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The Chemical Senses: Taste and Smell

Demonstrations (Direct Links)

Demonstration 13.1 Isolating Taste
Demonstration 13.2 Looking at the Papillae on Your Tongue
Demonstration 13.3 Adapting to Salt in Saliva
Demonstration 13.4 Water Taste
Demonstration 13.5 Olfactory Self-Adaptation
Demonstration 13.6 Recognizing Odors


Before You Start

• Initially, taste and smell are each examined separately, however there are many ways in which they are intertwined. Is the richness of your experience of "taste" consistent with the small number of taste receptors?

• How readily could you identify (name) an odor? What makes such a task difficult?

• How might flavor differ from taste and smell? To what extent are flavor preferences determined by experience?



Taste

Sensory Aspects of Taste

Taste Stimulus

Demonstration 13.1 Isolating Taste You'll need a friend to help you with this demonstration. First, gather a number of stimuli to test. For instance, cut up equally small pieces (without skin) of an apple, a pear, an onion, and a raw potato. Close your eyes and hold your nose. Have your friend take a piece at random and place it on your tongue. First, try to guess the food without chewing. Next, chew on the piece of food and try to guess what it is. In each case, try to label the taste experience. You should find that it's difficult to identify the foods based solely on taste. You are more likely to be able to distinguish the foods based on tactile information as you chew!

Taste Receptors

Demonstration 13.2 Looking at the Papillae on Your Tongue Take a small glass of milk or a spoonful of ice cream and stand in front of a mirror. Coat your tongue with the milk or ice cream and immediately look at your tongue in the mirror. You will notice rounded bumps that rise above the milky surface of your tongue. These are the papillae that contain taste buds. The smaller bumps do not contain taste buds.

Encoding Taste Qualities

Pathway from Receptors to the Brain

 

Modifying Taste Perception

Self-Adaptation and Cross-Adaptation

Demonstration 13.3 Adapting to Salt in Saliva Take a quart jar, fill it with water, and add 1/2 teaspoon of salt. Stir the solution until the salt dissolves. Take four small glasses and fill the first with the solution. Fill the second glass 3/4 full, the third 1/2 full, and the fourth 1/4 full. Add water to these last three glasses until they are equal to the first glass, and then mix the solutions thoroughly.
• Take a sip of the solution in the fourth glass, swish it around in your mouth, and determine whether or not you detect any trace of saltiness. If you do, mix up several glasses of increasingly diluted salty water; wait several minutes before proceeding.
• If you do not taste saltiness in the fourth glass, test the third glass to determine if you detect saltiness. Continue with the second and first glasses until you can just barely notice the salt. Keep in mind the glass that contains a barely noticeable amount of salt.
• Now rinse your mouth thoroughly with water. Use distilled water if possible, because tap water may be somewhat salty. Keep rinsing for about a minute. Repeat the threshold-measurement process. Your threshold should now be lower, so that you detect lower concentrations of salt.

Water Taste

Demonstration 13.4 Water Taste Here are three ways to produce a sweet water taste with an adaptation procedure:
• Take a mouthful of diluted vinegar and swirl it around your mouth for about 30 seconds. Spit it out and then take a drink of plain water.
• Swirl some strong caffeinated coffee around your mouth for about 30 seconds. (A teaspoon of instant coffee dissolved in a small amount of water works well.) Follow the coffee with a drink of plain water.
• Eat some artichokes (canned ones work well), making sure that they are thoroughly spread throughout your mouth. After swallowing them, sip some plain water.

Here are two ways to produce a sour or bitter water taste:
• Dissolve a teaspoon of salt in a small amount of water. Swirl it around your mouth for about 30 seconds, spit it out, and sip some plain water.
• Dissolve a teaspoon of sugar in a small amount of water. Swirl it around your mouth for about 30 seconds, spit it out, and sip some plain water.

Taste Modifiers

• Miracle fruit, which contains the protein miraculin, is a taste modifier. After tasting miracle fruit, even bitter substances taste sweet.
Link - New York Times article on miracle fruit, which can be ordered from Curtis Mozie or from this site.

• Gymnema sylvestre is another taste modifier. After tasting tea made from gymnema sylvestre, nothing tastes sweet.

Roger Cholewiak's demo of Gymnema sylvestre. Roger writes: "I've been working with a quite powerful (and safe) demonstration of the division of modalities in the sense of taste. Gymnema sylvestre has been described often by Linda Bartoshuk and others as a taste modifier. (Linda Bartoshuk talks about Gymnema sylvestre in her article "After Dinner Talk Taste Illusions: some demonstrations." Ann NY Acad Sci, 237, 279-285, 1974.) It selectively abolishes the sensation of sweet, leaving all other taste modalities intact. I don't tell the students what the experience will be, but I do insure that they understand that they are under no obligation to participate in the demonstration. Gymnema sylvestre is sold over the counter as a tea in Japan as a (misdirected) treatement for diabetes, and I assure the students that the tea can be swallowed or spit out if they want. There is a large pseudoscientific body of literature on the Web regarding gymnema's use in diabetes, and might be an interesting side discussion regarding research on the web. Anyway, the tea tastes very much like spinach tea, if you could imagine. After a water wash, there is no aftertaste, then the demonstrations begin as described below.

I use about 1/4 cup of the leaf per quart of boiling water and steep it for about 10 minutes. I kept it cold in the refrigerator. With my students I do a PTC demo first, then I have them swish (and drink if they want) about 1/3 of a dixie cup of the tea. Then they rinse with water to get rid of any residual taste. Then they first start with tasting salt, then sugar, SweeTarts, and finally M&Ms. I only tell them that it will affect their sense of taste without telling them how. I give them the opportunity NOT to taste if they don't want to (informed consent), and also preface the whole thing by telling them that the tea will taste like spinach tea, but it won't taste so bad if they hold their nose. I scratch my head when they all first report no change in the taste of salt from the Burger King packets that I collect over the year, saying, "Oh well, let's see if there's any effect on sugar." They each have a sugar packet. The looks on their faces are remarkable as they try the sugar. It tastes like melting sand on the tongue. There is no sweet taste, at all. The SweeTarts are purely sour, and the chocolate is quite bitter. I leave it to you to try the artificial sweetners. The effect lasts for 10 to 20 minutes, and they all seem to enjoy the clearly defined demarcation among the qualities defined by this psychophysical dissection."

You'll find many sources for Gymnema sylvestre on the web, but most of the results will be for tablets of Gymnema sylvestre leaf extract. I did locate one source of the leaves: Bouncing Bear Botanicals.

Measuring Taste Perception

Thresholds

Individual Differences in Taste Perception


Smell

Sensory Aspects of Smell

Link - Linda Buck and Richard Axel shared the 2004 Nobel Prize in Physiology or Medicine for their work on the olfactory system. You can watch their lectures (Buck - Axel) or read their lectures.

Cognitive Aspects of Smell

Modifying Olfactory Perception

Adaptation

Demonstration 13.5 Olfactory Self-Adaptation Find a substance with a strong odor, such as an onion, perfume, or shaving lotion. Place it near your nose as you read your book for the next 10 minutes. Notice how much fainter the odor seems after that time. Remove the substance for the next 5 minutes, then bring it back for a final whiff. At this point, the odor should seem just about as strong as it did when you first smelled it.

Measuring Olfactory Perception

Individual Differences in Olfactory Perception

Anosmia is a disorder in which the sense of smell is lost.

Link - Anosmia Foundation

Link - Karl Wuensch is an experimental psychologist who suffers from anosmia. He has developed a page with useful links to web information about anosmia.

• Link - An article about suffering from anosmia by Elizabeth Zierah.

Recognizing and Identifying Odors

Demonstration 13.6 Recognizing Odors For this demonstration, you will need about a dozen odorants. Use your imagination to find them. Some suggestions are pencil shavings, a green leaf, mud, partly chewed gum, various spices (e.g., cinnamon, bay leaves, cayenne pepper), ground coffee, cheese, onion, and mustard. Take each of the odorants and cover it with a sheet of paper. (It would be even better if you could find small paper containers, into which you could poke small holes.) Invite several friends to determine if they can identify the odors. Which odors are better identified? Do your friends differ in their ability to name the odors?

• In the 1/08 edition of Scientific American, a reader asked how we remember smells, given that olfactory neurons survive for only about 60 days. Donald Wilson responded that a single neuron responds to odors that activate the receptor protein found in that neuron. When that neuron dies, it is replaced by a neuron with the same genetic makeup, so in spite of the change, the pattern of neuron activity remains stable. And it's the overall pattern of firing that's important, so small changes in individual neurons are unlikely to affect the perceptual experience.

Link - PBS clip illustrating a human odor study by Wedekind.

Link - Jill Mateo (University of Chicago) does interesting work on kin recognition through smell.

 

Behavioral Influences of Odors

Link - The Pherobase is a web site that maintains a database of pheromones and semiochemicals.

Link - Synthetic pheromones play an important role in attempts to Slow-the-Spread of the gypsy moth.

Update - Richard Doty (2010, The Great Pheromone Myth, Johns Hopkins University Press) argues that pheromones do not affect mammals.

 


Flavor: Interactions Between Taste, Smell, and Other Senses

Contributions of Taste and Smell to Flavor

Contributions of Somatosensory Input to Flavor

Comtributions of Temperature to Flavor

Contributions of Vision to Flavor

Contributions of Cognition to Flavor

Putting It All Together: Hedonics of Food


Test Yourself

1. Distinguish between taste and flavor. What are some of the components of flavor? Imagine a person with total ageusia and anosmia. Try to describe that person’s experience of food.


2. Describe the five basic taste qualities that humans perceive. How are these qualities encoded in your mouth and then processed in your brain?


3. When attempting to understand human behavior, psychologists often look to the roles of nature (e.g., biology, genetics) and nurture (e.g., experience, memory). Examine the roles of nature and nurture in explaining taste, smell, and flavor.


4. Throughout the text, you’ve seen that the same person may perceive a stimulus as different depending on context. Furthermore, different people may perceive the identical stimulus as different. Use the chemical senses to provide further examples of the disparity between the physical stimulus and perceptual experience.


5. What are the important characteristics of stimuli that determine whether they are odorous? Suppose you sniff an orange and a lemon, and they smell similar. How would Henning’s and Amoore’s systems explain the similarities?


6. In a laboratory at your college, suppose that a professor measures a student’s threshold for a particular odor, and it is high. In a laboratory at another college, a different professor measures a different student’s threshold for a second odor, and it is low. Identify as many factors as possible that might explain the different results.


7. Point out aspects of the following topics that might be relevant for a perfumer: absolute thresholds, difference thresholds, adaptation, cross-adaptation, odor recognition, and odor constancy.


8. Two portions of this chapter discussed the importance of human body odors—the section on smell recognition and the section on pheromones. Summarize the results of these two sections. How would development of a human sex pheromone alter social interactions?


9. Researchers have made great strides in learning how the chemical sense receptors encode information. They have also learned a great deal about the areas of the brain responsible for encoding the information. Briefly describe these processes, but then illustrate that cognitive processes play an important role in your experience of flavor, taste, and smell.


10. Describe how flavor is a multisensory experience. You should emphasize the chemical senses, but also draw on other senses. Use examples from your own life to illustrate these principles where you can.

 


Teaching Materials

Link - Sense of Smell Institute has a number of links to articles about olfaction.

Link - The Fragrance Foundation has some links and publications regarding perfumes and manufactured fragrances.

Link - Berkeley Olfactory Research Project (BORP), directed by Noam Sobel.

Link - Ask the Whiff Guys has synopses of a number of different olfactory stories.

Link - Michael Mann (University of Nebraska) has placed his physiology textbook online, and it includes a chapter (Chapter 10) that discusses taste and smell.

Link - Leffingwell and Associates has a wealth of information about applications of the chemical senses.

Link - Sensonics, Inc. offers the University of Pennsylvania Smell Identification test (Doty) and other taste and smell stimuli.

Link - Edmund Rolls (Oxford Centre for Computational Neuroscience) is a computational neuroscientist who has made a number of contributions to the chemical senses.

 


Recommended Readings

Doty, R. L. (2001). Olfaction. Annual Review of Psychology, 52, 423-452.

Doty, R. L. (Ed.) (2003). Handbook of Olfaction and Gustation (2nd Ed.). New York: Marcel Dekker.

Gilbert, A. (2008). What the nose knows: The science of scent in everyday life. New York: Crown.

Herz, R. (2007). The scent of desire: Discovering our enigmatic sense of smell. New York: William Morrow.

Logue, A. W. (2004). The psychology of eating and drinking (3rd Ed.). New York: Routledge.

Wilson, D. A. & Stevenson, R. J. (2006). Learning to smell: Olfactory perception from neurobiology to behavior. Baltimore: Johns Hopkins.