Chapter 8: Visual–Spatial Perception & Cognition Disorders

The author begins by detailing early visual processing deficits, such as cortical blindness and hemianopia, which arise from damage to the primary visual cortex and result in systematic blind spots within the visual field. The text further explores specialized impairments of intermediate vision, including cerebral achromatopsia—a selective loss of color perception where the world appears in grayscale—and akinetopsia, a rare condition in which patients are unable to perceive motion, seeing moving objects as a series of frozen snapshots. Central to the chapter is the dual-stream framework, which distinguishes between the ventral "what" system in the temporal cortex, responsible for identifying objects and faces, and the dorsal "where" system in the parietal cortex, which manages spatial localization and visually guided actions. Disruptions to the ventral stream can lead to visual agnosia or prosopagnosia, while dorsal lesions often result in hemispatial neglect, visual disorientation, or optic ataxia, which impairs the ability to reach for objects accurately. Additionally, the chapter categorizes various forms of topographic disorientation, such as landmark agnosia and heading disorientation, which interfere with an individual's ability to navigate their environment. The final sections address mental imagery, presenting evidence that imagining an object utilizes many of the same neural substrates as perceiving one. This link is demonstrated by clinical cases where perceptual deficits, like narrowed visual fields or spatial neglect, are mirrored in the patient's mental images. The author also differentiates between the ability to represent images and the voluntary process of image generation, which may be independently impaired and is often associated with the brain's dominant hemisphere.