Knowledge Clips: Olfactory Social Neuroscience

The ability of smell in humans is actually very good, as demonstrated by research conducted by Porter et al (2007). In this study, people were required to follow a trail of chocolate oil using only their noses, with their eyes and ears covered. The research revealed that human beings are proficient at performing and following scent trails, guided by their sense of smell. Moreover, humans are capable of discriminating more than a trillion smells.

Why do we look down on our ability to smell?

→ We find it hard to talk about smells and name them. Odors operate below the surface, affecting lower brain regions shared with other mammals and reptiles (limbic brain regions and even lower, regions that produce survival instincts). This bias might explain why we perceive smells as less important to us.

The smell brain: anatomy

→ The sense of smell is chemical, consisting of molecules.
→ Remarkable features include:
  - Limbic overlap: a connection between emotions and odors, with many regions close together and overlapping. This makes smell memories highly emotional.
  - Ipsilateral projection: smell information entering the brain on one side is processed on that same side, with no thalamic intermediary or delay.
  - Feedback to the bulb: top-down modulation, learning, and plasticity, affecting the olfactory bulb, allowing us to rewire which smells we attend to.

Key features of the function of smell:

→ Metastudies have identified typical regions that are active in smell:
  - Piriform cortex (PC): registers stimulus identity (what is this smell) and extends to the amygdala, targeting emotional relevance.
  - Orbitofrontal cortex (OFC)
  - Anterior insula (AI): encodes the affective property of the smell (whether we want to approach or avoid the smell).

Plasticity of olfaction: we can quickly learn the importance (and dangers) of smell, for example, the smell of gas.

Odor discrimination after conditioning:

→ Research by Li et al (2008) showed that when a stimulus (hand) was given a shock, the brain could update the importance of a particular smell in connection with that stimulus. Participants could distinguish between two stimuli (hands) even though both emitted the same odor, emphasizing the role of conditioning.

Smell functions:

- Approach/Avoid (for example, food)
- Detecting environmental hazards
- Social communication

Chemical communication: types of information we can smell:

- Neuro Identity (we are all different)
- Strong genetic components to body odor (monozygotic twins)
- The ability to smell illnesses
- Smelling ages, gender
- Smelling emotions

Experimental set-up:

→ In two conditions, the sender watched a horror movie and a nature documentary. The receiver was then exposed to the fear odor and neutral odor of the sender.
- Emotional expressions were measured.
→ The hypothesis was that there would be more sensory intake in the fear condition (evolutionary functional to detect threats).
What they found: Exposure to a fearful body odor elicits a fearful facial expression in the receiver. This process is called emotional contagion.

Senders' results:

→ Also, the higher the fear experience, the more molecules are emitted. However, over multiple trials, the molecules decrease.

Receivers' method (n=31, double blind, within subjects):

- Olfactometer presented 4 body odors randomly: neutral, low fear, medium fear, high fear.
- Tested in fMRI scanner
- Face morph task (FMT): classify emotion expressions morphed between disgust and fear: individually calibrated for a more sensitive task. Calibration of emotional expressions was done to create a more sensitive experiment.

Example of trial:

Validating FMT: a disgusting artificial sweat-like odor causes participants to see more disgust in morphed faces morphed between disgust and fear.

Fear odor-induced bias to see more fear in ambiguous faces. No quantitative effects:

→ This suggests that emotional information from smells and faces interact, changing our perception of the world.

Fear odor-induced physiological effects: nasal aperture:

Fearful odors open the nose, allowing for increased detection (evolutionarily functional to detect threats).

Fear odor-induced brain activity (vs. neutral) in social brain regions:

→ Different brain regions are active in response to fearful odors, particularly the left amygdala.

Finding pheromones needs a multidisciplinary approach.

Emotional disorders and smell:

1. Social anxiety disorder (SAD):

→ The startle reflexes to the smell of fear are higher in highly anxious subjects.

2. Autism spectrum disorder (ASD):

→ Individuals with ASD show altered responses to (social) smells/stimuli, like the smell of fear.

3. Psychopathy and smell:

→ The more psychopathic traits, the poorer the smell ability and the poorer the level of empathy. Less empathy has been linked to a lesser ability to smell.

Oxytocin: reduces the smell of fear effect:

→ Investigation by Maier et al (2018) explored whether oxytocin diminishes behavioral and neural responses to social chemosensory stress cues. Oxytocin was found to reduce activation in certain areas of the brain, such as the amygdala, the anterior cingulate cortex in females, and the hippocampus in males.

Urbach-Wiethe disease (UWD) and smell:

- Rare recessive genetic disorder (~400 reported cases since discovery in 1929)
- 1/4th in South Africa. Thought due to founder effect (Dutch, German settlers)
- Located on chromosome 1, extracellular matrix protein 1 (ECM1)
- Causes thickening of the skin, hoarse voice, (basolateral) amygdala damaged
- Typically can test only small samples, but marked effects:
  → Hyper reaction to innate fear stimuli, hypo reaction to learned fear
- Investigation of the smell of fear is an interesting subject for future research.

Summary:

- Social anxiety: increased fear odor response
- Autism spectrum disorder: altered fear response
- Psychopathy: instrumental odor response
- Oxytocin: buffered response to fear