The cortical magnification factor is the apportioning of proportionally more space on the cortex to the representation of specific areas of sensory receptors. For example, a small area on the retina in or near the fovea receives more space on the cortex than the same area of peripheral retina. Smilarly, the fingertips receive more space on the somatosensory cortex than the forearm or leg. 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.
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, 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.