Our Sense of Smell

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The Chemical Senses: Taste and Smell
1991). Many animals easily learn taste aversions to particular foods when the taste is
associated with nausea, but humans learn aversions to odors more readily than to tastes
(Bartoshuk & Wolfe, 1990).
Variations in the state of our nutrition also affect our experience of taste and flavor,
as well as our motivation to eat particular foods. Food deprivation or salt deficiency
makes sweet or salty things taste better. Influences on protein and fat intake are less
direct. Protein and fat molecules have no particular taste or smell. So preferring or
avoiding foods that contain these nutrients is based on associations between scent cues
from other volatile substances in food and on the nutritional results of eating the foods
(Bartoshuk, 1991; Schiffman et al., 1999).
We experience warm foods as sweeter, but temperature does not alter our experience of saltiness (Cruz & Green, 2000). Warming releases aromas that rise from
the mouth into the nose and create more flavor sensations. This is why some people find hot pizza delicious and cold pizza disgusting. Spicy “hot” foods actually
stimulate pain fibers in the mouth because they contain a substance called capsaicin
(pronounced “kap-SAY-uh-sin”), which stimulates pain sensing neurons that are also
stimulated by heat.
Our Sense of Smell
Pinching your nose prevents you from smelling odors, and a dilator strip on the bridge
of your nose opens your nasal passages and intensifies odors (Raudenbush & Meyer,
2002). These effects occur because the nose (and the mouth, to some extent) acts as an
accessory structure that collects airborne odor molecules for coding and analysis by
the olfactory system (see Figure 3.16). As odor molecules pass into the moist lining of the
upper part of the nose—called the mucous membrane—they bind to receptors on the
dendrites of olfactory neurons, causing a biochemical change. This change, in turn,
leads to changes in the firing rates of these neurons, whose axons combine to form the
olfactory nerve (Dionne & Dubin, 1994). It takes only a single molecule of an odorous
substance to cause a change in the activity of an olfactory neuron, but detection of the
odor by a human requires about fifty such molecules (Menini, Picco, & Firestein, 1995).
The number of molecules needed to trigger an olfactory sensation can vary, however.
FIGURE
3.16
Olfactory bulb
The Olfactory System: The Nose
and the Rose
Airborne odor molecules reach the olfactory area either through the nose or
through an opening in the palate at the
back of the mouth. This opening allows us
to sample odors from our food as we eat.
Nerve fibers pass directly from the olfactory area to the olfactory bulb in the
brain, and from there signals pass to areas
that are involved in emotion. This arrangement helps explain why odors often
trigger strong emotional memories.
Receptor
cells
Olfactory bulb
Olfactory area
Tongue
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olfactory bulb A brain structure that
receives messages regarding smell.
pheromones Chemicals that are released by one creature and detected by
another, shaping the second one’s behavior or physiology.
Chapter 3 Sensation and Perception
For example, women are more sensitive to odors during certain phases of their menstrual cycles (Navarrete-Palacios et al., 2003).
There are thousands of different receptors for odors, but there are even more possible odors in the world. Any particular odor is sensed as a particular pattern of responses
by these odorant receptors (Kajiya et al., 2001; Zou & Buck, 2006). So a rose, a pizza,
and your favorite cologne each have a different smell because they stimulate their own
unique patterns of activity in your odorant receptors. The question of how smells are
coded has been of special interest since the September 11, 2001, terrorist attacks in the
United States. Researchers have intensified their efforts to develop an “electronic nose”
capable of detecting odorants associated with guns and explosives (Thaler, Kennedy, &
Hanson, 2001). Versions of these devices are already in use at some airports.
Unlike other senses, our sense of smell does not send its messages through the thalamus. Instead, axons from olfactory neurons in the nose extend through a bony plate
and directly into the brain, where they have a synapse in a structure called the olfactory bulb. Connections from the olfactory bulb spread throughout the brain (Zou, Li,
& Buck, 2005), but they are especially plentiful in the amygdala, a part of the brain
involved in emotional experience and learning. In humans, the amygdala is especially
active in response to disgusting odors (Zald & Pardo, 1997).
The unique anatomy of the olfactory system may help account for the unique relationship between smells and emotion (Stevenson & Boakes, 2003). Associations
between particular odors and experiences—especially emotional experiences—are not
weakened much by time or later experiences (Lawless & Engen, 1977). So catching a
whiff of the cologne once worn by a lost loved one can reactivate intense feelings of
love or sadness associated with that person. Odors can also bring back accurate memories of experiences linked with them, especially positive experiences (Engen, Gilmore,
& Mair, 1991; Mohr et al., 2001).
Species ranging from humans to worms have remarkably similar neural mechanisms
for sensing smell. And all mammals, including humans, have brain systems for detecting the source of smells by comparing the strength of sensory inputs reaching the left
and right nostrils (Porter et al., 2005). Different species vary considerably, however, in
their sensitivity to odor and in the degree to which they depend on it for survival.
Humans have about 9 million olfactory neurons, compared with about 225 million in
dogs, a species that is far more dependent on smell to identify food, territory, and receptive mates. Dogs and many other species also have an accessory olfactory system that
detects pheromones. Pheromones (pronounced “FAIR-oh-mohns”) are chemicals that,
when released by one creature and detected by another, can shape the second animal’s
behavior or physiology (Silvotti, Montanu, & Tirindelli, 2003). For example, when a
male snake detects a chemical on the skin of a female snake, it is stimulated to “court”
the female.
The role of pheromones in humans is much less clear, but it appears that we do have
some sort of pheromone-like system (Berglund, Lindström, Savic, 2006; Savic,
Berglund, & Lindström, 2005). A possible human gene for pheromone receptors has
been found (Rodriguez et al., 2000), and pheromones have been shown to cause reproduction-related physiological changes in humans (Grammer, Fink, & Neave, 2005).
Specifically, pheromonal signals secreted in women’s perspiration can influence nearby
women’s menstrual cycles. As a result, women living together eventually tend to menstruate at about the same time (Stern & McClintock, 1998). Furthermore, odorants that
cannot be consciously detected can nevertheless influence mood and stimulate activity
in nonolfactory areas of the brain (Jacob & McClintock, 2000; Jacob et al., 2001; Savic
et al., 2001).
Despite steamy ads for cologne and perfume, however, there is not yet any solid
evidence that humans give off or can detect pheromones that act as sexual attractants.
If a certain scent does enhance a person’s readiness for sex, it is probably because the
person has learned to associate that scent with previous sexual experiences. There are
many other examples of people using olfactory information in social situations. For
instance, after just a few hours of contact with their newborn babies, mothers can usually identify them by the infants’ smell (Porter, Cernich, & McLaughlin, 1983). And if