By Luisa Mayer Ph.D. Children's Hospital Boston What is the Visual Field? The visual field is the area of space in which all objects are visible simultaneously (see Bird's Eye View).
In common terms, the visual field is the area of peripheral or side vision, although the object that is being fixated is also within the visual field. The visual field is shown as an area drawn on a flat surface, with the boundary of the area being the farthest position that the person can detect a given object in any direction (EX binocular plot). However, the "true" visual field is actually a 3 dimensional space, conceptualized as a half-sphere with an equal radius from eye to any point on the surface of the sphere. This causes certain distortions of scale just like the distortions caused by projections of geography on maps.
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Another point: Notice as you look directly at the red "@" in the following example: images of the words to the right left and above are visible, although you cannot read them clearly unless they are quite close to the “@” symbol (or are larger then the “@”) This demonstrates that there is a fall off in visual acuity with more eccentric positions in the visual field. Visual acuity is sharpest near the fixation point and gradually becomes poorer the farther one tests in the periphery.
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The visual fields of the right and left eyes have a larger extent toward the temporal side (ear side of the eye's field) than the nasal side (EX monocular fields). The right and left eye fileds overlap out to about 60 degrees on either side of fixation, creating the field of single vision or "binocular vision." (EX Binocular with overlap areas shown) |
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Why is the visual field tested? Diseases or disorders of the visual pathway from eye to brain can cause visual field defects. A test of the visual field provides information relevant to diagnosis of eye and brain disorders. The visual field results can be used to monitor treatment for eye and brain disorders. Even when the diagnosis is known and the condition is stable without treatment, the visual field informs us of important effects on vision that are relevant to everyday functioning. |
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What types of visual field defects occur in different eye and brain disorders? To understand the types of visual field defects that occur one must know a little something about the projection of visual space on the pathway. Visual space is mapped in characteristic patterns at different levels of the visual pathway. This means that a specific visual field defect can reflect disease or damage to specific portion of the visual pathway. The visual fields of the right and left eyes are seperately projected; so that a disease of one eye only causes a field defect in that eye only. In some genetic conditions, such as congenital retinal degeneration however, the visual fields of both eyes are affected similarly. Visual field defects affecting the optic nerves and chiasm may produce very different field defects in right and left eyes. At the optic chiasm, about half the optic fibers from each eye project to the right and left cerebral hemispheres in a strictly topographic pattern. The fibers from the nasal half of the retina (temporal field of that eye) cross to the opposite hemisphere, while fibers from the temporal half of the retina (nasal field of that eye) project to the hemisphere on the side of the eye. Thus, beyond the optic chiasm, the projection of the visual field is split into right and left halves that are represented in the visual pathway of the left and right hemispheres, respectively. |
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So a lesion that damages all the visual fibers in the right hemisphere would cause a complete loss of the left field (to the left of the vertical). And the opposite would occur for a lesion of the left hemisphere, the right field would be blind. The following example illustrates hemispherical lesions:
For more detailed illustrations of visual field defects: Click here for the post chiasmal field defects simulation Click here for other visual field defects For more information about visual acuity and contrast sensitivity click here |
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