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U N I T 1 0
Nervous System
The visual field refers to the area that is visible during
fixation of vision in one direction
16,22,24
(see Fig. 38-13).
As with a camera, the simple lens system of the eye
inverts the image of the external world on each retina. In
addition, the right and left sides of the visual field also are
reversed. Most of the visual field is binocular, with both
eyes focusing on one object and then fusing two images
into one. The right binocular visual field (the nasal half
of the right eye and the temporal half of the left eye)
is seen by the left retinal halves of each eye. Likewise,
the left binocular field is seen by the right retinal halves
of each eye. This binocular field is subdivided into
central and peripheral portions. Central portions of the
retina provide high visual acuity and correspond to the
field focused on the central fovea. The peripheral and
surrounding portion provides the capacity to detect
objects, particularly moving objects. Beyond the visual
field shared by both eyes, the left lateral periphery of the
visual field is seen exclusively by the left nasal retina, and
the right peripheral field by the right nasal retina.
Disorders of the Optic Pathways
Among the disorders that can interrupt the visual pathway
are trauma, tumors, and vascular lesions. Trauma and
tumors can produce direct injury or impinge on the optic
pathways. Vascular insufficiency in any one of the arterial
systems of the retina or visual pathways can seriously
affect vision. For example, normal visual function
depends on the adequacy of blood flow in the ophthalmic
artery and its branches—the central retinal artery; the
anterior and middle cerebral arteries, which supply the
intracranial optic nerve, chiasm, and optic tracts; and the
posterior cerebral artery, which supplies the LGN, optic
radiation, and visual cortex. The adequacy of posterior
cerebral artery function depends on that of the vertebral
and basilar arteries that supply the brain stem.
Visual Field Defects
Visual field defects result from damage to the visual
pathways or the visual cortex (see Fig. 38-13). The
testing of the visual fields of each eye and of the two eyes
together is useful in localizing lesions affecting the system.
Perimetry or visual field testing, in which the visual field
of each eye is measured and plotted in an arc, is used to
identify defects and determine the location of lesions.
Blindness in one eye is called
anopia.
If half of
the visual field for one eye is lost, the defect is called
hemianopia
; if a quarter of the field is lost, it is called
quadrantanopia.
Loss of the temporal or peripheral
visual fields on both sides results in a narrow binocular
field, commonly called
tunnel vision.
The loss of different
Frontal eye field
(part of 8)
Somatosensory
(3, 1, 2)
Second
somatosensory
Visual
(17)
(18)
(19)
Visual
association
Somatosensory
association (5, 7)
Auditory (41)
Auditory
association (42, 22)
FIGURE 38-14.
Lateral view of the cortex illustrating the
location of the visual, visual association, auditory, and auditory
association areas.
Lesion 1
Lesion 2
Lesion 3
Left
Right
Whitened field
no vision
Right optic nerve
Optic chiasm
Right optic tract
Left
eye
Right
eye
Temporal
Nasal
Temporal
Left visual
field
Right visual
field
Optic
nerve
Optic
tract
Lateral
geniculate
nucleus
Optic
radiation
1
3
2
FIGURE 38-13.
Diagram of optic pathways.
The red lines indicate the right visual field and
the blue lines the left visual field. Note the
crossing of fibers from the medial half of each
retina at the optic chiasm. Lesion 1 (right optic
nerve) produces unilateral blindness. Lesion
2 (optic chiasm) may involve only those fibers
that originate in the nasal half of each retina
and cross to the opposite side in the optic
chiasm; visual loss involves the temporal half
of each field (bi-temporal hemianopia). Lesion
3 (right optic tract) interrupts fibers (and vision)
originating on the same side of both eyes
(homonymous), with loss of vision from half of
each field (hemianopia).
