Retinotopy
Encyclopedia
Retinotopy describes the spatial organization of the neuronal responses to visual stimuli. In many locations within the brain, adjacent neuron
s have receptive fields that include slightly different, but overlapping portions of the visual field
. The position of the center of these receptive fields forms an orderly sampling mosaic that covers a portion of the visual field. Because of this orderly arrangement, which emerges from the spatial specificity of connections between neurons in different parts of the visual system, cells in each structure can be seen as forming a map of the visual field (also called a retinotopic map, or a visuotopic map). Retinotopic maps are a particular case of topographic
organization. Many brain structures that are responsive to visual input, including much of the visual cortex
and visual nuclei
of the brain stem
(such as the superior colliculus
) and thalamus
(such as the lateral geniculate nucleus
and the pulvinar
), are organized into retinotopic maps, also called visual field maps.
The discovery of visual field maps in human can be traced to neurological cases, arising from war injuries, described and analyzed independently by Tatsui Inouye (a Japanese Ophthalmologist) and Gordon Holmes
(a British Neurologist). They observed correlations between the position of the entry wound and visual field loss (see Fishman, 1997 for an historical review).
Areas of visual cortex
are sometimes defined by their retinotopic boundaries, using a criterion that states that each area should contain a complete map of the visual field. However, in practice the application of this criterion is in many cases difficult (Rosa, 2002). Those visual areas of the brainstem and cortex that perform the first steps of processing the retinal image tend to be organized according to very precise retinotopic maps. The role of retinotopy in other areas, where neurons have large receptive fields, is still being investigated (Wandell et al., 2005).
Visual field maps (retinotopic maps) are found in many mammalian brains, though the specific size, number, and spatial arrangement of these maps can differ considerably between species.
Retinotopic maps in cortical areas other than V1 are typically more complex, in the sense that adjacent points of the visual field are not always represented in adjacent regions of the same area. For example, in the second visual area (V2), the map is divided along an imaginary horizontal line across the visual field, in such a way that the parts of the retina that respond to the upper half of the visual field are represented in cortical tissue that is separated from those parts that respond the lower half of the visual field. Even more complex maps exist in the third and fourth visual areas V3 and V4, and in the dorsomedial area
(V6). In general, these complex maps are referred to as second-order representations of the visual field, as opposed to first-order (continuous) representations such as V1 (Rosa, 2002).
Retinotopy mapping shapes the folding of the cerebral cortex
. In both the V1 and V2 areas of macaque
s and human
s the vertical meridian
of their visual field tends to be represented on the cerebral cortex's convex gyri folds whereas the horizontal meridian is tends to be represented in their concave sulci folds.
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s have receptive fields that include slightly different, but overlapping portions of the visual field
Visual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
. The position of the center of these receptive fields forms an orderly sampling mosaic that covers a portion of the visual field. Because of this orderly arrangement, which emerges from the spatial specificity of connections between neurons in different parts of the visual system, cells in each structure can be seen as forming a map of the visual field (also called a retinotopic map, or a visuotopic map). Retinotopic maps are a particular case of topographic
Topographic map (Neuroanatomy)
A topographic map is the ordered projection of a sensory surface, like the retina or the skin, or an effector system, like the musculature, to one or more structures of the central nervous system...
organization. Many brain structures that are responsive to visual input, including much 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....
and visual nuclei
Nucleus (neuroanatomy)
In neuroanatomy, a nucleus is a brain structure consisting of a relatively compact cluster of neurons. It is one of the two most common forms of nerve cell organization, the other being layered structures such as the cerebral cortex or cerebellar cortex. In anatomical sections, a nucleus shows up...
of the brain stem
Brain stem
In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...
(such as the superior colliculus
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
) and thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
(such as the lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
and the pulvinar
Pulvinar
The pulvinar nuclei are a collection of nuclei located in the pulvinar thalamus. The pulvinar part is the most posterior region of the thalamus....
), are organized into retinotopic maps, also called visual field maps.
The discovery of visual field maps in human can be traced to neurological cases, arising from war injuries, described and analyzed independently by Tatsui Inouye (a Japanese Ophthalmologist) and Gordon Holmes
Gordon Morgan Holmes
Sir Gordon Morgan Holmes CMG CBE FRS was a British neurologist. He is best known for carrying out pioneering research into the cerebellum and the visual cortex....
(a British Neurologist). They observed correlations between the position of the entry wound and visual field loss (see Fishman, 1997 for an historical review).
Areas of 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 sometimes defined by their retinotopic boundaries, using a criterion that states that each area should contain a complete map of the visual field. However, in practice the application of this criterion is in many cases difficult (Rosa, 2002). Those visual areas of the brainstem and cortex that perform the first steps of processing the retinal image tend to be organized according to very precise retinotopic maps. The role of retinotopy in other areas, where neurons have large receptive fields, is still being investigated (Wandell et al., 2005).
Visual field maps (retinotopic maps) are found in many mammalian brains, though the specific size, number, and spatial arrangement of these maps can differ considerably between species.
Retinotopic maps in cortical areas other than V1 are typically more complex, in the sense that adjacent points of the visual field are not always represented in adjacent regions of the same area. For example, in the second visual area (V2), the map is divided along an imaginary horizontal line across the visual field, in such a way that the parts of the retina that respond to the upper half of the visual field are represented in cortical tissue that is separated from those parts that respond the lower half of the visual field. Even more complex maps exist in the third and fourth visual areas V3 and V4, and in 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). In general, these complex maps are referred to as second-order representations of the visual field, as opposed to first-order (continuous) representations such as V1 (Rosa, 2002).
Retinotopy mapping shapes the folding of the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. In both the V1 and V2 areas of macaque
Macaque
The macaques constitute a genus of Old World monkeys of the subfamily Cercopithecinae. - Description :Aside from humans , the macaques are the most widespread primate genus, ranging from Japan to Afghanistan and, in the case of the barbary macaque, to North Africa...
s and human
Human
Humans are the only living species in the Homo genus...
s the vertical meridian
Meridian (perimetry, visual field)
Meridian is used in perimetry and in specifying visual fields. According to IPS Perimetry Standards 1978 : "Perimetry is the measurement of [an observer's] visual functions ... at topographically defined loci in the visual field...
of their visual field tends to be represented on the cerebral cortex's convex gyri folds whereas the horizontal meridian is tends to be represented in their concave sulci folds.
Sources
- Ronald S. Fishman (1997). Gordon Holmes, the cortical retina, and the wounds of war. The seventh Charles B. Snyder Lecture Documenta Ophthalmologica 93: 9-28, 1997.
- B. Wandell, A. A. Brewer and R.F. Dougherty (2005). Visual Field Map Clusters in Human Cortex. Phil. Trans. of the Royal Society London, vol. 360, pp. 693-707
- http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-879X2002001200008&lng=en&nrm=iso Rosa MGP, 2002, Visual maps in the adult primate cerebral cortex: some implications for brain development and evolution. Braz J Med Biol Res 35(12):1485-1498.
See also
- TonotopyTonotopyIn physiology, tonotopy is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain...
- Biological neural networkBiological neural networkIn neuroscience, a biological neural network describes a population of physically interconnected neurons or a group of disparate neurons whose inputs or signalling targets define a recognizable circuit. Communication between neurons often involves an electrochemical process...
- Cortical magnificationCortical magnificationCortical 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...
- Visual spaceVisual spaceVisual space is the perceptual space housing the visual world being experienced by an aware observer; it is the subjective counterpart of the space of physical objects before an observer's eyes.-Space of Physical Objects:...
- Frontal eye field