Stinky or fragrant? Predicting changing odor preferences

June 16, 2016, RIKEN
A fly navigating in a virtual flight simulator. Credit: RIKEN

Pleasant and unpleasant odors are a part of everyone's life, but how do our reactions to smells change when other odors are present? To answer this question, researchers at the RIKEN Brain Science Institute in Japan have combined experimental and modeling approaches to reveal the process through which smell preference is computed in the brain. Published in Neuron, the work shows how the activity of neurons in the olfactory processing center of the Drosophila brain can be decoded to predict behavioral responses to odors, and reveals that the relative preference of odors can flip depending on the situation.

For many animals, the sense of smell—the ability to detect and interpret chemicals in the environment—is fundamental to survival. From insects to mammals, olfaction is central to a range of behaviors, including foraging, habitat and prey location, predator avoidance, and social communication. While responding appropriately to odors requires the ability to distinguish those that are harmful from those that are beneficial, how this is achieved in the brain is an open question.

When an odor is smelled, it activates a population of small neuronal structures called glomeruli in the first olfactory center of the brain. While odor information is generally recognized to be encoded as patterns of glomerular activity across space and time, the sheer number of glomeruli—about 1,800 in mice and 5,500 in humans—is a major impediment to olfactory research.

To overcome this obstacle, Hokto Kazama and his team took advantage of the simpler olfactory system in the fruit fly Drosophila melanogaster, which is similar in function and organization to that of mammals, but contains only about 50 glomeruli. Explains Kazama, "Because of the limited number of glomeruli, we were able to use two-photon calcium imaging technology to systematically record odor-evoked activity from almost all fly glomeruli in response to a large number of odorants."

An image of a brain in which output neurons in glomeruli are labeledwith green fluorescence proteins. Credit: RIKEN

Fly behavior was monitored in a clever flight-simulator arena. In this virtual reality system, the fly's head is fixed and surrounded by an olfactory and visual landscape that is rotated in real-time in response to wing movements. The flies displayed a continuum of responses ranging from strong attraction to strong aversion—virtually flying into or away from the —and their judgments were made extremely quickly, sometimes in as short as 200 milliseconds.

By analyzing these behavioral and physiological data, researchers formulated a mathematical model that explains how attraction and repulsion to odorants can be computed from the activity of olfactory glomeruli. Their model suggests that each glomerulus contributes to attraction or aversion with a specific weight. Summing the transformed and weighted activity of all glomeruli not only matched the real to the odors used to make the model, but also accurately predicted responses to new odorants. Kazama notes that contrary to the prevalent hypothesis in the field, the results imply that this computation does not rely on a small subset of glomeruli, but likely requires most, if not all, of them.

The model also predicted that the relative preference of odors would vary, and could even switch, depending on the nature of other odorants present in the environment. The team performed a series of experiments in which the same odors were presented under different conditions, and successfully verified this prediction. Adds Kazama, "Not only does this demonstrate that even flies have the ability to adapt to their olfactory environment, it exemplifies the usefulness of our approach that combines physiological measurements with mathematical modeling of behavior and neural activity."

Because the basic function and wiring of the olfactory system are well conserved from flies to humans, the study is expected to provide a deeper understanding of the principles and mechanisms of olfactory processing in the human brain.

Explore further: New model show how the brain is organized to process odor information

More information: Neuron, DOI: 10.1016/j.neuron.2016.05.022

Related Stories

New model show how the brain is organized to process odor information

March 19, 2012
Just like a road atlas faithfully maps real-word locations, our brain maps many aspects of our physical world: Sensory inputs from our fingers are mapped next to each other in the somatosensory cortex; the auditory system ...

Understanding how neurons shape memories of smells

March 9, 2015
In a study that helps to deconstruct how olfaction is encoded in the brain, neuroscientists at University of California, San Diego School of Medicine have identified a type of neuron that appears to help tune, amplify and ...

Odor alternative: 'Olfactory necklace' detects scents in a way contrary to neurobiology dogma

May 26, 2016
Mammals have an exquisitely tuned sensory system that tells them whether they are smelling an orange or a rose. Like keys on a piano keyboard, each component of an odor blend strikes only one chord of olfactory neuron activation. ...

Revised view of brain circuit reveals how we avoid being overwhelmed by powerful odors

July 1, 2015
You've just encountered a frightened skunk, which has sprayed a generous quantity of its sulfur-containing scent directly in your path. The noxious odor is overpowering. As you run in the opposite direction, you are performing ...

Recommended for you

Your brain responses to music reveal if you're a musician or not

January 23, 2018
How your brain responds to music listening can reveal whether you have received musical training, according to new Nordic research conducted in Finland (University of Jyväskylä and AMI Center) and Denmark (Aarhus University).

New neuron-like cells allow investigation into synthesis of vital cellular components

January 22, 2018
Neuron-like cells created from a readily available cell line have allowed researchers to investigate how the human brain makes a metabolic building block essential for the survival of all living organisms. A team led by researchers ...

Finding unravels nature of cognitive inflexibility in fragile X syndrome

January 22, 2018
Mice with the genetic defect that causes fragile X syndrome (FXS) learn and remember normally, but show an inability to learn new information that contradicts what they initially learned, shows a new study by a team of neuroscientists. ...

Epilepsy linked to brain volume and thickness differences

January 22, 2018
Epilepsy is associated with thickness and volume differences in the grey matter of several brain regions, according to new research led by UCL and the Keck School of Medicine of USC.

Research reveals atomic-level changes in ALS-linked protein

January 18, 2018
For the first time, researchers have described atom-by-atom changes in a family of proteins linked to amyotrophic lateral sclerosis (ALS), a group of brain disorders known as frontotemporal dementia and degenerative diseases ...

Fragile X finding shows normal neurons that interact poorly

January 18, 2018
Neurons in mice afflicted with the genetic defect that causes Fragile X syndrome (FXS) appear similar to those in healthy mice, but these neurons fail to interact normally, resulting in the long-known cognitive impairments, ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.