Psychology 420 S

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Psychology 420 SP
Ch 15 The Chemical Senses Dr. David J.
Echevarria http//www.usm.edu/neurolab/
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Chapter 15 The Chemical Senses
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Overview of Questions

• Why is a dogs sense of smell so much better than
  a humans?
• Why does a cold inhibit the ability to taste?
• How do neurons in the cortex combine smell and
  taste?

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Gatekeeper Function

• Smell and taste are closely tied an with
  affective and/or emotional component(s)
• Identification of things necessary for survival
• Identifying and rejecting things that would be
  bad

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Functions of Olfaction

• Many animals are macrosmatic - having a keen
  sense of smell that is necessary for survival
  (Dogs, Cats, Rats)
• Humans are microsmatic - a less keen sense of
  smell that is not crucial to survive

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Functions of Olfaction and Pheromones

• Experiment by Stern and McClintock
• Underarm secretions were collected from 9 donor
  women
• These were wiped on the upper lips of recipient
  women

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Experiment by Stern and McClintock

• Results showed that menstrual synchrony occurred
  since
• Secretions from the donors taken at the beginning
  of their cycles led to a shortened length of the
  recipients cycles
• Secretions from the ovulatory phase lengthened
  recipients cycles
• Phermones in the secretions, even though the
  women did not report smelling them, led to the
  changes

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Anosmia

• A blow to the front of the head can result in the
  cribriform plate severing olfactory neurons
• Total absence of sense of smell, scarring
  prevents regeneration
• Stem cells in the olfactory epithelium regenerate
  every 28 days
• Infection and disease can also cause anosmia
• http//www.anosmiafoundation.org

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Specific Anosmia

• The inability to smell one specific compound with
  otherwise normal smell perception
• 50 of population has specific anosmia to
  androstenone
• Change in detection can occur with training

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Loss of Processing

• Olfactory loss can cause great suffering
• Sense of taste
• Danger warning
• Precursor to Alzheimer's and Parkinsons disease

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Detecting Odors

• Measuring the detection threshold
• Yes/no procedure - participants are given trials
  with odors along with blank trials
• They respond by saying yes or no
• This can result in bias in terms of when the
  participant decides to respond
• Forced-choice - two trials are given, one with
  odorant and one without
• Participant indicates which smells strongest

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• Table 15.1 Human odor detection thresholds

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Macrosmatic Animals

• Survival
• Orientation and spatial cues
• Marking territory
• Information about specific places, other animals,
  and food sources

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How do dogs do it???
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Transduction

• An action potential occurs when 7-8 odor
  molecules bind to a receptor
• Roughly 40 of theses nerve impulses must occur
  for a smell sensation to be reported
• 20 million olfactory sensory neurons split
  between the epithelia of our right and left
  nostrils
• Dogs about 220 million OSNs

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Dogsand rats
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The genetic basis of olfactory receptors

• In 1991 Buck and Axel showed that genome contains
  about 1000 different olfactory receptor genes,
  each codes for single type of olfactory receptor
• 60-70 are psuedogenes (human)
• 20 in dogs and rats
• Trade-off between vision and olfaction
  (efficiency of fixed available brain space)

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Detecting Odors - continued

• Rats are 8 to 50 times more sensitive to odors
  than humans
• Dogs are 300 to 10,000 times more sensitive
• However, individual receptors for all of these
  animals are equally sensitive
• The difference lies in the number of receptors
  they each have
• Humans have 10 million and dogs have 1 billion
  olfactory receptors

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Detecting Odors/ Change Detection

• Measuring the difference threshold
• Smallest difference in concentration that can be
  detected between two samples
• This research must be done with carefully
  controlled concentrations using a device called a
  olfactometer
• Research has shown the threshold to be
  approximately 11

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Identifying Odors

• Recognition threshold - concentration needed to
  determine quality of an odorant
• Humans can discriminate among 100,000 odors but
  they cannot label them accurately
• Subtle differences between sensation and
  perception
• Sensation occurs when scent is neurally
  registered
• Perception occurs when becoming aware of
  detecting it

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Olfactory Psychophysics, Identification, and
Adaptation

• Detection, discrimination, and recognition
• How much stimulation is required before we
  perceive something to be there?
• Olfactory detection thresholds Depend on several
  factors

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Olfactory Psychophysics, Identification, and
Adaptation

• Women Generally lower thresholds than men,
  especially during ovulatory period of menstrual
  cycles, their sensitivity may also be heightened
  during pregnancy
• Professionals can distinguish up to 100,000 odors
  (e.g., professional perfumers, wine tasters)
• Durability Our recognition of smells is durable
  even after several days, month, or year
• Your house after vacation

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Olfactory Psychophysics, Identification, and
Adaptation (contd)

• Adaptation
• Sense of smell is essentially a change detector
• Examples Walking into bakery, smelling strong
  perfume that person cannot smell
• Receptor adaptation
• Cross adaptation

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Olfactory Psychophysics, Identification, and
Adaptation (contd)

• Identification
• Attaching verbal label to smell is not easy
• Tip-of-the-nose phenomenon
• Compare to tip-of-the-tongue phenomenon
• Anthropologists found that there are fewer words
  for experience of smells as opposed to other
  sensations

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Olfactory Psychophysics, Identification, and
Adaptation (contd)

• Sense of smell and language Disconnected,
  possibly 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

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Description of Colors vs. Odors
How would you describe
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The Puzzle of Olfactory Quality

• Researchers have found it difficult to map
  perceptual experience onto physical attributes of
  odorants
• Hennings odor prism (1916)
• 6 corners with the qualities putrid, ethereal,
  resinous, spicy, fragrant, and burned
• Other odors located in reference to their
  perceptual relation to the corner qualities

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• Figure 15.1 The odor prism proposed by Henning
  (1916). As described in the text, the position of
  substances relative to the corners of the prism
  represents the relative contribution of the
  qualities at each corner to the substances
  smell. (Adapted from Woodworth, 1938)

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The Puzzle of Olfactory Quality - continued

• Unfortunately, Hennings prism has proven of
  little use in olfactory research
• Linking chemical structure to types of smells
• Initial attempts showed difficulties since
• Some molecules with similar shapes had very
  different smells
• Some similar smells came from molecules with
  different shapes

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Structure of the Olfactory System

• Olfactory mucosa is located at the top of the
  nasal cavity
• Odorants are carried along the mucosa coming in
  contact with the sensory neurons
• Cilia of these neurons contain the receptors
• Humans have about 350 types of receptors that
  each have a protein that crosses the membrane 7
  times

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• Figure 15.4 The structure of the olfactory
  system. See text for details.

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Structure of the Olfactory System - continued

• Signals are carried to the glomeruli in the
  olfactory bulb
• From there, they are sent to
• Primary olfactory (piriform) cortex in the
  temporal lobe
• Secondary olfactory (orbitofrontal) cortex in the
  frontal lobe
• Amygdala deep in the cortex

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• Figure 15.5 The underside of the brain, showing
  the neural pathways for olfaction. (Adapted from
  Chemosensory Neuroanatomy and Physiology, by M.
  E. Frank and M. D. Rabin, 1989, Ear, Nose and
  Throat Journal, 68.)

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Activating Receptor Neurons

• Calcium imaging method
• Receptors take up calcium ions when they respond
• Calcium can be detected by using a chemical that
  makes the neuron fluoresce
• Measuring the decrease in fluorescence indicates
  the strength of the response

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Activating Receptor Neurons - continued

• Combinatorial code for odor
• Proposed by Malnic et al. from results of calcium
  imaging experiments
• Odorants are coded by combinations of olfactory
  receptors called recognition profiles
• Specific receptors may be part of the code for
  multiple odorants

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Activating the Olfactory Bulb

• Olfactory mucosa is divided into 4 zones
• Each zone contains a variety of different
  receptors
• Specific types of receptors are found in only one
  zone
• Odorants tend to activate neurons within a
  particular zone
• Specific types of neurons synapse with only one
  or two glomeruli

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