Olfactory perception smell
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Olfactory Perception – Smell.

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Olfactory perception smell

Olfactory Perception – Smell

“…We have five senses, but only two that go beyond the boundaries of ourselves. When you look at someone, it’s just bouncing light, or when you hear them, it’s just sound waves, vibrating air, or touch is just nerve endings tingling. Know what smell is?... It’s made up of the molecules of what you’re smelling.”

Angels in America, p. 17


Olfactory perception smell1

Olfactory Perception – Smell

What is smell?

The substance that you’re “smelling” has been emitted from that source as gas molecules, which in turn you take into your body

Different from light, sound & cutaneous stimulation, which involves transformations of receptors

Molecule detectors


Olfactory perception smell2

Olfactory Perception – Smell

We have not yet defined the physical parameters the way we have in other senses (i.e., frequency, spatial contrast)

We have not yet defined the perceptual experiences like in other senses such as vision (brightness, color) or hearing (loudness, pitch)

Very difficult to manipulate the stimulus-medium (i.e., how do you control the concentration of airborne molecules?)

  • We know something about the receptors and neuroanatomy, but without understanding the stimuli we remain quite limited in what we know.

  • Net result: smell is still largely not understood


Significance of smell

Significance of Smell?

General alarm system (smoke, disease)

Component of flavor (quality of food)

Part of communication (pheromones)

Discrimination of social context (family, gender, attractiveness…)

  • General rules: smells good  “safe”

  • smells bad  “dangerous”


How can we classify odors

How can we classify odors?

Already stated, we do not have the yardsticks of frequency or spatial measurement

Largely done by description of examples

“Soap smells like lavender”


Olfactory perception smell

Olfactory system fun facts: human and dog receptors are the same

Human receptors work as good as dogs even though the dog’s ability to smell is 300 to 10,000 times more sensitive then humans

Difference? Number of receptors.

Humans = 10 million

Dogs = 1 billion


Olfactory perception smell

Olfactory system fun facts: human recognize their own odor and can distinguish between male v. female odors

Russell (1976) had subjects where a shirt for 24-hours consecutively, then sealed the shirts in plastic bags

Subjects could recognize there own shirt (from a set of 3) 75% of the time (chance: 33%)

Subjects could recognize the strangers (male and female) also 75% of the time


Olfactory perception smell

Sekuler & Blake, 2002 and can distinguish between male v. female odors


Olfactory perception smell

  • Six and can distinguish between male v. female odorsprimary (independent) odor qualities: fragrant, etheral, resinous, spicy, burned, putrid

  • They form a basis to describe any smell sensation as combination of these 6 components

  • (similar to Trichromatic color vision)


Olfactory perception smell

  • This corresponds to 6 and can distinguish between male v. female odors types of receptors, which are reflected by specific anosmias ('smell blindness') – though the actual number of receptors are still disputed


Olfactory perception smell

Can we localize smell? Comparisons to other senses, we are very poor at this

Experimentally, this problem is less easy to approach than in vision or hearing?!

Some researchers invented some ingenious sniffo-meters…

NOTE: other animals seem to perform much better (dogs)



Olfactory perception smell

Olfactory system from the top down very poor at this

  • Odors pass through the nose and mouth into the sinuses and across the Olfactory Mucosa (why don’t we smell much through our mouth?)

  • Olfactory bulbs are above the Mucosa & bone

  • Olfactory Receptor Neurons (ORN) run through the Mucosa

  • Cilia of ORN are imbedded in base of the Mucosa (look like roots growing down into Mucosa)


Olfactory perception smell

Receptor pathway of smell: very poor at this

  • From bottom up:

  • Olfactory Receptor (on the end of the cilia) 

  • Olfactory Receptor Neuron 

  • Olfactory Bulbs (which are the beginning of the Central Nervous System (CNS))

  • Olfactory Tract (to higher (CNS) sites)


Olfactory perception smell

The Olfactory Receptor very poor at this

  • The olfactory receptor at the end of the cilia are little proteins (like in the visual system)

  • The OR is the actual site of transduction from chemical reactions to electrical impulses

  • 1000 different protein patterns (each of which respond to the same group of odorants)

  • All OR’s are the same on each ORN (1000 different types)


Olfactory perception smell

Organization of olfactory mucosa and olfactory bulb: zones very poor at this

10 Million Olfactory Receptor Neurons (ORN) synapse on 1-to-2,000 Glomeruli

Translation: 5-to-10,000 ORN’s per Glomerulus  lots of transduction at Olfactory Bulb


Olfactory perception smell

Central Nervous System anatomy of smell: starting with the Olfactory Bulbs (in Brain) to…

  • Primary Olfactory Cortex: Piriform Cortex

  • Hypothalamus (lateral)

  • Thalamus (medial-dorsal)

  • Inferior Frontal Lobe

  • Secondary Olfactory Cortex: Orbitofrontal Cortex

  • Temporal Lobe


Olfactory perception smell

Sites in the Cortex for Olfactory Processing: a work in progress (Zatorre, et al., 1992)

  • Primary Olfactory Cortex: Piriform Cortex

  • Inferior Frontal Lobe

  • Secondary Olfactory cortex:Orbitofrontal Cortex

  • Temporal Lobe


What do we know about neural coding or central brain processing for odor
What do we know about neural coding or central (Brain) processing for odor?

  • Almost nothing.

  • Only thing clear? Different olfactory receptor neurons (ORN’s) fire in a coordinated pattern to an odorant.

  • Piriform cortex is the primary receiving area, but we don’t know what it is doing.

  • Orbital Frontal cortex is secondary receiving area where there is some evidence of differential responding to different combinations of odors.