Olfaction

Olfactory physiology

Odours and odorants

• Odour: olfactory sensations
• Odorant: the chemical compounds that form the stimuli for odours o To be smelled, odorant molecules must be able to float through air, small and hydrophobic

The human olfactory apparatus

• The inside of the nose has small ridges, called turbinates, that add turbulence to incoming air, causing a small puff of each breath to rise upward, pass through the olfactory cleft, and settle on the olfactory epithelium (figure 14.2 page 400)
• Olfactory cleft: a narrow space at the back of the nose into which air flows and where the olfactory epithelium is located
• Olfactory epithelium: a secretory mucous membrane in the human nose whose primary function is to detect odorants in inhaled air. Contains three types of cells:
  • Olfactory sensory neurons (OSN): small neurons, located beneath a watery mucous layer in the epithelium, that have cilia which are actually the OSN’s dendrites that contain olfactory receptors
  • Basal cells o Supporting cells
• Nasal dominance: our two nostrils take in different amounts of air, this alternates throughout the day which means that the two nostrils continually vary in their sensitivities to odorants as a function of the amount of air inhaled
• Anosmia: absence of a sense of smell o There is a profound connection between psychiatric depression and our sense of smell, the neurobiological connection between olfactory and emotional processing enables a bidirectional interaction between them
• The interaction between an odorant and the olfactory receptors produces an action potential that is transmitted along the axon of the OSN. These axons pass through the tiny sievelike holes of the cribriform plate, a bony structure at the level of the eyebrows that separates the nose from the brain. The OSN axons that pass the cribriform plate bundle to form the olfactory nerve, and enter a blueberry-sized extension of the brain just above the nose called the olfactory bulb. We have two olfactory bulbs, one in each hemisphere which receive info from the ipsilateral nostril. The olfactory bulb is the first relay for the OSNs in the brain, here the sensory nerve endings gather together to form tiny spheres called glomeruli. The distinct pattern of olfactory receptor activation for a specific odorant is then translated into a specific pattern of spatial activity across the glomeruli. The specific pattern of glomeruli activity is then interpreted by the brain as indicating a specific odour. Still within the bulb, the glomeruli pass odour info on to mitral cells and tufted cells, nerve cells that coordinate info from the glomeruli for further processing and distribution to higher brain centres. Axons of the mitral and tufted cells of each bulb combine and form the olfactory tract, that conveys info ipsilaterally to the primary olfactory cortex/piriform cortex. The primary olfactory cortex comprises the amygdala, parahippocampal gyrus and interconnected areas, and intimately interacts with the entorhinal cortex (figure 14.6 page 403)
• There is no fixed code for odour perception, rather your personal experience with an odour determines how it will be processed by the olfactory system, even at very early levels
• 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 I 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 genetic basis for olfactory receptors

• The more copies of a specific receptor you have, the more sensitive you will be to certain odorants, that is, certain specific chemicals will smell stronger to one person than they do to another because of the number of receptor copies they have. Whether you have a pseudogene or a functional gene for a given receptor will also alter odour perception
• Brains can only get so big, so to free up brain space necessary to house trichromatic colour vision, we have dropped the ability to analyse the odorants detected by certain olfactory receptor genes, and they became pseudo-genes

The feel of scent

• Most odorants stimulate the somatosensory system to some degree through polymodal nociceptors inside the nose. These sensations are mediated by the trigeminal nerve

From chemicals to smells

Theories of olfactory perception

• Shape-pattern theory: odorant molecules have different shapes and olfactory receptors have different shapes, and an odorant will be detected by a specific OR to the extent that the odorant’s molecules fit into the OR. When a given odorant is sniffed, a particular pattern is generated across the glomeruli. Difference in those spatial patterns provide the basis for the array of odours that we perceive (figure 14.8 page 408)
• Scents are detected by means of a combinatorial code: one odorant may bind to several different receptors and one receptor may bind several different odorants to varying degrees → different scents activate different arrays of ORs in the olfactory epithelia, producing specific firing patterns of neurons in the olfactory bulb, which then determines the particular scent we perceive, these various firing patterns for specific scents turn out to be highly consistent across individuals
• Vibration theory: proposes that, because of atomic structure, every odorant has a different vibrational frequency, and molecules that produce the same vibrational frequencies have the same smell
• Specific anosmia: the inability to smell one specific compound amid otherwise normal smell perception which are due to faulty odorant-receptor interactions and not odorant vibrations
• The same odorant can produce different scent sensations in different people, which could arise from differing odorant-receptor interactions (shape-pattern theory can explain this phenomenon and specific anosmia, vibration theory cannot)
• Stereoisomers: molecules that are mirror-image rotations of one another, and although they contain the same atoms, they can smell completely different. According to the shape-pattern theory this difference arise because the rotated molecules do not fit the same receptors, thus different receptors are activated for these two molecules, causing different scents to be perceived. Vibration-theory cannot explain this because the vibrations of the stereoisomers are the same

The importance of patterns

• We detect odours by the pattern of activity across various different receptor types. The intensity of an odour also changes which receptors will be activated, which is why weak and small concentrations of an odorant do not smell quite the same. The timing of OR activation also seems to be important: an odorant that activates several receptors will also stimulate them in a specific temporal sequence and speed. Another odorant might stimulate the same receptors in a different order and rate, and the difference might lead to the perception of a different smell

Odour mixtures

• The components in an odour mixture are processed by analysis and synthesis. An experiment showed that in a five-component mixture, the average amount of identified components was just three. However the more training one had had, the better he did. Thus olfaction is primarily a synthetic sense but a certain amount of analytical ability can be developed
• Binaural rivalry: when two different odours are presented to each of our nostrils, we alternate in our ability to smell one odour or the other.
  • A rose scent is presented to one nostril and a marker scent to the other: the participants switched between smelling markers or roses, but the marker scent was more intense and usually perceived first → the stronger scent tends to dominate
  • When the participants were later presented with a mixture of the rose and marker scent, most of them reports smelling markers and roses alternately

→ rivalry occurs both in the nose and in the brain

• There appears to be a strong connection between smell and vision: when two different pictures are shown to each of the eyes and a smell of one of them is presented simultaneously, the picture that corresponds to the smell is seen longer than the other picture in alternation

 

 

The power of sniffing

• Sniffing: consciously and forcibly inhaling burst of air into your nostrils, which increases the ability to detect odorants; sniffing produces greater activation in some parts of the brain, such as the cerebellum, than passive inhalation does

Olfactory psychophysics, identification and adaptation

Detection, discrimination and recognition

• Olfactory detection threshold depends on a number of factors:
  • Odorant molecules with longer carbon chains are easier to detect than those with shorter carbon chains
  • Women generally have lower olfactory detection thresholds than men o The ability to detect odorants declines with age, because of a change in the proportion of cell regeneration to cell death in ORs: as we age, the number of ORs that die off rises beyond the number that are regenerated
  • The ability to detect an odorant can be manipulated by experience: plasticity and modulation through environmental influences appear to be basic principles of olfaction
  • Our memory for odours is very resilient, especially when an odour is accompanied by an emotion
• It takes three times as many odorant molecules floating through the nose to recognize an odour as it does to simply detect the presence of an odour

Psychophysical methods for detection and discrimination

• Staircase method: an odorant is presented in ever-increasing concentration increments until the participant reports being able to smell something for several increments. Then the odorant’s concentration is decreased until the participant reports no detection. These reversals are repeated a number of times, and the odorant concentration at the point where reversals occur are averaged to determine the approximate concentration needed for that person to detect the odorant
• Triangle test: used to determine whether someone can discriminate between two odorants.

A participant is given three odorants to smell, of which two are the same and one is different

Identification

• Tip-of-the-nose phenomenon: the inability to name an odour, even though it is very familiar. We typically no nothing about the label we’re searching for, indicating that language and our sense of smell are deeply disconnected:
  • Olfactory info does not need to be integrated in the thalamus prior to processing in the cortex, and it is argued that the thalamus has relevance for language
  • The majority of olfactory processing occurs in the right hemisphere, whereas language is dominated by the left hemisphere
  • It is even suggested that odours are hard to name because of competition between odour and language processing for cognitive resources that share the same neural substrates → using brain-imaging techniques it is shown that the presence of an odorant alters the semantic processing of a word and degrades word encoding, but does not influence non-semantic processing

Adaptation

• Receptor adaptation: odorant binding on an OR causes the OR to be internalized into its cell body, where it becomes unbound from the odorant and is then recycled through the cell and emerges again in an number of minutes. The OR are therefor no longer physically available to respond to odorants during this period → you have adapted to the smell
• As the concentration of an odorant increases, the degree of adaptation decreases
• Sniffing enables us to filter out stable background odours because sniffing makes OR neurons less responsive to stable odours and more responsive to new odorants. Given the fact that background and foreground odours are slightly separated in time, sniffing helps us to separate components in an olfactory scene
• Cross-adaptation: the reduction in detection of an odorant following exposure to another odorant which is presumed to occur when the odours in question rely on similar sets of olfactory receptors

Cognitive habituation

• Cognitive habituation: the psychological process by which, after LT exposure to an odour, one no longer has the ability to detect that odour or has very diminished detection ability (takes weeks to undo in contrast to adaptation which can be undone by a sniff of another odour) o The OR that are internalized into their cell bodies during odour adaptation may be more hindered after continual exposure and take much longer to recycle than they normally would
  • From continual exposure, odorant molecules may be absorbed into the bloodstream, and then transported to the OR via nasal capillaries when we breathe out the nose
  • Cognitive-emotional factors, like those demonstrated in the experiment where participants were told that an odour was harmful and then perceived that they did not adapt, may be involved

Olfactory hedonics

• Odour hedonics: the liking dimension of odour perception, typically measured by ratings of an odour’s perceived pleasantness, familiarity and intensity

Familiarity and intensity

• We tend to like familiar odours more than unfamiliar ones, moreover we often perceive pleasant odours as familiar even if we haven’t smelled them before, thus ratings of odour pleasantness and familiarity show a linear relationship with odour liking -Intensity: figure 14.15 page 420

 

 

Nature or nurture?

• Innate: we are born with a predisposition to like or dislike various smells; if odour hedonics are innate, new-borns should display them, however research indicates that new-borns and children often display very different preferences from those of adults
• Learned: we are born merely with a predisposition to learn to like or dislike smells, and that whether a smell is liked or not is determined by the emotional value of the experience that have been associated with it; mothers who consumed distinctive-smelling volatiles during pregnancy or breast-feeding had infants who showed greater preferences for these smells

An evolutionary argument

• Specialist animal species have hardwired responses to particular odours, which are adaptive
• For generalist species it is not adaptive to have predetermined olfactory responses to any particular odour
• Learned taste aversion: the avoidance of a novel flavour after it has been paired with gastric illness. The smell, not the taste, of the substance is key for the learned aversion response
• The first association made to an odour is etched into the brain and produces a unique neural signature in the amygdala-hippocampal complex that predicts later memory for that odour association → the olfactory system of generalists does not come pre-programmed, but rather is geared to very effectively learn the meaning of smells based on experience

Caveats

• Trigeminally irritating odorants may elicit pain responses, and all humans have an innate drive to avoid pain
• The potential variability in the receptor genes and pseudo-genes that are expressed across individuals may influence odour intensity, and consequently the perceived pleasantness of an odour

Olfaction, memory and emotion

• When a recollection is triggered by an odour people experience more emotionally intense memories and feel more transported back to the original time and place of the event than when memories are triggered by another modality
• Aromatherapy: odours can influence and alter mood, performance, well-being and the physiological correlates of emotion, such as HR, blood pressure and sleep

Neuroanatomical and evolutionary connections between odour and emotion

• Odours are consciously perceived and experienced in the orbitofrontal cortex, which can be considered the secondary olfactory cortex o The right orbitofrontal cortex plays the most significant role in conscious olfactory perception; the right hemisphere is dominant for emotional processing

 

 

The vomeronasal organ, human pheromones and chemosignals

• In animals that rely on smell for survival, the olfactory system consists of two subdivisions whose neurons do not interconnect and which function separately in the integration of specific chemicals:
  1. Main olfactory bulb (MOB): first region of the brain where smells are processed
  2. Accessory olfactory bulb (AOB): depends on the engagement of the vomeronasal organ (VNO)
• Vomeronasal organ: a chemical sensing organ at the base of the nasal cavity with a curved tubular shape which evolved to detect chemicals that cannot be processed by the olfactory epithelium, such as large and aqueous molecules that constitute pheromones
• Pheromones: means of chemical communication o Releaser pheromones: triggers an immediate behavioural response among conspecifics

o Primer pheromones: produce a physiological change in the recipient over time

• Chemosignal: chemicals emitted by humans that are detected by the olfactory system and that may have some effect on the mood or behaviour of other humans