Cortical magnification describes how many

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...

in an

area Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an op ...

of the

visual cortex The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...

are 'responsible' for processing a

stimulus A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...

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 Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...

of the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...

, 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 The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight (the a ...

, 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 30 – 90 between the foveal and peripheral representation of the primary visual cortex (V1), depending on how the estimate is obtained. The inverse of M (i.e. degrees visual angle per millimeter cortical tissue) increases linearly with eccentricity in the visual field. Visual performance depends importantly on the amount of cortical tissue devoted to the task. As an example, spatial resolution (i.e.

visual acuity Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal ...

) is best in the center of the fovea and lowest in the far periphery. Consequently, visual performance variations across the visual field can often be equalized by enlarging stimuli depending on their location in the visual field by a factor that compensates for cortical magnification, which is referred to as ''M scaling'' (M=magnification). However, the variation of visual performance across the visual field differs widely between different functions (pattern recognition, motion perception, etc.), and cortical magnification is only one factor amongst others that determine visual performance.

References

{{reflist Cerebrum Visual system

See also

References