Odor Encoder

Ensembles of as few as 25 mushroom body neurons allow Drosophila to distinguish and react to specific odors. 

By | October 1, 2013

VISUALIZING SMELL: Neurons in the mushroom bodies of Drosophila, shown in this composite image, help the flies discriminate among a large number of odors.COURTESY OF ROB CAMPBELL AND GLENN TURNER


The paper
R.A.A. Campbell et al., “Imaging a population code for odor identity in the Drosophila mushroom body,” J Neurosci, 33:10568-81, 2013.

The finding
In the fruit fly (Drosophila melanogaster), pheromones and other highly salient odors have direct neural links to the brain from antennal olfactory receptor neurons. However, there are more smells that the flies must distinguish than there are odor-processing neurons, says Glenn Turner, a neuroscientist at Cold Spring Harbor Laboratory in New York. Imaging the brains of flies exposed to different smells, Turner and colleagues discovered that many odors are encoded by small, nonlocalized ensembles of neurons, allowing the insects to discriminate among vast numbers of closely related scents.

The discrimination       
The researchers used two-photon calcium imaging to record neural activity in immobilized flies exposed to various odors. Focusing on the mushroom body, an area that plays a role in olfactory learning and memory, the experiments revealed sets of neurons that encoded particular odors. Behavioral experiments demonstrated that the less the neural ensembles overlapped, the better the flies were able to differentiate the odors.

The neural network
As few as 25 of the fly’s 2,000 or so mushroom body neurons encode enough information about an odor to account for the fly’s performance in the behavioral discrimination task. The researchers also demonstrated that flies could generalize a learned aversive association when smells were represented by largely overlapping neural sets.

The link
“This work provides a powerful correlation between population cell activity and behavior,” says Vivek Jayaraman, a neuroscientist at the Janelia Farm Research Campus in Ashburn, Virginia, who was not involved in the study.


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