From odour to action – how smells are processed in the brain and influence behaviour

While humans primarily use their vision to navigate their environment, the vast majority of organisms on Earth communicate and experience the world through olfaction – their sense of smell.

We are members of Odor2Action, an international network of over 50 scientists and students using olfaction to study brain function in animals. Our goal is to understand a fundamental question in neuroscience: How do animal brains translate information from their environments to changes in their behaviours?

Here, we trace the interconnections between smells and behaviours – looking at how behaviour influences odour detection, how the brain processes sensory information from smells and how this information triggers new behaviours.

https://www.youtube.com/watch?v=58U52lDTuvk

Detecting odour in the environment

When the odour of a flower is released into the air, it takes the shape of a wind-borne cloud of molecules called a plume. It encounters physical obstacles and temperature differences as it flows through space. These interactions create turbulence that splits the odour plume into thin threads that spread out as the scent moves away from its source. These filaments eventually reach an animal’s nose or an insect’s antenna.

Odours that are broken up into filaments present a challenge to animals using them to find food or mates or avoid threats. It becomes difficult to predict precisely where the odour is coming from. Is the source directly ahead, to the left or right, above or below?

https://www.youtube.com/watch?v=jQaHbHMrqmE

To work around this, animals have evolved what are called active sensing behaviours that improve their ability to detect and find odours in the environment.

When a fly detects the smell of fruit or a mosquito detects carbon dioxide from a possible host, for example, both insects first move upwind to get closer to the odour of the food source. They then move in a meandering, back-and-forth motion called casting to find more odour threads before surging upwind again. If they lose the scent, they’ll start casting again until they find the scent. Larger animals, such as mice and dogs, also alternate between more directed movements and more exploratory searching actions.

Animals also move their noses and antennae to improve the chances that they’ll encounter an odour. This is why dogs raise their noses in the air to increase the amount of odour they can sniff, and why insects move their antennae to stir up and penetrate the air to make better contact with odour molecules.

Once information from odours tell the animal that they’re close to the source, visual searching then comes into play.

Making sense of odours

When an animal comes into contact with an odour plume, it detects the presence of these odour molecules through tiny proteins called odorant receptors. These receptors are embedded in the sensory neurons lining its nasal cavity or antennae.

Each sensory neuron contains only one type of odorant receptor. And each type of odorant receptor has a different shape and set of chemical properties that determine which odours can bind to and activate it. Most of these receptors recognize multiple odours, and most odours can bind to multiple different receptors. What encodes the identity of a specific odour in the brain is determined by which combination of receptors are activated, and their relative strength of activation.

https://www.youtube.com/watch?v=MyHR6a-zJM0

An animal like a mouse has about a thousand types of odorant receptors. Having a large number of these receptors with diverse shapes allows the system to detect and distinguish between a very large number of chemically unique odours, including ones the animal has never encountered before. Most odours in the environment are often a mix of many different types of molecules. The smell of some flowers can be a blend of over 100 different chemical compounds.

Once an odour molecule binds to a receptor, sensory neurons send specific electrical signals into compartments of the brain called olfactory glomeruli. Different odours elicit distinct patterns of electrical activity across these regions, and this generates a specific neural representation of the odour in the brain.

An important step toward understanding olfaction is figuring out how different classes of odours map to different patterns of electrical signals in the brain.

Neuroscientists hypothesize that as these signals undergo successive stages of processing deep in the brain, sensory representations of odour are reformatted in ways that extract information most useful to survival. This could be whether the smell is coming from something nutritious, indicating a potential source of food, or it could help the animal identify whether the smell is coming from a potential competitor or predator.

These reformatted sensory representations form the basis for how animals perceive smell and determine what actions they take in response to this information.

Image: Hannah Gibbs / Unsplash