Cortical Magnification

Sensory areas, especially those early in the cortical processing sequence, are said to represent the receptor sheets. Thus, stimuli falling on different parts of the receptor sheet activate neurons in different parts of sensory areas in a pattern that is isomorphic with the receptor sheet. However, not all parts of the receptor sheet are equally important. For example, we only see well within a few degrees of central vision corresponding to the specialized center of the retina, the fovea, and the immediate surround, where receptor cells are very densely packed. In humans and monkeys, much of the visual cortex is devoted to processing information from the fovea. In part this is no more than a reflection of the greater receptor density of the fovea. However, the amount of cortex devoted to the fovea exceeds the proportional relationship to retinal receptors. Thus, there is a distortion within sensory representations in the neocortex that follows the variable density of receptors across the sensory surface (receptor-based cortical magnification) and a distortion that exceeds that dictated by receptor density (functionally based cortical magnification). Other mammals may distort sensory maps in other ways. For example, ground squirrels benefit from accurately seeing predators coming from the side as well as from the front. Their retinas have a horizontal strip of high receptor density, and their visual cortex is largely devoted to processing information from this strip of receptors. Rats devote most of their somatosensory cortex to processing information from the long whiskers on the sides of the face that they whisk forward to contact objects, whereas humans devote much of their somatosensory cortex to information coming from our sensitive finger tips. Thus, mammals vary in sensory and perceptual abilities, not only from specialization and alteration of the receptor surfaces but also by changes in the proportions of cortical areas that are devoted to different parts of the receptor sheet.

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