It has long been thought that olfaction plays a role in emotion. In fact, Lieff and Alper (1988) even went as far as to call it our “most emotional sense”. This is because odors have such a strong influence over our moods that they can evoke feelings of both pleasure and displeasure, where one whiff can transport us back to memories of times gone by... (Ehrlichman & Bastone, 1992).
Where olfaction meets emotion
Researchers spent years looking into the brain to try to identify this link between olfaction and emotion. They showed that the olfactory system is directly connected to regions of the brain that are involved in emotional processing, such as the amygdala and the orbitofrontal cortex (Gottfried, 2010). They concluded that these connections allow odors to elicit emotional responses directly by bypassing conscious cognitive processing and evoking immediate emotional reactions (Herz, 2009).
Olfactometers: The more you nose
One Dutch physiologist, Hendrik Zwaardemaker, was curious about these ideas of cognition and olfaction. So, he redirected his research to focus on questions like:
“What are our principal and most elementary sensations of smell"
"How do we become conscious of these sensations?"
"Why do these sensations influence our thinking?”
To explore this, he designed what he later dubbed an 'Olfactometer' (Nicolas & Bensafi, 2021). His rudimentary device was designed to deliver controlled odor stimuli to either animal or human subjects. With his understanding that odor perception is only possible when the odor molecules coming off of the odorous materials are brought into the upper cavity of the nose by atmospheric air, his earliest olfactometer was designed so that odor molecules would flow from the outer tube into the inner tube and finally up into the nostrils.
Zwaardemaker's early olfactometer design with three parts: (1) a cylindrical outer tube containing the odorous material, (2) a glass inner tube positioned inside the outer tube which extends 90 degrees into the participant’s nose, and (3) a wooden, hand-held mount for the inner tube with a handle for better orientation.
The modern olfactometer
Major advancements in the field of olfaction have led to far more sophisticated systems in recent years, designed for insect, human and animal testing. Today, olfactometers largely consist of a series of tubes or chambers through which controlled amounts of odorants could be delivered to the participant's nose (Nicholas & Bensafi, 2021). This control is most often achieved by automated computer-driven systems which regulate the air flow using solenoid valves connected to odor jars (Aviles-Rosa et al.I, 2021).
An example of a commercially available olfactometer: the Aurora Scientific 220A
Today, they are widely applicable across various fields, including environmental monitoring, food science, psychology, and neuroscience, and enable the assessment of odor perception thresholds, discrimination abilities, and preferences. They typically consist of a delivery system for odorants, such as air dilution systems or liquid odor reservoirs, along with a means for subjects to respond to stimuli, such as response buttons or rating scales (Rabin and Cain, 1986; Smeets and Dalton, 2005).
Applications: how we can harness this potential
Olfaction has prospects to be used as a powerful cognitive tool for the treatment of mental health conditions (PTSD; Daniels & Vermetten, 2016). For example, virtual reality (VR) technology incorporating odors has emerged as a promising tool for treating post-traumatic stress disorder (PTSD). Olfactory virtual reality (OVR) capitalises on the emotional aspects of scent-triggered memories, making it uniquely effective for PTSD treatment, relaxation, and emotional well-being. Strategies like desensitisation and exposure therapy can be augmented with OVR, while preemptive measures such as habituation and reappraisal training may help prevent PTSD. However, challenges like odor habituation and technological limitations need to be addressed. With the anticipated rise in PTSD due to the COVID-19 pandemic, OVR treatment methods are expected to play a crucial role in diverse settings. Beyond PTSD, OVR shows promise in improving overall emotional well-being. This suggests a broad potential for OVR technology in clinical practice, paving the way for further innovation and research to enhance quality of life and mental health outcomes.(Herz, 2021).
Ethics: ensuring nothing smells fishy
To lay this foundation, we must first explore odor discrimination using brain imaging techniques to understand how different regions of the brain respond to various odors. Understanding the neural mechanisms underlying odor discrimination in mice can provide valuable insights into the emotional state of both mice and humans, and how it may be altered in conditions such as PTSD. But, to study the brain in this way requires an invasive surgery to allow for miniscope imaging access to the olfactory bulb. To do this, we work in mouse models. So, it is crucial to minimise harm and distress to the mice, adhere to high standards of animal care, and prioritise the principles of Replacement, Reduction, and Refinement (3Rs) across all research.
If you want to know more
Below, you can find a video that can give you more about this topic, if you are interested.