In
neuroscience
Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions, and its disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, ...
, cortical magnification describes how many
neurons
A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
in an
area
Area is the measure of a region's size on a 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 open surface or the boundary of a three-di ...
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 thalam ...
are 'responsible' for processing a
stimulus of a given size, as a function of
visual field
The visual field is "that portion of space in which objects are visible at the same moment during steady fixation of the gaze in one direction"; in ophthalmology and neurology the emphasis is mostly on the structure inside the visual field and it i ...
location. In the center of the visual field, corresponding to the center of the
fovea of the
retina
The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
, 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.
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 visual perception, vision, but technically rates an animal's ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the eye ...
) 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.
See also
*
Retinotopy
Retinotopy () is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'retinally mapped'.
Visual f ...
Notes
References
{{reflist
Cerebrum
Visual system