Smell Is A Democracy, Not A Dictatorship

Odors activate neuronal structures called glomeruli in the brain; the fruit fly’s smell preference depends on most, if not all, of them.

AsianScientist (Jun. 17, 2016) – Researchers at the RIKEN Brain Science Institute in Japan have revealed that smell preference is not decided by a small number of structures called glomeruli in the brain, but depends on most, if not all, of them.

The research, which is published in Neuron, shows how the activity of neurons in the fruit fly (Drosophila melanogaster) brain can be decoded to predict behavioral responses to odors, and reveals that depending on the situation, the relative preference of odors can flip.

For many animals, the sense of smell—the ability to detect and interpret chemicals in the environment—is fundamental to survival. 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.

Odors activate a population of small neuronal structures called glomeruli in the first olfactory center of the brain. However, 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, study leader Dr. Hokto Kazama and his team instead focused on the simpler olfactory system in Drosophila melanogaster, which is similar in function and organization to that of mammals but contains only about 50 glomeruli.

Fly behavior was monitored in an innovative flight-simulator arena, where the flies displayed a continuum of responses ranging from strong attraction to strong aversion—virtually flying into or away from the odor. Their judgments were made extremely quickly, sometimes in as short as 200 milliseconds.

From these data, researchers formulated a mathematical model that explains how attraction and repulsion to odors 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 behavioral responses to the odors used to make the model, but also accurately predicted responses to new odors.

Hence, 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 odors present in the environment. Through further experiments, the team successfully verified this prediction.

“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,” said Kazama.

As the basic function and wiring of the olfactory system are well-conserved from flies to humans, the study provides a deeper understanding of the principles and mechanisms of olfactory processing in the human brain.

The article can be found at: Badel et al. (2016) Decoding of Context-Dependent Olfactory Behavior in Drosophila.


Source: RIKEN; Photo: Ash Berlin/Flickr/CC.
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