1. 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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2. 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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3. 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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4. Measuring the detection threshold
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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5. Detecting Odors - continued
Measuring the difference threshold
Measure the smallest difference that can be detected between two samples
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6. Table 15.1 Human odor detection thresholds
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7. 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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8. 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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9. Structure of the Olfactory System
glomeruli
Olfactory mucosa is located at the top of the nasal cavity
Odorants are carried along the mucosa coming in contact with the sensory neurons
Cilia of these neurons contain the receptors
Humans have about 350 types of receptors.
Signals are carried to the glomeruli in the olfactory bulb
Sensory neurons
receptors
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10. 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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11. 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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12. 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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13. Figure 15. 6 Recognition profiles for some odorants
Figure 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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14. 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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15. 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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16. 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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17. Figure 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, )
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18. Patterns of activation in the rat olfactory bulb (Linster, et al
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 related to both chemical structure and to perception
Figure These molecules have the same chemical formula, but the molecular group at the 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 chemicals as well as their perceived odors
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19. Somewhat similar to Retinotopic map  V1 Tonotopic map  A1
Topological locations specify the characteristics of stimuli (light, sounds, ordorants)
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20. Taste
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21. Salty taste indicates the presence of sodium
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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22. Five basic taste qualities: Salty Sour Sweet Bitter
Umami - described as meaty, brothy or savory and associated with MSG
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23. 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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24. Tongue circumvllate foliate 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
filiform
fungiform
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25. 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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26. Table 15.2 Structures in the taste system
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27. 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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28. 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
Orbital frontal cortex
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29. 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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30. 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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31. 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
Wikipedia.org
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Ellis, Logan, & Dixon “human cross-section”

32. Figure 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, , Fig. 2. Reprinted with permission from Oxford University Press.)
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