of the visual field to which that neuron responds.
• Information about location of an object in visual space is transmitted through magnocellular (M) pathway ganglion cells; information about color and shape of an object is transmitted through parvocellular (P) pathway ganglion cells.
• Images that fall on the two retinas are merged in stages along the visual pathway, culminating in the visual cortex with an integrated, unified perception of visual space.
• Because all parts of the primary visual pathway maintain a retinotopic organization, lesions at specific points in the pathway result in specific visual field deficits.
• Cells within ocular dominance layers of the lateral geniculate project to cells clustered in ocular dominance columns in the visual cortex.
• Functional clustering of other cortical cells establishes cortical modules containing orientation-specific columns and color-specific blobs within the visual cortex.
• Perception of the color of an object or light source results from a comparative assessment of the hue, saturation, and brightness of the direct or reflected light.
• Specialized arrangements of extraocular muscles continually align the eyeballs, allowing the fovea to capture a focused image of the object of interest.
Humans are highly visual animals. The sense of sight is usually acknowledged as the most important of all the senses based on the vast amount of usable information it provides about the environment. The relative amount of brain tissue devoted to visual information processing is proportionally large, accounting for more than half of the total brain mass. Pathways linking the retina and the primary visual cortex in the occipital lobe provide conscious perception of visual input; storage of visual memory occupies significant regions of the parietal and temporal lobes; visual reflex pathways within the brain stem and spinal cord control eye movement and provide protective reflexes; and light-driven circadian rhythms regulate general metabolic rates, control hormonal function, and influence mood through pathways involving the retina, pineal gland, and portions of the diencephalon. The extensive coverage given to the the visual system in this chapter reflects its dominance within the structure of the brain as well as its significance in the clinical setting. Visual testing is included in most standard physical exams of the patient because it can be easily administered and it can be used to pinpoint possible lesions in the many different visual areas of the brain as well as to evaluate other functions such as cognitive status (visual memory), motor reflexes (postural reflex), autonomic function (pupil dilation), and even inner ear problems (eye movement). Visual testing in the experimental setting can also be accomplished with relative ease. The retina, in particular, has been intensively studied as one of the most approachable parts of the brain, and it has served well as a widely used model for understanding many aspects of nervous system function.
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