Page 628 - Fundamentals of anatomy physiology

nerves, one from each eye, reach the diencephalon at the optic Chapter 17 The Special Senses 615
chiasm (Figure 17–20). From that point, approximately half
the fibers proceed toward the lateral geniculate nucleus of the The bundle of projection fibers linking the lateral geniculates
same side of the brain, whereas the other half cross over to reach with the visual cortex is known as the optic radiation. Col-
the lateral geniculate nucleus of the opposite side. p. 501 laterals from the fibers synapsing in the lateral geniculate con-
From each lateral geniculate nucleus, visual information travels tinue to subconscious processing centers in the diencephalon
to the occipital cortex of the cerebral hemisphere on that side. and brain stem. For example, the pupillary reflexes and reflexes
that control eye movement are triggered by collaterals carrying
Figure 17–20 The Visual Pathways. The crossover of some information to the superior colliculi.

nerve fibers occurs at the optic chiasm. As a result, each hemisphere The Field of Vision. You perceive a visual image due to the
receives visual information from the medial half of the field of vision integration of information that arrives at the visual cortex of
of the eye on that side, and from the lateral half of the field of vision of the occipital lobes. Each eye receives a slightly different visual
the eye on the opposite side. Visual association areas integrate this in- image. One reason is that the foveae in your two eyes are 5–7.5
formation to develop a composite picture of the entire field of vision. cm (2–3.0 in.) apart. Another reason is that your nose and eye
socket block the view of the opposite side. Depth perception
Combined Visual Field is the ability to judge depth or distance by interpreting the
three-dimensional relationships among objects in view. Your
Left side Right side brain obtains it by comparing the relative positions of objects
within the images received by your two eyes.
Left eye Right eye
only Binocular vision only When you look straight ahead, the visual images from your
left and right eyes overlap (Figure 17–20). The image received by
The Visual Retina
Pathway Optic disc 17the fovea of each eye lies in the center of the region of overlap.
Photoreceptors
Diencephalon Suprachiasmatic A vertical line drawn through the center of this region marks
in retina and nucleus the division of visual information at the optic chiasm. Visual
information from the left half of the combined field of vision
Optic nerve brain stem reaches the visual cortex of your right occipital lobe. Visual
(II) information from the right half of the combined field of vision
Superior arrives at the visual cortex of your left occipital lobe.
Optic chiasm colliculus
The cerebral hemispheres thus contain a map of the entire
Optic tract field of vision. As in the case of the primary sensory cortex, the
map does not faithfully duplicate the areas within the sensory
Lateral field. For example, the area assigned to the macula and fovea cov-
geniculate ers about 35 times the surface it would cover if the map were pro-
portionally accurate. The map is also upside down and reversed,
nucleus duplicating the orientation of the visual image at the retina.
Projection fibers
(optic radiation) The Brain Stem and Visual Processing. Many centers in the
brain stem receive visual information, either from the lateral
Visual cortex geniculate nuclei or through collaterals from the optic tracts.
of cerebral Collaterals that bypass the lateral geniculates synapse in the su-
perior colliculi or in the hypothalamus. The superior colliculi of
hemispheres the midbrain issue motor commands that control u­ nconscious
eye, head, or neck movements in response to visual stimuli.
Left cerebral Right cerebral
hemisphere hemisphere Visual inputs to the suprachiasmatic nucleus of the hy-
pothalamus affect the function of other brain stem nuclei.

p. 506 The suprachiasmatic nucleus and the pineal gland of
the epithalamus receive visual information and use it to estab-
lish a circadian rhythm (circa, about + dies, day), which is a
daily pattern of visceral activity that is tied to the day–night
cycle. This circadian rhythm affects your metabolic rate, endo-
crine function, blood pressure, digestive activities, sleep–wake
cycle, and other physiological and behavioral processes.