Olfaction Lecture Notes

Olfaction is important for hazard detection. A survey showed that individuals with smell loss experienced more food-related, gas, and work incidents.
Olfaction plays social functions, such as:
- Infants recognizing parents' smell, promoting bonding.
- Adult relationships through partner's odor, improving sleep quality.
Memory functions: Smells evoke powerful memories (Proust Effect).
- Olfactory evoked memories are automatically evoked and highly emotional.
Wellbeing: Odors impact physiology and feelings.
- Lavender can lead to relaxation (decreased blood pressure, heart rate, skin temperature).
- Lemon can improve cognitive performance in memory tasks.
Eating/Drinking: Smell is needed for flavor perception and regulates eating behavior.
- Food odors can increase appetite for specific food groups.

Orthonasal vs Retronasal Route: Orthonasal is sniffing a flower, retronasal is tasting a lollipop.
Peripheral Olfactory System: Odor molecules are transported through nasal passages to olfactory receptors in the epithelium.
- Odor perception is based on specific patterns of receptor neuron activation.
Central Olfactory Network: Olfactory signals are transferred from the olfactory bulb to primary and secondary olfactory cortices.
Trigeminal System:
- Most odors activate the trigeminal system, leading to somatosensory sensations (pain, temperature).

Piriform Cortex: Processes the chemical structure and perceptual dimensions of odors (pleasantness, quality).
- May also play a role in semantic processing of odor-related information.
Entorhinal Cortex: Acts as a gateway to the hippocampus and is impacted early in neurodegenerative diseases.
Amygdala: Processes emotion, influencing the emotional quality of olfactory memories, odor pleasantness/intensity, motor responses, and cross-modal processing.
Hippocampus: Shows strong connectivity with primary olfactory regions, explaining the unique nature of olfactory memories, stores episodic and semantic knowledge of olfactory concepts.
Thalamus: Relays olfactory signals to higher-order regions like the orbitofrontal cortex; unique as a secondary processing region.
Insula: Evaluates the valence of olfactory stimuli; lesions can cause pleasant odors to be perceived as unpleasant.
Orbitofrontal Cortex: Key role in conscious and unconscious olfactory processing, valence processing, and multisensory integration.

Olfactory Cross-Modal Interactions: Occur when smell interacts with other senses (e.g., vision).
- Shaped by repeated exposure, physiological similarities, semantic similarities, and emotion.

Electrophysical Methods: Measure electrical activity in the brain when smelling odors (OERPs).
Imaging Methods:
- MRI measures the volume of olfactory structures (e.g., olfactory bulb) to indicate olfactory loss.
Psychophysical Methods:
- Test conscious perception; subjective measures classifying normal vs. abnormal function.
- Tests measure olfactory threshold, discrimination, and identification.
- Examples: University of Pennsylvania Smell Identification Test, Sniffin’ Sticks Olfactory Test Battery.
Self-Rated Questionnaire Methods:
- Subjective measure through questionnaires (e.g., English Olfactory Disorders Questionnaire).

Olfactory dysfunction is often understudied and lacks treatment options.
Impacts patients by:
- Altering eating habits (increased or decreased eating).
- Causing anxiety about hazards and hygiene.
- Triggering emotions like frustration, isolation, and sadness.
- Affecting intimate relationships and bonding with children.

Types: Impacts strength (quantitative) or quality (qualitative) of odor perception.
- Includes hyperosmia, normosmia, hyposmia, anosmia, parosmia, and phantosmia.
Causes:
- Age, Infections, Head Injury, Sinonasal disorders.
- Neurodegenerative diseases, Idiopathic causes.

Cognitive Effects:
- Impacts memory for events and facts.
- Leads to poorer olfactory imagery quality.
Treatments:
- Treatment options are lacking and vary based on the cause.
- Olfactory training improves olfactory test scores in post-infectious cases.