The contention that odors can influence, improve, and alter mood, performance, and well-being, as well as the physiological correlates of emotion such as heart rate, blood pressure, and sleep.
The fifth pair of cranial nerves, which transmit 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).
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.
The bundle of axons of the mitral and tufted cells within the olfactory bulb that sends odor information to the primary olfactory cortex.
The translation of a chemical stimulus into a smell sensation. For example, “The cake has a chocolate odor.”
olfactory receptor (OR)
The region on the cilia of olfactory sensory neurons where odorant molecules bind.
Any of the spherical conglomerates containing the incoming axons of the olfactory sensory neurons. Each OSN converges onto two glomeruli (one medial, one lateral).
A molecule that is defined by its physicochemical characteristics, which are 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 sensory neuron (OSN)
One of three cell types—the main one—in the olfactory epithelium. OSNs are small neurons located beneath a watery mucous layer in the epithelium. The cilia on the OSN dendrites contain the receptor sites for odorant molecules.
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.
A narrow space at the back of the nose into which air flows and where the olfactory epithelium is located.
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.
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.
The sense of taste.
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 and intimate connection to the limbic system.
The sense of smell.
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.
One of the three types of cells in the olfactory epithelium. Supporting cells provide metabolic and physical support for the olfactory sensory neurons.
One of the three types of cells in the olfactory epithelium. Basal cells are the precursor cells to olfactory sensory neurons.
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.
Fetal development during pregnancy.
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.
The science of defining quantitative relationships between physical and psychological (subjective) events.
Any of the main projective output neurons in the olfactory bulb.
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.
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.
Referring to the same side of the body (or brain).
Any of a secondary class of output neurons in the olfactory bulb.
The reduction in detection of an odorant following exposure to another odorant. Cross-adaptation is presumed to occur because the two odorants share one or more olfactory receptors for their transduction, but the order in which odorants are presented also plays a role.
The inability to smell one specific compound amid otherwise normal smell perception.
The biochemical phenomenon, occurring after continual exposure to an odorant, whereby receptors stop responding to an odorant and detection ceases.
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.”
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.
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.
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 repeated several times for greater accuracy.
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.
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.
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 the olfactory epithelium, such as large and/or aqueous molecules, the types of molecules that constitute pheromones. Also called Jacobson’s organ.
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.
The liking dimension of odor perception, typically measured by ratings of an odors’s perceived pleasantness, familiarity, and intensity.
A pheromone that triggers a physiological (often hormonal) change among conspecifics. This effect usually involves prolonged pheromone exposure.
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.
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.
A pheromone that triggers an immediate behavioral response among conspecifics.
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.
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.
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.
The total inability to smell, most often resulting from sinus illness or head trauma.
The conjoined regions of the amygdala and hippocampus, which are key structures in the limbic system. This complex is critical for the unique emotional and associative properties of olfactory cognition.
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.
accessory olfactory bulb (AOB)
A smaller neural structure located behind the main olfactory bulb that receives input from the vomeronasal organ.
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.