Perceptual processes chapter 14

Olfaction Fun Facts

• Roughly 5% of the population cannot smell
• Viral infections can permanently alter the sense of smell
• Parosmia is the inability to accurately identify smells where familiar smells are distorted and repulsive
• Smell training can improve verbal function, attention, and well-being and quality of life
• Humans can tracking the scent of chocolate across a 10 acre field
• Moles use stereo smelling to locate objects
• Smell and genetic differences largely govern the attractiveness of sexual partners

Chemical Senses

• Chemical sense detects specific chemicals in the environment
• Olfaction is the sense of smell
• Gustation is the sense of taste
• Though often separated, taste and smell are very closely related

Olfactory Physiology: Odor Detection Routes

• Odors are detected through two routes.
• Orthonasal olfaction involves sniffing and perceiving odors through the nostrils
• Retronasal olfaction involves perceiving odors through the mouth while breathing and chewing
• Most of the sense of flavour comes from retronasal olfaction

Olfactory Physiology: Odorants

• Odor is the translation of a chemical stimulus into a smell sensation
• Odorant is a molecule defined by its physiochemical characteristics which allows translation into the nervous system for the perception of smell
• In order to be smelled, odorants must be:
  • Volatile to float through the air
  • Small
  • Hydrophobic to repel water

Olfactory Physiology: The Nose

• Unlike other senses, smell relies on an organ with another purpose besides smell - the nose
• The primary purpose of the nose is to filter, warm, and humidify air
• The nose contains: - Turbinates: Curved bony protrusions inside the nasal cavity create air turbulence to ensure air passes through the olfactory cleft. - Olfactory cleft: A narrow space at the back of the nose that houses the olfactory epithelium. - Olfactory epithelium: The mucous membrane in the nose that captures odorants in the air

Olfactory Physiology: Olfactory Epithelium Cells

• Olfactory epithelium is known as the retina of the nose
• There are three types of cells:
  • Supporting cells provide metabolic and physical support for olfactory sensory neurons.
  • Basal cells are precursor cells to olfactory sensory neurons.
  • Olfactory sensory neurons (OSNs) are the main cell type, small neurons located beneath a watery mucous layer, and have a 30 day life cycle.

Olfactory Physiology: Cilia and Olfactory Receptors

• Cilium (plural cilia) refers to hairlike protrusions on the dendrites of OSNs
  • Cilium contain receptor sites for odorant molecules
  • They are the first structures involved in olfactory signal transduction
• Olfactory Receptor (OR) is the region on the cilia of OSNs where odorant molecules bind
  • It takes seven or eight odor molecules binding to a receptor to initiate an action potential.
  • Approximately 40 nerve impulses are required for a smell sensation to be reported

Olfactory Physiology: Cribriform Plate, Olfactory Nerves, and Olfactory Bulbs

• Cribriform plate is a bony structure riddled with tiny holes at the level of the eyebrows that separates the nose from the brain
  • Axons from OSNs pass through the tiny holes to enter the brain
• Olfactory nerves is the first pair of cranial nerves comprised of bundled OSN axons that pass through the cribriform plate
• Olfactory bulb is a blueberry-sized extension of the brain just above the nose which first processes olfactory information
  • There are two olfactory bulbs, one in each brain hemisphere that correspond to the left and right nostrils with ipsilateral connections

Olfactory Physiology: Glomeruli

• Glomeruli are spherical conglomerates containing incoming axons of the OSNs
  • Each OSN converges on two glomeruli (one medial, one lateral)
  • Glomeruli are the first relay for the OSNs in the olfactory bulb
  • Patterns of activity in the glomeruli determine which odor is experienced

Olfactory Physiology: Neurons

• Juxtaglomerular neurons are excitatory and inhibitory cells that respond to a wide range of odorants
• Tufted cells respond to fewer odorants than juxtaglomerular cells, but to more neurons than at the deepest layer
• Mitral cells respond to only a few specific odorants
• Granule cells are an extensive network of inhibitory neurons
  • The integrate input from all projections and are thought to govern odorant identification

Neurophysiology of Olfaction: Primary Olfactory and Entorhinal Cortex

• Primary olfactory cortex is the neural area where olfactory information is first processed, and includes:
  • The amygdala
  • Parahippocampal gyrus
  • Entorhinal cortex
  • These regions are known as the limbic system
• Entorhinal cortex is a phylogenetically old cortical region with major sensory association input into the hippocampus, and receives direct projections from olfactory regions

Neurophysiology of Olfaction: Limbic System

• The limbic system:
  • is involved in many aspects of emotion and memory
  • is uniquely connected to olfaction by a direct and intimate connection
• The unique connection between olfaction and the limbic system explains why scents have strong emotional associations

Sense of Smell: Capacity

• The sense of smell can detect over one trillion smells
• Humans can only detect 7.5 million colors
• Humans have about 5-10 million OSNs
• Dogs have at least 100 times more OSNs, more brain dedicated to olfaction, and sense smells in lower concentrations

Perception of Smell: The Feel of Scent

• The feel of scent:
  • Odorants can stimulate the somatosensory system through polymodal nociceptors (touch, pain, temperature receptors)
  • Sensations are mediated by the trigeminal nerve (cranial nerve V)
  • It is often impossible to distinguish between sensations traveling up cranial nerve I (olfactory receptors) and those traveling up cranial nerve V (somatosensory receptors)

Olfactory Testing: The Alberta Smell Test

• The Alberta Smell Test was developed by Green and Iverson in 1998
• A set of 12 scents are prepared ahead of time
• Participants are exposed to each scent, once in each rostril in random order
• They are asked to identify the scent from a list of 12 options
• Scores are tabulated for each nostril and then calculated to a grand score.
  • 0-4 constitutes poor or impaired olfaction
  • 5-15 constitutes average olfaction
  • 16-20 constitutes superlfaction

Theories of Smell Perception: Shape-Pattern Theory

• Shape-pattern theory states that different scents, as a function of odorant-shape to OR-shape fit, activate different arrays of olfactory receptors in the olfactory epithelium.
  • These arrays produce specific firing patterns of neurons in the olfactory bulb, which then determine the particular scent the organism perceives

Theories of Smell Perception: Vibration Theory

• Vibration theory proposes that every perceived smell has a different vibrational frequency
• Molecules that produce the same vibrational frequencies will smell the same

Theories of Smell Perception: Specific Anosmia

• Specific anosmia is the inability to smell one specific compound amid otherwise normal smell perception
  • 20-40% of the population have specific anosmia to androstenone (found in armpit sweat and pork) - Those who can smell androstenone perceive it as a urinous odor - Those insensitive to it perceive it as a sweet musky-floral scent
  • Sensitivity to androstenone can be increased with training - This cannot be explained by vibration theory - Shape-pattern theory can explain these findings due to genetic differences in receptor expression

Theories of Smell Perception: Stereoisomers

• Study of stereoisomers:
  • Molecules are mirror-image rotations of one another, although they contain the same atoms and can smell completely different
    • D-cavone smells like caraway, while L-cavone smells like spearmint
  • Vibration theory cannot explain this phenomenon
  • Shape theory can, as mirror-reversed shapes mean they fit into different receptors

Olfactory Perception: Patterns

• Detection relies on understanding patterns
• To detect thousands of scent types, genes can code for 1000 olfactory receptors
• Activity patterns across receptors can be detected
• The intensity of odorant changes which receptors are activated
  • Weak concentrations of an odorant may not smell the same as strong concentrations
• Specific time order of receptor activation is important

Olfactory Perception: Odor Mixtures

• Pure odorants are rarely smelled
• The processing of components in an odorant mixture can be identified in two ways:
  • Analysis, as in auditory mixtures. High and low notes can be played together, but each can be detected
  • Synthesis, as in color mixtures. Mixing red and green light results in yellow light, but you can't separately perceive the red and green in the yellow
• Olfaction is primarily a synthetic sense, but some analytical abilities may be developed

Olfactory Perception: Binaral Rivalry

• Binaral rivalry means that there is a competition between the two nostrils for odor perception
• When presented with a different scent in each nostril, one scent is experienced at a time instead of both together

Olfactory Perception: Sensory Competition

• What is smelled affects what is seen
  • In binocular rivalry stimulus of markers in one eye and a rose in the other, for example, subjects switch back and forth between seeing one or the other stimulus
    • Senses presented to the nostrils influence vision

Olfactory Perception: Odorants

• The more odorants in a mixture, the harder it becomes to tell different mixtures apart
• Olfactory white is the olfactory equivalent of white noise, or the color white
  • It is a set of at least 30 different odorants
    • that are all of equal intensity
    • and span the olfactory physiochemical and perceptual space
  • People cannot tell one mixture of 30 odorants from another mixture, even if the various mixtures do not share any common odorants

Olfactory Perception: Mixtures

• Bacon and rose are both smells made up of 100s of chemical components
• Discrimination relies on intensity and span
  • Components in stimuli are of unequal intensity
  • They do not span the entirety of the olfactory space

Olfactory Psychophysics: Staircase Method

• Staircase method is used to determine concentration of a stimulus needed for detection at a threshold level
  • It is similar to the method of limits
  • Stimulus is presented in increasing concentrations until detection is indicated
  • Then concentration is decreased until detection ceases
  • Ascending and descending sequences are repeated several times and concentrations at which reversals occur are averaged to determine threshold detection level

Olfactory Psychophysics: Triangle Test

• Triangle test involves presenting a participant with three odors
  • Two are the same and one that is different
  • Participant must identify the odd odor
  • The order of the three odors is varied and tested several times

Olfactory Psychophysics: Identification

• With identification, language and olfactory perception are deeply disconnected
• Tip-of-the-nose phenomenon is the inability name an odorant (even though it is familiar)
  • Unlike the tip-of-the-tongue phenomenon, an individual will have no lexical access to the name of the odorant (first letter, rhyme, number of syllables, etc.).
• Anthropologists have found that there are fewer words for language used to describe smells, language is underutilized in this area

Olfactory Psychophysics: Language

• The 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

Disorders of Olfaction: Definitions

• Anosmia is the complete inability to detect odors
• Agnosia is the inability to recognize or identify odors despite retention of ability to detect them
• Hyposmia is reduced sensitivity to odors
• Hyperosmia is heightened sensitivity to odors
• Dysomia is a distorted perception of smells (ex. lemon smells like baby powder, roses smell like dirt)
• Cacosmia means that familiar and pleasant odors are perceived as vile and intensely unpleasant
• Phantosmia means smelling odors that are not present

Olfactory Psychophysics: Individual Differences

• Olfactory detection thresholds depend on several factors
  • Women generally have lower thresholds than men, especially during the ovulatory period of menstrual cycles, but sensitivity is not heightened during pregnancy
  • Professional perfumers and wine tasters can distinguish up to 100,000 odors
  • With age, 50% of population is effectively anosmic age 85

Olfactory Psychophysics: Adaptation

• The sense of smell is essentially a change detector
• Receptor adaptation is the biochemical phenomenon of ceased odor detection after continuous exposure to an odorant
• Cross-adaptation is the reduction in detection of an odorant following exposure to another odorant
• It is presumed to occur because the two odors share one or more olfactory receptors for their transduction, order of odorants also has a role.

Olfactory Psychophysics: Cognitive Habituation

• Cognitive habituation is the psychological process through which long-term exposure to an odorant diminishes the ability to detect it, or results in very diminished detection ability
• Cognitive habituation is also known as nose-blindness.
• Three mechanisms are 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 also play a role.

Odor Hedonics

• Odor hedonics pertains to the liking dimension of odor perception
• Measured with scales pertaining to an odorant's perceived pleasantness, familiarity, and intensity
• Odors that have been smelled many times are generally liked
• The relationship between intensity and odor liking may be based on:
  • the inverted U-shape function of pleasant odors
  • the linearly decreasing function of tolerable, but not pleasant, odors.

Odor Hedonics: Evidence from Infants

• Odor preferences of infants are very different from adults
• Infants do not find the odors of feces or sweat offensive
  • Experience and exposure changes preference
• Some animals exhibit an instinctive aversion to certain smells

Odor Hedonics: Memory

• Memories are most intense when triggered by olfactory stimuli
• Memories triggered by odor cues are distinctive in their emotionality
  • This leads the emotion and evocativeness of odor-elicited memories to lead to the false impression that memories are especially accurate
  • Orbitofrontal cortex and limbic areas process olfaction.
    • These areas are the cortical regions for assigning affective value (i.e., hedonic judgment).
  • These factors explain increased emotionality of smells versus other senses

Odor Hedonics: Aromatherapy

• Aromatherapy states that odors can influence, improve, and alter mood, performance, and well-being
• It can also influence physiological correlates of emotion such as heart rate, blood pressure, and sleep
• Effects of aromatherapy can be explained by memory associations, not pharmacological effects of the odors themselves
• Aromas can influence emotions which lead to improved well-being