1. Olfaction
The Sense of Smell

2. Smell helps us: Avoid dangerous situations (fumes, smoke)
Avoid consuming poisonous substances and spoiled food (sour milk)
Choose mates?

3. What are olfactory stimuli?
Odorants (airborne molecules)
Odorants must be volatile (they give off vapors)
More vapors are given off when an odorant is heated!! (warm soup smells better than cold soup)

4. Odorants reach olfactory receptors by:
1. Being inhaled through the nose.
2. Through the mouth (vapors circulate up through throat)

5. Olfaction- originates in the nasal cavity

6. Olfactory system structures

7. Olfactory Receptor Neurons
Carry impulses directly to the brain
Live for 5- 8 weeks & then die.

8. Summary: olfactory pathway
Olfactory receptor neurons detect odorants in mucosa.
Signals are sent via olfactory receptor neurons to bulb structures (glomeruli).
Mitral and tufted cells carry signals to orbitofrontal cortex, temporal lobe, and the limbic system.

9. Which animal is most sensitive to smell?
Dogs ,000 times more sensitive to smell than humans
Why? Humans have million olfactory receptor neurons, dogs have over a billion!!

10. Odor Hedonics
What are “pleasant” and “unpleasant” odors? Variability is high.
Most universally-rated “pleasant” odorant is vanilla.

11. How is perception affected by odor?
Odor enhances our experience of the world (food, surroundings, people).
We associate memories with certain odorants.
We lose the ability to experience “flavors” if our sense of smell is impaired.

12. Anosmia: loss of sense of smell
May be to specific odors or all odors.
Caused by infection to nasal cavity or brain injury (frontal lobe).
About 2 million people in US are anosmic.

13. Olfaction and behavior
A. Infant perception & attachment
Babies enthusiastically “orient” to pleasant odors (banana, vanilla) and cry/grimace to unpleasant odors (shrimp; rotten eggs).
Babies can discriminate their mother’s scent from other women (Macfarlane, 1975).

14. Olfaction and Behavior
B. Smell may alter hormonal activity
McClintock Effect: women’s menstrual cycles synchronize when they live together (dorms).

15. Olfaction and behavior
C. Mate Selection
Females rate males with immunity profiles more dissimilar to their own (Wederkind et al., 1995) as smelling “more pleasant.” Males with very similar immunity profiles are rated as smelling “less pleasant.”
What is the current theory underlying this finding?

16. Smell may provide a clue about our suitability as a mate?
It’s possible that choosing a mate with an immunity profile very similar to our own, may result in the expression of unwanted recessive genes in our offspring.
Smell may provide us with a “cue” in which we can detect suitable mates who will produce healthy viable offspring.