- 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 objects and light sources in the external world that impinge the retina". In other words, field of view is everything that (at a given time) causes light to fall onto the retina. This input is processed by the visual system, which computes the visual field as the output.
The term is often used in optometry and ophthalmology, where a visual field test is used to determine whether the visual field is affected by diseases that cause local scotoma or a more extensive loss of vision or a reduction in sensitivity (threshold).
The normal human visual field extends to approximately 60 degrees nasally (toward the nose, or inward) from the vertical meridian in each eye, to 100 degrees temporally (away from the nose, or outwards) from the vertical meridian, and approximately 60 degrees above and 75 below the horizontal meridian. In the United Kingdom, the minimum field requirement for driving is 60 degrees either side of the vertical meridian, and 20 degrees above and below horizontal. The macula corresponds to the central 13 degrees of the visual field; the fovea to the central 3 degrees.
Measuring the visual field
The visual field is measured by perimetry. This may be kinetic, where points of light are moved inwards until the observer sees them, or static, where points of light are flashed onto a white screen and the observer is asked to press a button if he or she sees it. The most common perimeter used is the automated Humphrey Field Analyzer.
Another method is to use a campimeter, a small device designed to measure the visual field.
Patterns testing the central 24 degrees or 30 degrees of the visual field, are most commonly used. Most perimeters are also capable of testing the full field of vision.
Visual field loss
- Altitudinal field defects, loss of vision above or below the horizontal – associated with ocular abnormalities
- Bitemporal hemianopia, loss of vision at the sides (see below)
- Central scotoma, loss of central vision
- Homonymous hemianopia, loss at one side in both eyes – defect behind optic chiasm (see below)
In humans, confrontational testing and other forms of perimetry are used to detect and measure visual field loss. Different neurological difficulties cause characteristic forms of visual disturbances, including hemianopsias (shown below without macular sparing), quadrantanopsia, and others.
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