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Much can be learned about the visual system by studying clinical defects and abnormalities. Damage to the eyes or the visual cortex results in defects confined to a particular region of visual space. Damage to higher centers can produce visual neglect, which is an inability to attend to particular regions of space. Brain damage can also produce stereoanomalies or affect the way in which information is transferred between the eyes. Genetic defects such as albinism can also produce defects of stereoscopic vision. This chapter reviews all these pathologies.

This chapter examines the computational neuropsychology of visual attention. It investigates the specific visual cognitive impairment in brain-damaged patients known as visual spatial neglect and attempts to model the neglect syndrome. The chapter concludes that computational neuroscience provides a mathematical framework for studying the mechanisms involved in brain function and allows complete simulation and prediction of neuropsychological syndromes, and that simulations provide useful support for the explanation offered of the functional impairments resulting from brain damage in patients.

There is evidence that mere activity within perceptual systems is not sufficient for consciousness. This chapter begins by reviewing evidence for subliminal perception. This raises the question, when do perceptual states become conscious? The answer defended here is that we are conscious when and only when perception is modulated by attention. Evidence for the necessity and sufficiency of attention is presented, and empirical results that aim to dissociated attention and consciousness are critically reviewed. The chapter also offers an account of the nature of attention, according to which attentional modulation is a change in information processing that allows perceptual states to gain access to working memory. It is argued that accessibility to working memory is the psychological correlate of consciousness; actual encoding in working memory is not necessary.

This chapter deals with visual neglect and the neglect syndrome. It shows that the clinical syndrome of neglect may help shed more light on the functional specialization of the right hemisphere, and in particular the right posterior parietal cortex (PPC). The chapter reveals that specific cognitive deficits within the neglect syndrome can articulate and interact with each other. The PPC is an important point of convergence for various other types of information: sensory, motor, and goal-related. The chapter suggests that the right PPC does not appear to be purely concerned with spatial aspects of attention. Examination of the neglect syndrome and the results of functional imaging studies in healthy control subjects indicate that this region is also critical for sustaining attention and for the detection of novel or behaviorally salient events.

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.

This chapter explains fundamental differences between seeing and recognition, notably how and why our brains use conscious seeing to control actions like looking and reaching, while we learn both view-, size-, and view-specific object recognition categories, and view-, size-, and position-invariant object recognition categories, as our eyes search a scene during active vision. The dorsal Where cortical stream and the ventral What cortical stream interact to regulate invariant category learning by solving the View-to-Object Binding problem whereby inferotemporal, or IT, cortex associates only views of a single object with its learned invariant category. Feature-category resonances between V2/V4 and IT support category recognition. Symptoms of visual agnosia emerge when IT is lesioned. V2 and V4 interact to enable amodal completion of partially occluded objects behind their occluders, without requiring that all occluders look transparent. V4 represents the unoccluded surfaces of opaque objects and triggers a surface-shroud resonance with posterial parietal cortex, or PPC, that renders surfaces consciously visible, and enables them to control actions. Clinical symptoms of visual neglect emerge when PPC is lesioned. A unified explanation is given of data about visual crowding, situational awareness, change blindness, motion-induced blindness, visual search, perceptual stability, and target swapping. Although visual boundaries and surfaces obey computationally complementary laws, feedback between boundaries and surfaces ensure their consistency and initiate figure-ground separation, while commanding our eyes to foveate sequences of salient features on object surfaces, and thereby triggering invariant category learning. What-to-Where stream interactions enable Where’s Waldo searches for desired objects in cluttered scenes.

The two systems evolved together and play complementary roles in the control of behavior. The constant interplay between the two streams is something that can be seen in almost everything we do. A good metaphor for the interaction between the two systems is tele-assistance in which a human operator controls a semi-autonomous robot. The notions of conscious and unconscious vision are discussed.