Page 336 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Visual Pathway and Processing of Visual Information
The information from both eyes is transmitted (→ B4b). Yet if there is a loss of sympathetic
to the visual cortex via the visual pathway stimulation, the pupil is also constricted in
(→ A). On each side the visual tracts cross the dark (→ B5); under massive sympathetic
over in the optic chiasm from the nasal half of stimulation it is dilated even under the influ-
the retina, while the nerves from the temporal ence of light (→ B6). If the lesion is in the re-
Systems sides pass on without crossing over. After syn- gion of the pretectal area, the pupils remain di-
apting in the lateral geniculate body of the
lated even under the influence of light, but
thalamus, the information reaches the primary
they are constricted by near-response (light-
near dissociation; → B7a,b).
visual cortex in the occipital lobe. A lesion in
Neuromuscular and Sensory causes a deficit in the nasal half of this eye’s vi- sults in an inability consciously to perceive vi-
Loss of the primary visual cortex (→ C) re-
the temporal part of the retina of the left eye
sual stimuli, even though the retina, thalamus,
sual field (→ A1). If the optic nerve of the left
eye is interrupted, the entire visual field of
and subcortical visual centers are intact and,
for example, pupillary reflexes are maintained
this eye is lost (amaurosis; → A2). Interruption
of the pathway in the optic chiasm especially
(cortical blindness). The phenomenon of blind
sight is caused by lesions in the visual cortex:
affects the crossing fibers, the consequence
the person can point at the source of the local-
being that the lateral portion of the visual field
“blinker blindness”; → A3). Complete lesion
flash of light. The ability depends on connec-
of the optic tract on the left results in loss of
tions between the subcortical visual centers
10 is lost in both eyes (bitemporal hemianopsia, ized light flash without being conscious of the
the right half of the visual field in both eyes
and the somatomotor areas.
(homonymous hemianopsia; → A4). Homony- If there are lesions in the occipitotemporal
mous anopsia also results from destruction of association fields, neither objects (object agno-
the lateral geniculate body. Interruptions in sia), faces and facial expressions (prosopag-
the optical radiation (e.g., upper and lower nosia), nor colors (achromatopsia) can be rec-
quadrant anopsia; → A5,6) and in the primary ognized.
visual cortex (→ A7; see below) lead to further Lesions in the occipitotemporal association
characteristic visual field deficits, depending fields can, in addition, lead to hemineglect, a
on their localization. condition in which perceptions from one half
Pupillary reflex. The afferent fibers from the of a room or the body are ignored. It is more
retina serve not only the flow of visual infor- marked with lesions of the right hemisphere
mation to the visual cortex, but also to pro- (ignoring objects on the left hand side) than
mote the contraction of the pupillary sphincter those of the left hemisphere, because the right
via the pretectal area of the mid-brain and the hemisphere is dominant in spatial orientation.
oculomotor nerve (acetylcholine). Conversely, In addition, such patients are often incapable
the pupils are widened by contraction of the of perceiving the movement of objects (akine-
pupillary dilator muscles stimulated by sym- topsia).
pathetic fibers (→ B1). When light is shone With lesions in the visual association fields,
into one eye, not only is the pupil of this eye faulty spatial and three-dimensional percep-
constricted (direct reaction), but also that of tion also often occurs, objects being perceived
the other eye (consensual reaction; → B2). If as distorted (dysmorphopsia, metamorphop-
one eye is blind, both pupils remain dilated sia), as too small (micropsia), or too large (mac-
when light is shone into the blind eye ropsia). Other lesions cause asynthesia (inabil-
(→ B3b). However, when light is shone into ity to combine different properties of one ob-
the healthy eye, the pupil of the blind eye con- ject).
stricts consensually (→ B3b). If the patient has If the connection from the visual cortex to
a unilateral lesion of the oculomotor nerve area 39 is interrupted (→ p. 345), the patient
(→ B4a), the pupil of the diseased eye remains is no longer able to read (alexia).
326 dilated to light, but there is consensual con-
traction of the pupil of the healthy eye
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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