447
CHAPTER TWELVE
Nervous System III
“The song was full of glittering orange
diamonds.”
“The paint smelled blue.”
“The sunset was salty.”
“The pickle tasted like a rectangle.”
One in 1,000 people has a condition called
synesthesia, in which the brain interprets a stimu-
lus to one sense as coming from another. Most
common is
grapheme-color type synesthesia,
in
which letters, numbers, or time evoke specific
colors, or music may evoke perception of a strong
taste or odor (
lexical-gustatory synesthesia
). These
associations are involuntary, are specific, and
persist over a lifetime. For example, a person
might report that three is always mustard yellow,
Thursday brown, or a symphony bittersweet.
Most synesthesia is present for as long as
a person can remember, but it may develop fol-
lowing brain damage. One woman began to feel
touch sensations in response to certain sounds
after su±
ering a stroke. Brain imaging has traced
the formation of unusual connections between
her midbrain and the tactile part of her cerebral
cortex.
Synesthesia seems to be inherited and is
more common in women. One of the authors of
this book (R. L.) has it—to her, days of the week
and months are speci²
c colors. Synesthesia has
been attributed to an immature nervous system
that cannot sort out sensory stimuli or to altered
brain circuitry that routes stimuli to the wrong
part of the cerebral cortex. It may be a common
inability in infants, but tends to be lost over time.
Positron emission tomography (PET) scan-
ning reveals a physical basis to synesthesia. Brain
scans of six nonsynesthetes were compared with
those of six synesthetes who reported associat-
ing words with colors. Cortical blood flow was
monitored while a list of words was read aloud
to both groups. Interestingly, cortical blood ³
ow
was greatly elevated in the synesthetes com-
pared with the nonsynesthetes. Furthermore,
while blood ³
ow was increased in word-process-
ing areas for both groups, the scans revealed that
areas important in vision and color processing
were also lit up in those with synesthesia.
12.2
CLINICAL APPLICATION
Mixed-Up Senses—Synesthesia
the f rst cranial nerves) synapse with neurons located in the
enlargements oF the
olfactory bulbs,
structures that lie on
either side oF the crista galli oF the ethmoid bone (see f
gs.
7.24 and 12.5).
In the olFactory bulbs, the sensory impulses are ana-
lyzed, and as a result, additional impulses travel along the
olfactory tracts
to portions oF the limbic system (see chapter
11, p. 407), a brain center For memory and emotions. This is
why we may become nostalgic over a scent From the past. A
whiFF oF the perFume that grandma used to wear may bring
back a fl
ood oF memories. The input to the limbic system also
to sensory stimuli. Membrane receptors are molecules such
as proteins and glycoproteins on the cell membranes. They
allow cells, such as neurons and olFactory receptor cells, to
respond to specif
c molecules. Thus, the olFactory receptors
are cells that respond to chemical stimuli, but they require
cell membrane receptors to do so.
Olfactory Nerve Pathways
Once olFactory receptor cells are stimulated, nerve impulses
travel along their axons through tiny openings in the cribri-
Form plates oF the ethmoid bone. These f
bers (which Form
Olfactory
bulb
Olfactory
tract
Olfactory area of
nasal cavity
Nasal cavity
Superior nasal
concha
Nerve fibers within
the olfactory bulb
Cribriform
plate
Cribriform
plate
Columnar
epithelial cells
Olfactory
receptor cells
Cilia
(b)
(a)
FIGURE 12.5
Olfactory receptors. (
a
) Columnar epithelial cells support olfactory receptor cells, which have cilia at their distal ends.
(
b
) The olfactory area is associated with the superior nasal concha.
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