Chapter 15
The Special Senses
565
15
Te “where” processing stream takes a dorsal path through
the parietal cortex all the way to the postcentral gyrus and
uses information from the primary visual cortex to assess the
location of objects in space.
Output from both these regions then passes to the frontal
cortex, which uses that information to direct activities that,
among other things, can guide movements such as reaching for
a juicy peach.
Check Your Understanding
9.
Which part of the visual field would be affected by a tumor
in the right visual cortex? By a tumor compressing the right
optic nerve?
For answers, see Appendix H.
The Chemical Senses:
Smell and Taste
Describe the location, structure, and afferent pathways of
smell and taste receptors, and explain how these receptors
are activated.
Smell and taste are gritty, primitive senses that alert us to
whether that “stuff” nearby (or in our mouth) is to be savored or
avoided. Te receptors for smell (olfaction) and taste (gustation)
are
chemoreceptors
(they respond to chemicals in an aqueous
solution). Tey complement each other and respond to differ-
ent classes of chemicals. Smell receptors are excited by airborne
chemicals that dissolve in fluids coating nasal membranes, and
taste receptors are excited by food chemicals dissolved in saliva.
Olfactory Epithelium and the Sense
of Smell
Although our olfactory sense (
olfact
5
to smell) is far less acute
than that of many other animals, the human nose is still no
slouch in picking up small differences in odors. Some people
capitalize on this ability by becoming wine tasters.
Location and Structure of Olfactory Receptors
Olfaction detects chemicals (
odorants
) in solution. Te organ
of smell is a yellow-tinged patch (about 5 cm
2
) of pseudostrati-
fied epithelium, called the
olfactory epithelium
, located in the
roof of the nasal cavity
(Figure 15.20a)
. Air entering the nasal
cavity must make a hairpin turn to stimulate olfactory receptors
before entering the respiratory passageway below, so the human
olfactory epithelium is in a poor position for doing its job. (Tis
is why sniffing, which draws more air superiorly across the ol-
factory epithelium, intensifies the sense of smell.)
Te olfactory epithelium covers the superior nasal concha
on each side of the nasal septum, and contains millions of bowl-
ing pin–shaped receptor cells—the
olfactory sensory neurons
.
Tese are surrounded and cushioned by columnar
support-
ing cells
, which make up the bulk of the penny-thin epithelial
(for example, nearer objects appear larger, and parallel lines
converge with distance).
Homeostatic Imbalance
15.11
Te visual system’s pathways explain patterns of blindness that
follow damage to different visual structures. Loss of an eye or
destruction of one optic nerve eliminates true depth percep-
tion entirely, and peripheral vision on the damaged side. For
example, if the “le± eye” in Figure 15.19 was lost in a hunting
accident, nothing would be seen in the visual field area that is
stippled yellow in that figure.
If neural destruction occurs beyond the optic chiasma—in
an optic tract, the thalamus, or visual cortex—then part or all of
the opposite half of the visual field is lost. For example, a stroke
affecting the le± visual cortex leads to blindness in the right half
of the visual field. However, since the right (undamaged) visual
cortex still receives inputs from both eyes, depth perception in
the remaining half of the visual field is retained.
Visual Processing
How does information received by rods and cones become vi-
sion? Visual processing begins in the retina.
Retinal cells simplify and condense the information from rods
and cones, splitting it into a number of different “channels,” each
with its own type of ganglion cell. Tese “channels” include infor-
mation about color and brightness, but also about more complex
aspects of what we see—the angle, direction and speed of move-
ment of
edges
(sudden changes in brightness or color). Extracting
information about edges depends on a type of processing called
lateral inhibition
, a kind of contrast enhancement, which is the
job of the amacrine and horizontal cells mentioned on p. 551.
Te ganglion cells pass the processed information to the lat-
eral geniculate nuclei of the thalamus. Tere, the retinal axons
from each visual field of the two eyes are combined in prepara-
tion for depth perception, input from cones is emphasized, and
contrast is further sharpened.
Te
primary visual cortex
, also called the
striate cortex
,
receives the thick bundle of fibers coming in from the lateral
geniculate nucleus. Tis area contains an accurate topographi-
cal map of the retina, with the le± visual cortex receiving input
from the right visual field and vice versa. Visual processing here
occurs at a relatively basic level, with the processing neurons
responding to dark and bright edges (contrast information) and
object orientation.
Te striate cortex also provides form, color, and motion in-
puts to
visual association areas
collectively called the
prestriate
cortices
. Te more anterior prestriate cortices are occipital lobe
centers that continue processing visual information concerned
with form, color, and movement.
Functional neuroimaging of humans has revealed that com-
plex visual processing extends well forward into the temporal,
parietal, and frontal lobes via two parallel streams:
Te “what” processing stream extends through the ventral
part of the temporal lobe and specializes in identifying ob-
jects in the visual field.
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