This article is about the
.
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
In perimetry, the observer's
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 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
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
References
IPS Perimetry Standards 1978. (2002). Author. Department of Ophthalmology & Visual Sciences, The University of Iowa. Retrieved March 4, 2005, from http://webeye.ophth.uiowa.edu/ips/GEN-INFO/standards/STANDARD.HTM
