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Cortical magnification describes how many neurons in an area of the visual cortex are 'responsible' for processing a stimulus of a given size, as a function of visual field location. In the center of the visual field, corresponding to the fovea of the retina, a very large number of neurons process information from a small region of the visual field. If the same stimulus is seen in the periphery of the visual field (i.e. away from the center), it would be processed by a much smaller number of neurons. The reduction of the number of neurons per visual field area is achieved in several steps along the visual pathway, starting already in the retina.
A reduction of the number of neurons for a given area of the visual field implies an increase of the size of the receptive fields of the neurons, since each neuron has to cover a larger part of the visual field. As a consequence, visual performance (e.g. visual acuity) is best in the center and worse in the periphery. In primary visual cortex (V1), the scaling of cortical magnification with eccentricity is also known as M scaling (M=magnification).
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