Chapter 14 Olfaction PDF

14 Olfaction Chapter 14 Master Click to edit Olfaction title style 14.1 Olfactory Physiology 14.2 Neurophysiology of Olfaction 14.3 From Chemicals to Smells 14.4 Olfactory Psychophysics, Identification, and Adapta...

14 Olfaction Chapter 14 Master Click to edit Olfaction title style 14.1 Olfactory Physiology 14.2 Neurophysiology of Olfaction 14.3 From Chemicals to Smells 14.4 Olfactory Psychophysics, Identification, and Adaptation 14.5 Olfactory Hedonics 14.6 Associative Learning and Emotion: Neuroanatomical and Evolutionary Considerations © Oxford University Press 14.0 Introduction Click to edit Master title style Olfaction is one of our chemical senses. (The other is taste) There are two routes for chemicals that will result in olfactory experiences: Orthonasal olfaction Retronasal olfaction (mediate flavor) NPR website © Oxford University Press 14.0 Introduction Click to edit Master title style chemical compounds that we smell are referred to as Smells like methanol Odorants: and green peppers! To be smelled odorants must be: volatile – travel through the air. small – between 25-300 daltons. 1 Dalton = 1.660539040×10−27 kg Hydrophobic – repellant to water. No odor! Not all chemicals satisfying these criteria can be smelled. Figure. Odorants. (A) Most small, volatile, and hydrophobic molecules activate the sense of smell, (B) but there are notable exceptions to the rule, such as methane and carbon monoxide. © Oxford University Press 14.1 Olfactory Click to Physiology edit Master title style Functions of the nose? olfactory cleft & olfactory epithelium. Nasal dominance © Oxford University Press 14.1 Olfactory Click to Physiology edit Master title style This schematic illustrates the fact that different OSNs The olfactory epithelium is made expressing the same up of 3 types of cells: receptors converge on the same type of Supporting cells glomerulus, no Basal cells matter where they are Olfactory sensory neurons in the olfactory (OSNs) epithelium. “One to one to one” rule: © Oxford University Press 14.1 Olfactory Click to Physiology edit Master title style Cilia and olfactory receptors! Odorant binding activates a G protein– anosmia (“smell coupled receptor blindness”). (GPCR) signaling pathway that will At level of trigger an action eyebrows. Separates the nose potential. from the brain. Experienced smells are thus the results of a unique pattern of OR binding and OSN firing. Figure. The pathway of olfactory perception, from odorant molecule to the © Oxford University Press olfactory bulb. Olfactory bulb (Part 1) Human olfactory bulb (OB) is much larger than in mouse. But 200 times more of the mouse brain is devoted to smell. The number of OB neurons is relatively conserved across mammals. Figure 14.7 From odorant to odor perception OSN axons passing through cribiform plate and Figure. The entering the olfactory bulb form the olfactory pathway of nerve (1st cranial nerve). olfactory perception, from Unlike other sensory systems, each olfactory odorant molecule bulb receives input from ipsilateral OSNs. to the olfactory bulb. Within the olfactory bulb the OSN nerve endings coalesce to form glomeruli. Each glomerulus receives axons from several different receptor types. Patterns of activity in the glomeruli determine which odor is experienced. Human olfactory bulbs have approximately 6000 glomeruli, nearly twice as many as the mouse (~3600).  Olfactory Bulb Processing 3 Juxtaglomular neurons: excitatory and inhibitory neurons surrounding the glomeruli. Respond to much 2 wider range of odorants. 1 Tufted cells: second layer for specificity of odorant. Mitral cells: The deepest layer of neurons in the olfactory bulb. Each mitral cell responds to only a few specific odorants. (the above two axons combine to form olfactory tract!) Granule cells: Also, in the deepest layer of neurons in the olfactory bulb, along with the mitral cells. Granule cells 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.  Olfactory Bulb Processing 3 From “juxtaglomular neurons” (not shown) to “tufted cells” to “mitral cells” the olfactory 2 selectivity of neurons becomes increasingly 1 sharper. At the deepest level of the olfactory bulb, the inhibitory “granular cells (3),” integrate the input from lower processing stages. These are believed to functions as “feature detectors” that are capable of detecting and learning about specific patterns of activity in tufted(1) and mitral cells (2), and thereby responding in a specific way to odorants. 14.2 Neurophysiology Click to edit Master titleofstyle Olfaction Olfactory information is transmitted from the olfactory bulb to the primary olfactory cortex Primary olfactory cortex: (Piriform cortex) The neural area where olfactory information is first processed, which includes the amygdala, parahippocampal gyrus, and interconnected areas, and also entorhinal cortex. Entorhinal cortex: A phylogenetically old cortical region that provides the major sensory association input into the hippocampus. Also receives direct projections from olfactory regions. The orbitofrontal cortex is considered to be the secondary Figure 14.8 The neuroanatomy of olfactory cortex and is responsible for conscious odor olfaction perception. A cross-sectional view of the neural organization of olfaction. © Oxford University Press 14.2 Neurophysiology Click to edit Master titleofstyle Olfaction Limbic system: The encompassing group of neural structures that includes the olfactory cortex, the amygdala, the hippocampus, the piriform cortex, and the entorhinal cortex. Involved in many aspects of emotion and memory Olfaction—unique among the senses for its direct and intimate connection to the limbic system Olfaction’s unique connection to the limbic system explains why scents tend to have such strong emotional associations. Figure 14.8 The neuroanatomy of olfaction A cross-sectional view of the neural organization of olfaction. © Oxford University Press Figure 14.9 Olfactory receptor (OR) genes by species Whether a person has a particular functional gene or pseudogene determines whether they smell a certain aroma. People that possess a nonfunctioning gene for the herbal floral component of cilantro hate its smell. The more intense a food’s retro nasal aroma is, the less people tend to consume it.. A person’s preference for certain smells depends on the number of corresponding Heavy alcohol consumption decreases olfactory olfactory receptors. More olfactory receptors sensitivity, but one drink improves olfactory increase sensitivity to scents and make them acuity (one drink may improve the flavor of less pleasurable. foods). Activation of endocannabinoid receptors increases olfactory sensitivity and appetite in mice. 14.2 Neurophysiology Click to edit Master titleofstyle Olfaction: The Feel of a scent Most odorants also stimulate polymodal nociceptors Trigeminal Innervation (touch, pain, and temperature receptors) inside the nose. This is due to the trigeminal nerve (cranial never V), which innervates most of the face (mouth, nose, and eyes). Sensations relayed via the olfactory and trigeminal nerve may be indistinguishable. Trigeminal nerve innervation explains why: The smell of menthol is cooling and ammonia burns. Cutting onions makes us cry. Pepper makes us sneeze. Photic sneeze reflex Figure 14.10 The trigeminal nerve’s role in the perception of odors. (A) The trigeminal nerve carries information from somatosensory receptors in the nose and other areas of the face to the thalamus and then on to the somatosensory cortex. (B) Your eyes tear when you chop onions because of stimulation of the trigeminal nerve. © Oxford University Press 14.3 From Click to editChemicals Master titletostyle Smells: How do odorants produce olfactory experience Theories of olfactory perception Shape pattern theory match between the shape of odorants and odorants receptor Combinatorial code: one odorant binds to several receptors and one receptor may bind to several odorants! Different scents activate different arrays of olfactory receptors, producing specific glomerular activity in the olfactory bulb. Inhibitory neurons in the olfactory bulb affect the spatial and temporal patters based on physical and contextual states. Figure Odorant-receptor binding and odorant activation, as predicted by shape-pattern theory. (A) Chemicals of But how molecules with very different shapes produce specific shapes fit receptors with shapes that best same odor? (eg. Phenylethyl alcohol vs rose accommodate them. (B) Odorant molecules activate feature detectors on various receptor types. The specific pattern of activity elicited by a given set of receptors determines the specific scent perceived. © Oxford University Press  Olfactory Bulb Processing Click to edit Master title style The specific spatial pattern of glomerular activity in the olfactory bulb serves as a neural representation for a specific odorant. However, these spatial patterns can be modified by experience, for example, following a learned flavor aversion. Areas in the olfactory bulb that are activated by various chemicals: (a) a series of carbolic acids; (b) a series of aliphatic alcohols. (From Uchida et al., 2000.) © Oxford University Press 14.3 to Click From editChemicals Master titletostyle Smells: How do odorants produce olfactory experience Theories of olfactory perception Vibration theory: Atomic structure→ vibrational frequency Molecules with same vibrational frequency have same smell Cannot explain specific anosmias- cannot smell one compound but normal smell for all others. Cannot explain why stereoisomers smell different! Stereoisomers → molecules that are mirror Figure 14.12 The stereoisomers D-carvone (A) and L-carvone (B) contain the image rotations of another same atoms, yet they smell completely different: D-carvone smells like caraway; L-carvone, like spearmint. Shape-pattern theory can account for this © Oxford University Press fact. 14.3 From Click to editChemicals Master titletostyle Smells: How do odorants produce olfactory experience Importance of patterns: Order and speed Rose & Phenylethyl alcohol? Pattern overlap Minty (wintergreen /Spearmint Citrusy (lemon and lime) Figure 14.13 The hypothetical role of OR activation timing and order. A single molecule binds first to receptor type 1, then a split second later to receptor type 2, and then to receptor type 3. Brains are especially well suited to recognizing patterns of responses such as this, so the number of odors we can recognize greatly exceeds the number of receptor types available. © Oxford University Press ClickRole The of Analysis to edit andstyle Master title Synthesis in Sensory Perception There is evidence that odors are processed both analytically (like auditory stimuli) and synthetically (like colors). Analytical processing (detecting the individual components that form the mixture): Binaral rivalry occurs when different scents are delivered to each nostril. People’s perception alternate between the two scents. Red and green cannot be separately When a mixture of rose and marker pen were High and low notes can be perceived from yellow. smelled, people’s perception also alternated separately perceived. between the two scents, suggesting that rivalry occurs in the brain as well. Figure 14.14 The roles of analysis and synthesis in sensory perception. We can separately perceive the three tones of the musical chord being Synthetic processing (the mixture produces played in (A), but not the highand medium-wavelength light rays something different from either of its mixing in the center of (B). When we mix odorants (C), we perceive the components): mixture primarily synthetically, but some degree of analytical A mixture of 30 separate odorants will smell the perception is possible. Analytical ability varies with prior training and same as a mixture of 30 different odorants. Such a with the odorants that constitute the mixture. mixture is referred to as olfactory noise. © Oxford University Press ClickRole The of Analysis to edit andstyle Master title Synthesis in Sensory Perception Its both! Additionally, there are 10 perceptual odour clusters- fragrant, woody, chemical, fruity, lemony, sour, minty, sweet, nutty, sickening/sulfurous. (tentative) And, most odour mixtures are perceived as unitary wholes. Eg. Bacon -Complex smells like “bacon” and “rose” only stimulate part of the physiochemical and perceptual space, which is why they smell distinct. (There is no single scent that is “bacon.” Rather the aroma of bacon is comprised of a mixture of approximately 150 different aromatic compounds) © Oxford University Press The Role of Analysis and Synthesis in Sensory Perception Nasal Power: People are much better at remembering smells through nose than through mouth. Nasal respiration cycles lock in neural oscillations that enhance sensory information Odor Imagery: Poor! No such thing in minds eye. Dreaming of odors? Perfumers? Yes! 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Detection, discrimination, and recognition How much stimulation is required before we perceive something to be there? o Longer carbon chains (eg. Vanilla) are easier to detect than shorter ones (eg. Nail polish remover) o Odor exposure and experience o Genetic factors o Attention: conscious focus on smell enhances it o Inattentional anosmia (coffee and visual search!) © Oxford University Press Figure 14.17 Long-term memory for odors Discrimination: Better with smell than other modalities. Does not require recognition→ but have experience with the odor For Recognition to work , 3X odor concentration is required Durability: Our recognition of smells is durable even after several days, months, or years. 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Psychophysical methods for detection and discrimination Staircase method: Method for determining the concentration of a stimulus required for detection at a threshold level (method of limits!) o Stimulus is presented in increasing concentrations until detection is indicated. o Then, concentration is decreased until detection ceases. o Ascending and descending sequence is repeated several times and concentrations at which reversals occur are averaged to determine threshold detection level. © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Psychophysical methods for detection and discrimination Triangle test: Participant is given three odors to smell, two of which are the same and one that is different. o Participant must identify the odd odor. o The order of the three odors is varied and tested several times to increase accuracy. © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Psychophysical methods for detection and discrimination Odor Recognition: c) Verbal descriptors: choose from the list d) Free recall: come up with the correct name! © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Identification Attaching a verbal label to a smell is not always easy. Language and olfactory perception are deeply disconnected. Tip-of-the-nose phenomenon: The inability name an odorant, even though it is very familiar. o Contrary to tip-of-the-tongue phenomenon, one has no lexical access to the name of the odorant, such as first letter, rhyme, number of syllables, etc. Anthropologists have found that there are fewer words for experiences of smells compared to other sensations. END HERE © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Sense of smell and language may be so disconnected because Olfactory information is not integrated in thalamus prior to processing in cortex; Majority of olfactory processing occurs in right side of brain, while language processing occurs in left side of brain. © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Individual differences in olfaction Olfactory detection thresholds depend on several factors. o Women generally have lower thresholds than men, especially during the ovulatory period of menstrual cycles, but sensitivity is not heightened during pregnancy o Professional perfumers and wine tasters can distinguish up to 100,000 odors. o Age: By 85, 50% of population is effectively anosmic. © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation A New Test to Diagnose Parkinson’s Disease Question: People with Parkinson’s disease (PD) have reduced olfactory sensitivity and find smells to be less pleasant. Can the New Test of Odor Pleasantness (NTOP) distinguish between people with PD and healthy controls? Test: 30 PD patients and 30 healthy control patients took the NTOP, which involved rating 32 odorants on their pleasantness. Results: PD patients had reliably lower ratings of pleasantness on the NTOP test than the healthy controls. Conclusions: The NTOP is a promising new tool for distinguishing individuals with PD from healthy controls, and it is well accepted in this patient group. © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Adaptation Sense of smell is essentially a change detector. ▪ Examples: Walking into a bakery, one can only smell fresh bread for a few minutes; someone who wears perfume every day cannot smell it and might put on a lot. Receptor adaptation: The biochemical phenomenon that occurs after continuous exposure to an odorant, whereby the receptors stop responding to the odorant and detection ceases. Because the ORs bound to the odorant are internalized into cell bodies where they are detached and then the ORs are recycled back to the surface to bind to new molecules Cross-adaptation: The reduction in detection of an odorant following exposure to another odorant. Presumed to occur because the two odors share one or more olfactory receptors for their transduction, but the order of odorants also plays a role. © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation Cognitive habituation: 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. Example: Going out of town, coming back and noticing how your house smells Three mechanisms involved -Olfactory receptors internalized into cell bodies during odor adaptation may be hindered after continuous exposure, taking longer to recycle. -Odorant molecules may be absorbed into bloodstream, causing adaptation to continue. -Cognitive-emotional factors (refer to effects like increased sensitivity to a smell when it is labeled ”harmful” compared to when the same smell is labeled “healthful”.) © Oxford University Press 14.4 Olfactory Click to Psychophysics, edit Master title style Identification, and Adaptation The importance of attention and conscious perception We cannot smell while we are asleep. Attention increases our ability to detect odors. Attention is cut off during sleep, so is our ability to respond to odors. © Oxford University Press 14.5 Olfactory Click to Hedonics edit Master title style Odor hedonics: The liking dimension of odor perception, typically measured with scales pertaining to an odorant’s perceived pleasantness, familiarity, and intensity. Familiarity and intensity -We tend to like odors we have smelled many times before. -Concentration alone does not predict intensity -Intensity has a more complicated relationship with odor liking: Inverted U-shape function Linearly decreasing function © Oxford University Press 14.5 Olfactory Click to Hedonics edit Master title style Nature or nurture? Are hedonic responses to odors innate or learned? Debated Evidence from infants: Odor preferences often very different from adults Cross-cultural data support associative learning An evolutionary argument: Some animals exhibit an instinctive aversion to smells from predators, etc. Learned taste aversion: Avoidance of a novel flavor after it has been paired with gastric illness. Natto is regularly eaten by Japanese but seems disgusting to Westerners. Cheese is regularly eaten by Westerners but seems disgusting to Japanese. © Oxford University Press 14.5 Olfactory Click to Hedonics edit Master title style Two caveats for theory that odor hedonics are mostly learned: Trigeminally irritating odors may elicit pain responses, and all humans have an innate drive to avoid pain. There is potential variability in receptor genes and pseudogenes that are expressed across individuals. © Oxford University Press