Meridian (plural: "meridians") 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 degrees 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 through 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 macaques and humans 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.[1]
^Rajimehr, R.; Tootell, R. B. H. (2009). "Does retinotopy influence cortical folding in primate visual cortex?". J. Neurosci. 29 (36): 11149–52. doi:10.1523/JNEUROSCI.1835-09.2009. PMC2785715. PMID 19741121.
meridian, perimetry, visual, field, this, article, about, perimetry, concept, other, uses, meridian, disambiguation, meridian, plural, meridians, used, perimetry, specifying, visual, fields, according, perimetry, standards, 1978, 2002, perimetry, measurement, . This article is about the perimetry concept For other uses see Meridian disambiguation Meridian plural meridians 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 degrees 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 through 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 macaques and humans 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 1 See also EditAngle of view Field of view Peripheral visionReferences Edit Rajimehr R Tootell R B H 2009 Does retinotopy influence cortical folding in primate visual cortex J Neurosci 29 36 11149 52 doi 10 1523 JNEUROSCI 1835 09 2009 PMC 2785715 PMID 19741121 IPS Perimetry Standards 1978 2002 Author Department of Ophthalmology amp Visual Sciences The University of Iowa Retrieved March 4 2005 from http webeye ophth uiowa edu ips GEN INFO standards STANDARD HTM Retrieved from https en wikipedia org w index php title Meridian perimetry visual field amp oldid 839156383, wikipedia, wiki, book, books, library,
