Sensation & Perception
Ch. 15: The Chemical Senses
© Takashi Yamauchi (Dept. of Psychology, Texas A&M
University)
Main topics
Olfactory system
odors
Neural codes
Neural receptors
Cortical structure
Taste
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Functions of Olfaction
• Many animals are macrosmatic
– having a keen sense of smell that is
necessary for survival
• Humans are microsmatic
– a less keen sense of smell that is not
crucial to survive
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Detecting Odors - continued
• Rats are 8 to 50 times more sensitive to
odors than humans
• Dogs are 300 to 10,000 times more
sensitive
• The difference lies in the number of
receptors they each have
– Humans have 10 million and dogs
have 1 billion olfactory receptors
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Detecting Odors
• Measuring the detection threshold
– Yes/no procedure - participants are given
trials with odors along with “blank” trials
• They respond by saying yes or no
– Forced-choice - two trials are given, one
with odorant and one without
• Participant indicates which smells
strongest
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Detecting Odors - continued
• Measuring the difference threshold
– Measure the smallest difference that can
be detected between two samples
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Table 15.1 Human odor detection thresholds
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Identifying Odors
• Humans can discriminate among 100,000
odors but they cannot label them accurately
– This appears to be caused by an inability
to retrieve the name from memory, from a
lack of sensitivity
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The Puzzle of Olfactory Quality
• Researchers have found it difficult to map
perceptual experience onto physical
attributes of odorants.
• Linking chemical structure to types of smells
• Some molecules with similar shapes had
very different smells
• Some similar smells came from
molecules with different shapes
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Structure of the Olfactory System
• Olfactory mucosa is located
at the top of the nasal cavity
glomeruli
– Odorants are carried
along the mucosa
coming in contact with
the sensory neurons
Sensory
neurons
– Cilia of these neurons
contain the receptors
receptors
– Humans have about 350
types of receptors.
– Signals are carried to the
glomeruli in the
olfactory bulb
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Structure of the Olfactory System continued
– Signals are sent to
• Primary olfactory
(piriform) cortex in
the temporal lobe
• Secondary
olfactory
(orbitofrontal)
cortex in the frontal
lobe
• Amygdala deep in
the cortex
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Activating Receptor Neurons
• Calcium imaging method
– Soak up the receptor neurons with a
chemical.
– This causes the receptors fluoresce with a
green glow when exposed to ultraviolelet
light.
– When the receptors are responding to
ordorants, they take up a lot of calium (Ca++)
inside the receptor.
– The increase in Ca++ decreases this
fluorescence.
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Activating Receptor Neurons - continued
• Combinatorial code
for odor
– Odorants are
coded by
combinations of
olfactory
receptors
– Specific receptors
may be part of the
code for multiple
odorants.
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Figure 15.6 Recognition profiles for some odorants. Large dots indicate that
the odorant causes a high firing rate for the receptor listed along the top;
small dots indicate lower firing rates for the receptor. The structures of the
compounds are shown on the right. (Adapted from Malnic et al., 1999.)
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Combinatorial coding?
• Remember distributed coding in face
representation?
• Representing color by S, M, L cones.
• Even language is combinatorial.
• Combining 26 alphabets  many many
words.
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Activating the Olfactory Bulb
• Olfactory mucosa is divided into 4 zones
– Each zone contains a variety of different receptors
– Specific types of receptors are found in only one zone
– Odorants tend to activate neurons within a particular
zone
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Activating the Olfactory Bulb - continued
• Optical imaging method
– Cortical cells consume oxygen when
activated
– Red light is used to determine the
amount of oxygen in the cells
– More oxygen reflects less red light
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Figure 15.9 Areas in the rat olfactory bulb that are activated by various
chemicals: (a) a series of carbolic acids: (b) a series of aliphatic alcohols.
(Uchida, N., Takahaski, Y. K., Tanifuji, M., & Mori, K. (2000). Odor maps in the
mammalian olfactory bulb: Domain organization and odorant structural
features. Nature Neuroscience, 3, 1035-1043.)
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Patterns of activation in the rat olfactory bulb (Linster, et
al., 2001)
• 2-deoxyglucose (2DG)
technique
– 2DG, which contains
glucose, is ingested into
an animal
– Animal is exposed to
different chemicals
– Neural activation is
measured by amount of
radioactivity present
• This technique shows the
pattern of neural activation is
Figure 15.10 These molecules have the same
related to both chemical
chemical formula, but the molecular group at the
structure and to perception
bottom is rotated to a different position.
The black arrows indicate that the two forms of
liminone activate similar areas in the olfactory
bulb.
The pattern of activation on the OB is related
to functional
groups and structures of the
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chemicals as well as their perceived odors
• Somewhat similar to
– Retinotopic map  V1
– Tonotopic map  A1
– Topological locations specify the characteristics
of stimuli (light, sounds, ordorants)
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Taste
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Functions of Taste
• Sweetness is usually associated with
substances that have nutritive value
• Bitter is usually associated with substances
that are potentially harmful
• Salty taste indicates the presence of sodium
• However, there is not a perfect connection
between tastes and function of substances
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Basic Taste Qualities
• Five basic taste qualities:
– Salty
– Sour
– Sweet
– Bitter
– Umami - described as meaty, brothy or
savory and associated with MSG
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Human Tongue (from wikipedia)
The surface of the tongue is
very bumpy: many ridges
and valleys.
This structure is called
Papillae (singular:
papilla)
There are 4 types of
papillae:
Filiform, Fungiform,
Circmvallate, foliate
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Tongue
circumvllate
• papillae:
– Filiform - shaped
like cones and
located over entire
surface
– Fungiform - shaped
like mushrooms and
found on sides and
tip
– Foliate - series of
folds on back and
sides
– Circumvallate shaped like flat
mounds in a trench
located at back
foliate
filiform
fungiform
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Structure of the Taste System - continued
• Taste buds are located in
papallae except for filiform
– Tongue contains
approximately 10,000
taste buds
– Each taste bud has taste
cells with tips that extend
into the taste pore
– Transduction occurs
when chemicals contact
the receptor sites on the
tips
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Table 15.2 Structures in the taste system
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Structure of the Taste System - continued
• Signals from taste cells travel along a set
of pathways:
– Chorda tympani nerve from front and
sides of tongue
– Glossopharyngeal nerve from back of
tongue
– Vagus nerve from mouth and throat
– Superficial petronasal nerve from soft
palate
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Structure of the Taste System - continued
• These pathways make
connections in the
nucleus of solitary tract
in the spinal cord
• Then they travel to the
thalamus
• Followed by areas in
the frontal lobe:
– Insula
– Frontal opervulum
cortex
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– Orbital frontal
cortex
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Neural Coding for Taste - continued
• Evidence exists for both specificity and
distributed coding
• Some researchers suggest that the neural
system for taste may function like the visual
system for color
• Currently there is no agreed upon explanation
for the neural system for taste
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The Perception of Flavor
• Combination of smell, taste, and other
sensations (such as burning of hot peppers)
• Odor stimuli from food in the mouth reaches
the olfactory mucosa through the retronasal
route
• The taste of most compounds is influenced by
olfaction, but a few, such as MSG are not
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The Physiology of Flavor Perception
• Responses from taste and
smell are first combined in the
orbital frontal cortex (OFC)
• OFC also receives input from
the primary somatosensory
cortex and the inferotemporal
cortex in the visual what
pathway
– Bimodal neurons in this
area respond to taste and
smell as well as taste and
vision
– Firing of these neurons is
also affected by the level of
hunger of the animal for a
specific food
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Wikipedia.org
31
Ellis, Logan, & Dixon “human crosssection”
Figure 15.22 The orbital frontal cortex (OFC) receives inputs from
vision, olfaction, and touch, as shown. It is the first area where signals
from the taste and smell systems meet. (Adapted from E. T. Rolls
(2000). The orbitofrontal cortex and
reward. Cerebral Cortex, 10, 284ch 15
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294, Fig. 2. Reprinted with permission from Oxford University Press.)