Meridian (perimetry, visual field)
Encyclopedia
Meridian is used in perimetry and in specifying visual fields. According to IPS Perimetry Standards 1978 (2002): "Perimetry
is the measurement of [an observer's] visual functions ... at topographically defined loci in the visual field. The visual field
is that portion of the external environment of the observer [in which when he or she is] steadily fixating ...[he or she] can detect visual stimuli."
In perimetry, the observer's eye
is considered to be at the centre of an imaginary sphere
. More precisely, the centre of the sphere is in the centre of the pupil
of the observer's eye. The observer is looking at a point, the fixation point, on the interior of the sphere. The visual field
can be considered to be all parts of the sphere for which the observer can see a particular test stimulus. If we consider this surface to be that on which an observer can see anything, then it is a section of the sphere somewhat larger than a hemisphere
. In reality it is smaller than this, and irregular, because when the observer is looking straight ahead, his or her nose blocks vision of some possible parts of the surface. In perimetric testing, a section of the imaginary sphere is realized as a hemisphere in the centre of which is a fixation point. Test stimuli can be displayed on the hemisphere.
To specify loci in the visual field, a polar coordinate system is used, all expressed from the observer's perspective. The origin
corresponds to the point on which the observer is fixating. The polar angle
is considered to be zero degree
s when a locus is horizontally to the right of the fixation point and to increase to a maximum of 360 degrees going anticlockwise. Distance from the origin is given in degrees of visual angle
; it's a measure of eccentricity
. Each polar axis is a meridian of the visual field. For example, the horizontal meridian runs from the observer's left, through the fixation point, and to the observer's right. The vertical meridian runs from above the observer's line of sight, through the fixation point, and to below the observer's line of sight.
Another way of thinking of the maximum visual field is to think of all of the retina
that can be reached by light from the external environment. The visual field in this case is all of the external environment that can project light onto the retina. Meridians correspond to sections of great circles passing though the centre of the fovea
. In an analogy to Meridian (geography)
, in which meridians are lines of longitude
, the North pole
might correspond to the fovea, Greenwich would correspond to a retinal location about 39 degrees to the left of the fovea (because the retinal image is inverted, this corresponds to a location in the visual field to the observer's right), and the South pole
would correspond to the centre of the pupil.
The meridian of the visual field has been found to influence 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.
Perimetry
Perimetry or campimetry is the systematic measurement of differential light sensitivity in the visual field by the detection of the presence of test targets on a defined background. Visual field testing can be performed clinically with confrontational field testing keeping the subject's gaze fixed...
is the measurement of [an observer's] visual functions ... at topographically defined loci in the visual field. 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...
is that portion of the external environment of the observer [in which when he or she is] steadily fixating ...[he or she] can detect visual stimuli."
In perimetry, the observer's eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
is considered to be at the centre of an imaginary sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
. More precisely, the centre of the sphere is in the centre of the pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
of the observer's eye. The observer is looking at a point, the fixation point, on the interior of the sphere. 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...
can be considered to be all parts of the sphere for which the observer can see a particular test stimulus. If we consider this surface to be that on which an observer can see anything, then it is a section of the sphere somewhat larger than a hemisphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
. In reality it is smaller than this, and irregular, because when the observer is looking straight ahead, his or her nose blocks vision of some possible parts of the surface. In perimetric testing, a section of the imaginary sphere is realized as a hemisphere in the centre of which is a fixation point. Test stimuli can be displayed on the hemisphere.
To specify loci in the visual field, a polar coordinate system is used, all expressed from the observer's perspective. The origin
Origin (mathematics)
In mathematics, the origin of a Euclidean space is a special point, usually denoted by the letter O, used as a fixed point of reference for the geometry of the surrounding space. In a Cartesian coordinate system, the origin is the point where the axes of the system intersect...
corresponds to the point on which the observer is fixating. The polar angle
Polar angle
In geometry, the polar angle may be* one of the two coordinates of a two-dimensional polar coordinate system;* one of the three coordinates of a three-dimensional spherical coordinate system; in this case it is also called the zenith....
is considered to be zero degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s when a locus is horizontally to the right of the fixation point and to increase to a maximum of 360 degrees going anticlockwise. Distance from the origin is given in degrees of visual angle
Visual angle
The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.It also is called the object's angular size....
; it's a measure of eccentricity
Eccentricity (mathematics)
In mathematics, the eccentricity, denoted e or \varepsilon, is a parameter associated with every conic section. It can be thought of as a measure of how much the conic section deviates from being circular.In particular,...
. Each polar axis is a meridian of the visual field. For example, the horizontal meridian runs from the observer's left, through the fixation point, and to the observer's right. The vertical meridian runs from above the observer's line of sight, through the fixation point, and to below the observer's line of sight.
Another way of thinking of the maximum visual field is to think of all 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...
that can be reached by light from the external environment. The visual field in this case is all of the external environment that can project light onto the retina. Meridians correspond to sections of great circles passing though the centre of 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....
. In an analogy to Meridian (geography)
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...
, in which meridians are lines of longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
, the North pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
might correspond to the fovea, Greenwich would correspond to a retinal location about 39 degrees to the left of the fovea (because the retinal image is inverted, this corresponds to a location in the visual field to the observer's right), and the South pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
would correspond to the centre of the pupil.
The meridian of the visual field has been found to influence 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 of their visual field tends to be represented on the cerebral cortex's convex gyri
Gyrus
A gyrus is a ridge on the cerebral cortex. It is generally surrounded by one or more sulci .-Notable gyri:* Superior frontal gyrus, lat. gyrus frontalis superior* Middle frontal gyrus, lat. gyrus frontalis medius...
folds whereas the horizontal meridian is tends to be represented in their concave sulci
Sulcus (neuroanatomy)
In neuroanatomy, a sulcus is a depression or fissure in the surface of the brain.It surrounds the gyri, creating the characteristic appearance of the brain in humans and other large mammals....
folds.