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Visual field defect

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Visual field defect

The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments"[1]

Visual field is sometimes confused with a field of view. The 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.

In optometry and ophthalmology 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 even a reduction in sensitivity (threshold).

Normal limits

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 and Heidelberg Edge Perimeter.

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.

Another method is for the practitioner to hold up 1,2, or 5 fingers in the four quadrants and center of a patient's visual field (with the other eye covered). If the patient is able to report the number of fingers properly as compared with the visual field of the practitioner, the normal result is recorded as "full to count fingers" (often abbreviated FTCF). The blind spot can also be assessed via holding a small red object between the practitioner and the patient. By comparing when the red object disappears for the practitioner, a patient's abnormally large blind spot can be identified. There are many variants of this type of exam (e.g. wiggling fingers at visual periphery in cardinal axes).

Visual field loss

Visual field loss may occur due to disease or disorders of the eye, optic nerve, or brain. Classically, there are four types of visual field defects:[2]

  1. Altitudinal field defects, loss of vision above or below the horizontal – associated with ocular abnormalities
  2. Bitemporal hemianopia, loss of vision at the sides (see below)
  3. Central scotoma, loss of central vision
  4. 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.






The good vision island

The visual field is uneven. It is not uniform and has its maximum definition in its central part.

Harry Moss Traquair describes in 1927 that our visual field as "an island of vision or hill of vision surrounded by a sea of blindness".[3] The 'island of vision' corresponds to a sudden change of definition we have. Its empirical elliptical limits in the longest (horizontal) axis, are our Blind Spots.

Kim Lloveras i Montserrat, argues[4] that in the Romanesque age, people were aware of the particularities of our blinds spots as horizontal limits of the central good vision. Further, they know that more than a change in definition there is a strong change of perception of space. The observer feels inside the central area and understands their borders as 'their enveloping', making them a very helpful principle for architectural design. The exhibit La experiencia del Espacio Personal[5] at the Barcelona School of Architecture explored this phenomenon, using a large ellipse (TK), whch is 3.10 meters tall (twice the height of a person) and has a major axis of 3.94, located 6.38 m from the observer (these dimensions are very similar to those proposed by Traquair for the limits of the "visual island").

See also

References

External links

  • Visual Field
  • Patient Plus
  • Quadrantanopsia Visual Fields Teaching Case from MedPix
  • Free visual field test
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