The area within which objects can be seen
by the eye
without moving the head
or the eye. Each eye has its own visual field. Human vision
is normally best
at the center of the visual field, because images from that region fall on the fovea
an area of the retina
that has the highest concentration of light receptors
and the highest resolution. We generally also have some degree of peripheral vision
although objects in the periphery are seen in much less detail.
Stroke, head injury, and diseases of the eye can change the visual field.
For example, brain injury caused by stroke or trauma can result in hemianopsia
(blindness in half the visual field). (Changes to the visual field resulting from
brain injury don't reflect damage to the eye, but rather are due to changes in the
brian's ability to process visual information.) Glaucoma and retinitis pigmentosa
are eye diseases that can
cause loss of part of the visual field. In extreme cases, all peripheral vision
can be lost, resulting in tunnel vision.
Ophthalmologists will perform a rough test of the visual field by asking you to focus
on something directly ahead while they move their hand in from the sides, top, and
bottom, and ask you to say when you first see their hand. Fancier tests can be performed
using special equipment. In the automated perimetry exam, you sit facing a white
hemispherical screen while a computer flashes small points of light on the screen,
and push a button anytime you see one. This test results in a map of the visual
field, showing any holes or loss at the edges.