Cortical magnification
Encyclopedia
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 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 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 field
s 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:
Cortical area
A cortical area is a part of the cerebral cortex.-Functionally Defined:Often, a cortical area is functionally defined, i.e. its neurons share certain distinguishing functional properties...
of the visual cortex
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
are 'responsible' for processing a stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
of a given size, as a function of visual field location. In the center of the visual field, corresponding to the fovea
Fovea
The fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....
of the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, 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
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
, 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)
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
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 field
Receptive field
The receptive field of a sensory neuron is a region of space in which the presence of a stimulus will alter the firing of that neuron. Receptive fields have been identified for neurons of the auditory system, the somatosensory system, and the visual system....
s 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
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
) is best in the center and worse in the periphery.
In primary visual cortex (V1)
Visual cortex
The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
, 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
Dorsomedial area
The Dorsomedial area, also known as DM or V6, is a subdivision of the visual cortex of primates first described by John Allman and Jon Kaas in 1975...
(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.