietal higher order areas of the dorsal visual stream and describes spatial information processing in the parietal lobe. The chapter considers the neuropsychological syndromes that result from parietal lesions, such as visual neglect, Balint's syndrome, and Gerstmann's syndrome in order to provide a better understanding of the functions of the parietal cortex in spatial cognitive functions.

The neuroscientific approach to peripersonal space (PPS) stems directly from electrophysiological studies assessing the response properties of multisensory neurons in behaving non-human primates. This multisensory context fostered frameworks which i) stress the PPS role in actions (including defensive reactions) and affordances, which are optimally performed through multiple sensory convergence; and ii) largely make use of tasks that are multisensory in nature. Concurrently, however, studies on spatial attention reported proximity-related advantages in purely unisensory tasks. These advantages appear to share some key PPS features. Activations in brain areas reported to be multisensory, indeed, can also be found using unimodal (visual) paradigms. Overall, these findings point to the possibility that closer objects may benefit from being processed as events occurring in PPS. The dominant multisensory view of PPS should therefore be expanded accordingly, as perceptual advantages in PPS may be broader than previously thought.