564
UNIT 3
Regulation and Integration of the Body
15
slightly different view (Figure 15.19a). Te visual cortex fuses
the slightly different images delivered by the two eyes, providing
us with
depth perception
(or
three-dimensional vision
), an
accurate means of locating objects in space.
In contrast, many animals (pigeons, rabbits, and others) have
panoramic vision
. Teir eyes are placed more laterally on the
head, so that the visual fields overlap very little, and crossover
of the optic nerve fibers is almost complete. Consequently, each
visual cortex receives input principally from a single eye and a
totally different visual field.
Depth perception depends on the two eyes working together.
If only one eye is used, depth perception is lost, and the person
must learn to judge an object’s position based on learned cues
visual reflex centers controlling the extrinsic muscles of the eyes.
Another set comes from a small subset of ganglion cells in the
retina that contain the visual pigment
melanopsin
, dubbed the
circadian pigment. Tese ganglion cells respond directly to light
stimuli and their fibers project to the
pretectal nuclei
, which
mediate pupillary light reflexes, and to the
suprachiasmatic nu-
cleus
of the hypothalamus, which functions as the “timer” to set
our daily biorhythms.
Depth Perception
Notice that both eyes are set anteriorly and look in approxi-
mately the same direction. Teir visual fields, each about 170
degrees, overlap to a considerable extent, and each eye sees a
L
e
f
t
e
y
e
o
n
l
y
Both e
yes
R
i
g
h
t
e
y
e
o
n
l
y
Pretectal
nucleus
Right eye
Left eye
Fixation point
Optic
radiation
Superior
colliculus
(sectioned)
Lateral
geniculate
nucleus
Optic tract
Optic chiasma
Uncrossed
(ipsilateral) fiber
Crossed
(contralateral) fiber
Optic nerve
Lateral
geniculate
nucleus of
thalamus
Superior
colliculus
Occipital
lobe
(primary visual
cortex)
(a) The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field
do not cross at the optic chiasma.
(b) Photograph of human brain, with the right side
dissected to reveal internal structures.
Corpus callosum
Supra-
chiasmatic
nucleus
Figure 15.19
Visual pathway to the brain and visual fields, inferior view.
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