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Cortical magnification describes how many neurons in an area of the visual context 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 center of 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 from foveal to peripheral representations is achieved in several steps along the visual pathway, starting already in the retina.

The receptive field of a visual neuron comprises a two-dimensional region in visual space whose size can range from a few minutes of arc (a dot in this page at reading distance) to tens of degrees (the entire page). The receptive field size increases at successive processing stages in the visual pathway and, at each processing stage, it increases with the distance from the point of fixation (eccentricity).

Retinal ganglion cells located at the center of vision, in the fovea, have the smallest receptive fields and those located in the visual periphery have the largest receptive fields. The large receptive field size of neurons in the visual periphery explains the poor spatial resolution of our vision outside the point of fixation (other factors are photoreceptor density and optical aberrations). To become aware of the poor spatial resolution in our retinal periphery, try to read this line of text while fixating your eyes in a single letter. The letter that you are fixating is being projected at the center of your fovea where the receptive fields of retinal ganglion cells are smallest. The letters that surround the point of fixation are being projected in the peripheral retina. You will notice that you can identify just a few letters surrounding the point of fixation and that you need to move your eyes if you want to read the entire line of text.

Visual receptive fields are sometimes described as 3-dimensional volumes in visual space to include depth in addition to planar space. However, this use of ‘receptive field’ is less common and it is usually restricted to cortical neurons whose responses are modulated by visual depth.

References:

Born, R.T., Trott, A., Hartmann, T. (2014, October 16). Cortical Magnification Plus Cortical Plasticity Equals Vision? National Center for Biotechnology Information. Retrieved April 7, 2020 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4400204/