Cortical magnification

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.

For quantitative purposes, the cortical magnification factor is normally expressed in millimeters of cortical surface per degree of visual angle. When expressed in this way, the values of cortical magnification factor vary by a factor of approximately 100 between the foveal and peripheral representation of the primary visual cortex (V1) of primates (Daniel and Whitteridge 1961).

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). Different cortical areas have different emphases on the representation of the fovea. Areas involved in fine analyses of shape and texture (such as V4) typically show a very high foveal magnification factor, and very little representation of the peripheral visual field. In contrast, other areas show a more gradual decay of magnification factor from fovea to periphery (for example, the dorsomedial area (V6).

References:
 * Daniel PM, Whitteridge D (1961) The representation of the visual field on the cerebral cortex in monkeys. Journal of Physiology 159:203-21.