Chapter 14

olfactory bulb

A blueberry-sized extension of the brain just above the nose, where olfactory information is first processed. There are two olfactory bulbs, one in each brain hemisphere, corresponding to the right and left nostrils.

cribriform plate

A bony structure riddled with tiny holes, at the level of the eyebrows, that separates the nose from the brain. The axons from the olfactory sensory neurons pass through the tiny holes of the cribriform plate to enter the brain.

Pheromone

A chemical emitted by one member of a species that triggers a physiological or behavioral response in another member of the same species. Pheromones are signals for chemical communication and do not need to have any smell.

vomeronasal organ (VNO)

A chemical sensing organ at the base of the nasal cavity with a curved tubular shape. The VNO evolved to detect chemicals that cannot be processed by ORs, such as large and/or aqueous molecules, the types of molecules that constitute pheromones. Also called Jacobson's organ.

19. What is a learned taste aversion?

A learned taste aversion is the avoidance of a novel flavor after it has been paired with gastric illness. It is the smell, not the taste, of the substance that is the key for learned aversion response in humans.

Odorant

A molecule that is defined by its physicochemical characteristics, which is capable of being translated by the nervous system into the perception of a smell. For example, "You were given the odorant methyl salicylate to smell, which has the odor of wintergreen mint."

olfactory cleft

A narrow space at the back of the nose into which air flows and where the olfactory epithelium is located.

22. What is a pheromone

A pheromone is a chemical emitted by one member of a species that triggers a physiological or behavioral response in another member of the same species. Pheromones do not necessarily have any smell.

primer pheromone

A pheromone that triggers a physiological (often hormonal) change among conspecifics. This effect usually involves prolonged pheromone exposure.

releaser pheromone

A pheromone that triggers an immediate behavioral response among conspecifics.

entorhinal cortex

A phylogenetically old cortical region that provides the major sensory association input into the hippocampus. The entorhinal cortex also receives direct projections from olfactory regions.

staircase method

A psychophysical method for determining the concentration of a stimulus required for detection at the threshold level. The staircase method is an example of a method of limits. A stimulus (e.g., odorant) is presented in an ascending concentration sequence until detection is indicated, and then the concentration is shifted to a descending sequence until the response changes to "no detection." This ascending and descending sequence is typically repeated several times, and the concentrations at which reversals occur are averaged to determine the threshold detection level of that odorant for a given individual. Also called reverse staircase method.

olfactory epithelium

A secretory mucous membrane in the human nose whose primary function is to detect odorants in inhaled air. Located on both sides of the upper portion of the nasal cavity and the olfactory clefts, the olfactory epithelium contains three types of cells: olfactory sensory neurons, basal cells, and supporting cells.

accessory olfactory bulb (AOB)

A smaller neural structure located behind the main olfactory bulb that receives input from the vomeronasal organ.

triangle test

A test in which a participant is given three odorants to smell, of which two are the same and one is different. The participant is required to state which is the odd odor out. Typically, the order in which the three odorants are given (e.g., same, same, different; different, same, same; same, different, same) is manipulated and the test is re-peated several times for greater accuracy.

vibration theory

An alternative to shape-pattern theory for describing how olfaction works. Vibration theory proposes that every odorant has a different vibrational frequency, and that molecules that produce the same vibrational frequencies will smell the same.

G protein-coupled receptor (GPCR)

Any of the class of receptors that are present on the surface of olfactory sensory neurons. All GPCRs are characterized by a common structural feature of seven membrane-spanning α-helices.

Cilium

Any of the hairlike protrusions on the dendrites of olfactory sensory neurons. The receptor sites for odorant molecules are on the cilia, which are the first structures involved in olfactory signal transduction.

Glomerulus

Any of the spherical conglomerates containing the incoming axons of the olfactory sensory neurons. Each OSN converges onto two glomeruli (one medial, one lateral).

Chemosignal

Any of various chemicals emitted by humans that are detected by the olfactory system and that may have some effect on the mood, behavior, hormonal status, and/or sexual arousal of other humans.

15. What is cognitive habituation?

Cognitive habituation is the psychological process by which, after long-term exposure to an odorant, one is no longer able to detect that odorant or has very diminished detection ability.

binaral rivalry

Competition between the two nostrils for odor perception. When a different scent is presented to each nostril simultaneously, we perceive each scent to be alternating back and forth with the other, and not a blend of the two scents.

14. Why is cross-adaptation presumed to occur?

Cross-adaptation is the successive reduction in detection of an odorant following exposure to another odorant. It is presumed to occur because the two odorants share one or more olfactory receptors for their transduction, but the order of odorant presentation also plays a role.

Stereoisomers

Isomers (molecules that can exist in different structural forms) in which the spatial arrangements of the atoms are mirror-image rotations of one another, like a right and left hand.

17. Provide evidence that hedonic responses to odors are learned.

Evidence that hedonic responses are learned comes from cross-cultural data, where people acquire a taste for various foods that are considered "disgusting" by other cultures.

18. Provide evidence that hedonic responses to odors are innate.

Evidence that hedonic responses to odors are innate come from the idea that there is a cross-cultural agreement for some hedonic responses to common everyday smells, labeling them as either "good" or "bad."

Gestation

Fetal development during pregnancy.

trigeminal (V) nerve

The fifth cranial nerve, which transmits information about the "feel" of an odorant (e.g., mint feels cool, cinnamon feels warm), as well as pain and irritation sensations (e.g., ammonia feels burning).

granule cells

Like mitral cells, granule cells are at the deepest level of the olfactory bulb. They comprise an extensive network of inhibitory neurons, integrate input from all the earlier projections, and are thought to be the basis of specific odorant identification.

16. What is odor hedonics?

Odor hedonics is the liking dimension of odor perception. It is typically measured with scales pertaining to an odorant's perceived pleasantness, familiarity, and intensity.

11. Provide one example of the difference in processing between olfaction and vision.

Odor imagery is not possible within the realm of olfaction. It is not possible to imagine a particular smell very clearly. However, it is possible to imagine a visual image very clearly and easily.

12. Explain whether odor recognition is durable over time or not.

Odor recognition is durable over time. That is, when exposed to a particular odor once, its recognition is relatively stable over the course of one day, a week, or even a year.

20. Why do odors have a reputation of being the "best cues" for memory?

Odors have a reputation of being the "best cues" for memory because very often a smell elicits a strong emotion or memory that other modalities, such as vision or audition, do not.

basal cell

One of the three types of cells in the olfactory epithelium. Basal cells are the precursor cells to olfactory sensory neurons.

supporting cell

One of the three types of cells in the olfactory epithelium. Supporting cells provide metabolic and physical support for the olfactory sensory neurons.

olfactory sensory neuron (OSN)

One of three cell types—the main one—in the olfactory epithelium. OSNs are small neurons located beneath a mucous layer in the epithelium. The cilia on the OSN dendrites contain the receptor sites for odorant molecules.

8. Provide one piece of evidence in favor of the shape-pattern theory of olfactory perception.

One piece of evidence in favor of this theory is the study of stereoisomers. Stereoisomers are molecules that are mirror-image rotations of one another, and although containing the same atoms, they can smell completely different. According to the shape-pattern theory, this difference arises because the rotated molecules do not fit the same receptors.

retronasal olfaction

Perceiving odors through your mouth while breathing and chewing. This occurs when we are smelling something that is inside our mouth and is what gives us the experience of flavor.

Ipsilateral

Referring to the same side of the body (or brain).

orthonasal olfaction

Sniffing in and perceiving odors through our nostrils, which occurs when we are smelling something that is outside of us.

13. What is the "tip-of-the-nose" phenomenon?

The "tip-of-the-nose" phenomenon is the inability to name an odorant, even though it is very familiar. The phenomenon is evidence that language and olfaction are deeply disconnected.

nasal dominance

The asymmetry characterizing the intake of air by the two nostrils, which corresponds to differing sensitivity to odorants between the two nostrils. Nasal dominance alternates nostrils throughout the day, but there is no predictability about when the nostrils alternate.

learned taste aversion

The avoidance of a novel flavor after it has been paired with gastric illness. The smell, not the taste, of the substance is key for the learned aversion response in humans.

receptor adaptation

The biochemical phenomenon, occurring after continual exposure to an odorant, whereby receptors stop responding to an odorant and detection ceases.

olfactory tract

The bundle of axons of the mitral and tufted cells within the olfactory bulb that sends odor information to the primary olfactory cortex.

mitral cell

The deepest layer of neurons in the olfactory bulb. Each mitral cell responds only to a few specific odorants.

limbic system

The encompassing group of neural structures that includes the olfactory cortex, the amygdala, the hippocampus, the piriform cortex, and the entorhinal cortex. The limbic system is involved in many aspects of emotion and memory. Olfaction is unique among the senses for its direct connection to the limbic system.

juxtaglomerular neurons

The first layer of cells surrounding the glomeruli. They are a mixture of excitatory and inhibitory cells and respond to a wide range of odorants. The selectivity of neurons to specific odorants increases in a gradient from the surface of the olfactory bulb to the deeper layers.

olfactory (I) nerves

The first pair of cranial nerves. The axons of the olfactory sensory neurons bundle together after passing through the cribriform plate to form the olfactory nerve, which conducts impulses from the olfactory epithelia in the nose to the olfactory bulb.

specific anosmia

The inability to smell one specific compound amid otherwise normal smell perception.

odor hedonics

The liking dimension of odor perception, typically measured by ratings of an odors's perceived pleasantness, familiarity, and intensity.

4. How is the limbic system involved in odor perception?

The limbic system is involved in many aspects of emotion and memory. It has direct and strong connections to the olfactory system, which explains the strong relationship between emotion, memory, and odor perception.

Aromatherapy

The manipulation of odors to influence, mood, performance, and well-being, as well as the physiological correlates of emotion such as heart rate, blood pressure, and sleep.

primary olfactory cortex or piriform cortex

The neural area where olfactory information is first processed. It comprises the amygdala, parahippocampal gyrus, and interconnected areas; and it interacts closely with the entorhinal cortex.

tufted cell

The next layer of cells after the juxtaglomerular neurons. They repond to fewer odorants than the juxtaglomerular cells, but more than neurons at the deepest layer of cells.

main olfactory bulb (MOB)

The olfactory bulb; the blueberry-sized extension of the brain just above the nose; the first region of the brain where smells are processed. In humans we refer simply to "olfactory bulb(s)"; in animals with accessory olfactory bulbs, we distinguish between "main" and "accessory."

3. Explain the role of the olfactory bulbs in odor perception.

The olfactory bulbs are the first sites in the brain where olfactory information is processed.

1. How is the olfactory epithelium involved in odor perception?

The olfactory epithelium contains three types of cells that allow it to detect odorants in the inspired air.

olfactory white

The olfactory equivalent of "white noise" or the color white. When at least 30 odorants of equal intensity that span olfactory physiochemical and psychological (perceptual) space are mixed, they produce a resultant odor percept that is the same as every other mixture of 30 odorants meeting the span and equivalent intensity criteria, even though the various mixtures do not share any common odorants.

21. Explain the role of the orbitofrontal cortex in odor perception.

The orbitofrontal cortex is responsible for processing olfaction. It is also the area of the brain that is critical for assigning affective value to stimuli (i.e., liking or disliking), and for determining hedonic meaning.

orbitofrontal cortex (OFC)

The part of the frontal lobe of the cortex that lies behind the bone (orbit) containing the eyes. The OFC is responsible for the conscious experience of olfaction, as well as the integration of pleasure and displeasure from food; and it has been referred to as the secondary olfactory cortex and the secondary taste cortex. The OFC is also involved in many other functions, and it is critical for assigning affective value to stimuli—in other words, determining hedonic meaning.

Lordosis

The position that females of some species (e.g., pigs and rats) need to assume in order to be impregnated. It involves the inward curving of the spinal column and exposure of the genitals.

cognitive habituation

The psychological process by which, after long-term exposure to an odor, one no longer has the ability to detect that odor or has very diminished detection ability.

odorant receptor (OR)

The region on the cilia of olfactory sensory neurons where odorant molecules bind.

Psychophysics

The science of defining quantitative relationships between physical and psychological (subjective) events.

Olfaction

The sense of smell.

Gustation

The sense of taste.

6. What does the shape-pattern theory contend?

The shape-pattern theory contends that odorant molecules and olfactory receptors have specific shapes, and that an odorant will be detected by a specific olfactory receptor to the extent that the odorants' molecules fit into the olfactory receptor like a key fits into a lock.

2. Name and describe the three cells found in the epithelium.

The three cells found in the epithelium are: 1) supporting cells, which perform metabolic and physical supportive functions; 2) basal cells, which are precursor cells to olfactory sensory neurons; and 3) olfactory sensory neurons, which possess cilia that contain the receptor sites for odorant molecules.

Anosmia

The total inability to smell, most often resulting from sinus illness or head trauma.

Odor

The translation of a chemical stimulus into a smell sensation. For example, "The cake has a chocolate odor."

5. What kinds of sensations are mediated by the trigeminal nerve?

The trigeminal nerve transmits information about the "feeling" dimension of smell. For instance, menthol feels cool, and ammonia feels burning.

9. What two kinds of processes are thought to be involved in the perception of odor mixtures?

The two processes involved in the perception of odor mixtures are analysis and synthesis.

7. What does the vibration theory contend?

The vibration theory contends that there is a different vibration frequency for every perceived smell, and that molecules that produce the same vibration frequencies will produce the same smell.

10. What is binaral rivalry?

When one scent is presented to the left nostril and another scent is presented to the right nostril at the same time, they compete with each other to be perceived. Only one scent at a time is perceived rather than a combination of the two scents, and which scent is perceived switches back and forth over time.

cross

adaptation-The reduction in detection of one odorant following exposure to another odorant. Cross-adaptation is presumed to occur because the components of the odors (or the odorants) in question share one or more olfactory receptors for their transduction, but the order in which odorants are presented also plays a role.

amygdala

hippocampal complex-The conjoined regions of the amygdala and hippocampus, which are key structures in the limbic system. This complex is critically involved in the unique emotional and associative properties of olfactory cognition.

tip

of-the-nose phenomenon-The inability to name an odor, even though it is very familiar. Contrary to the tip-of-the-tongue phenomenon, one has no lexical access to the name of the odor, such as first letter, rhyme, number of syllables, and so on, when in the tip-of-the-nose state. This is an example of how language and olfactory perception are deeply disconnected.

shape

pattern theory-The current dominant biochemical theory for how chemicals come to be perceived as specific odors. Shape-pattern theory contends that different scents—as a function of the fit between odorant shape to OR shape—activate different arrays of olfactory receptors in the olfactory epithelia. These various arrays produce specific firing patterns of neurons in the olfactory bulb, which then determine the particular scent we perceive.