The discerning fruit fly: Linking brain-cell activity and behavior in smell recognition

June 18, 2013 by Edward Brydon Ph.d.
Fruit Fly
Fruit Fly. Credit: UCSD

Behind the common expression "you can't compare apples to oranges" lies a fundamental question of neuroscience: How does the brain recognize that apples and oranges are different? A group of neuroscientists at Cold Spring Harbor Laboratory (CSHL) has published new research that provides some answers.

In the fruit fly, the ability to distinguish smells lies in a region of the brain called the mushroom body (MB). Prior research has demonstrated that the MB is associated with learning and memory, especially in relation to the sense of smell, also known as olfaction.

CSHL Associate Professor Glenn Turner and colleagues have now mapped the activity of in the MB, in flies conditioned to have Pavlovian to different odors. Their results, outlined in a paper published today by the Journal of Neuroscience, suggest that the activity of a remarkably small number of neurons—as few as 25—is required to be able to distinguish between different odors.

They also found that a similarly small number of are involved in grouping alike odors. This means, for instance, that "if you've learned that oranges are good, the smell of a tangerine will also get you thinking about food," says Robert Campbell, a in the Turner lab and lead author on the new study.

These intriguing new findings are part of a broad effort in contemporary neuroscience to determine how the brain, easily the most complex organ in any animal, manages to make a mass of raw intelligible to the individual—whether a person or a fly—in order to serve as a basis for making vital decisions.

Looking closely at Kenyon cells

The neurons in the fly MB are known as Kenyon , named after their discoverer, the neuroscientist Frederick Kenyon, who was the first person to stain and visualize individual neurons in the insect brain. Kenyon cells receive from organs that perceive smell, taste, sight and sound. This confluence of sensory input in the MB is important for memory formation, which comes about through a linking of different types of information.

Kenyon cells make up only about 4% of the entire fly brain and are extremely sensitive to inputs triggered by odors, in which only two connections between neurons, called synapses, separate them from the receptor cells at the "front end" of the olfactory system.

But in contrast to other regions of the brain, such as the vertebrate hippocampus, the sensory responses in the MB are few in number and relatively weak. It is the sparseness of the signals in the Kenyon cell neurons that makes studying memory formation in flies so promising to Turner and his team. "We set out to learn if these signals were really informative to the animal's with regard to smell," Turner says.

In particular, Turner's group wanted to see if they could link these signals with actual behavior in flies. The team used an imaging technique that allowed them to view the responses of over 100 Kenyon cells at a time and, importantly, quantify their results. They found that even the very sparse responses in these cells that are triggered by odors provide a large amount of information about identity. Turner suspects the very selectiveness of the response helps in the accurate formation and recall of memories.

When the researchers used two odors blended together in a series of increasingly similar concentrations, they found that two very similar smells could be distinguished as a result of the activity of as few as 25 Kenyon cells. This correlated well with the behavior of the flies: when brain activity suggested the flies had difficulty discerning the odors, their behavior also showed they could not choose between them.

The activity of these cells also accounts for flies' ability to discern novel odors and group them together. This was determined in a "generalization" test, in which the degree to which flies learned a generalized aversion to unfamiliar test odors could be predicted based upon the relatively similar activity patterns of Kenyon cells that the odors induced.

"Being able to do this type of 'mind-reading' means we really understand what signals these activity patterns are sending," says Turner. Ultimately, he and colleagues hope to be able to relate their findings in the fly brain with the operation of the in mammals.

Explore further: Neuroscientists show activity patterns in fly brain are optimized for memory storage

More information: "Imaging a population code for odor identity in the Drosophila mushroom body" is published online in Journal of Neuroscience on June 19, 2013. The authors are: Robert Campbell, Kyle Honegger, Hongtao Qin, Wanhe Li, Ebru Demir, & Glenn Turner. The paper can be obtained online at DOI: dx.doi.org/10.1523/JNEUROSCI.0682-12.2013

Related Stories

Neuroscientists show activity patterns in fly brain are optimized for memory storage

August 18, 2011
We know from experience that particular smells are almost inseparable in our minds with memories, some vague and others very specific. The smell of just-baked bread may trigger an involuntary mental journey, even if for a ...

Flies reveal that a sense of smell, like a melody, depends upon timing

April 10, 2013
(Medical Xpress)—The sense of smell remains a mystery in many respects. Fragrance companies, for instance, know it is crucial that chemical compounds in perfumes reach nostrils at different rates to create the desired sensory ...

Researcher finds elderly lose ability to distinguish between odors

November 10, 2011
Scientists studying how the sense of smell changes as people age, found that olfactory sensory neurons in those 60 and over showed an unexpected response to odor that made it more difficult to distinguish specific smells, ...

How memory is read out in the brain: MB-V2 nerve cells enable the read-out of associative memories

July 8, 2011
What happens if you cannot recall your memory correctly? You are able to associate and store the name and face of a person, yet you might be unable to remember them when you meet that person. In this example, the recall of ...

A giant interneuron for sparse coding

May 13, 2011
A single interneuron controls activity adaptively in 50,000 neurons, enabling consistently sparse codes for odors.

Study shows how brain cells shape temperature preferences

January 29, 2013
While the wooly musk ox may like it cold, fruit flies definitely do not. They like it hot, or at least warm. In fact, their preferred optimum temperature is very similar to that of humans—76 degrees F.

Recommended for you

Brain stimulation may improve cognitive performance in people with schizophrenia

July 24, 2017
Brain stimulation could be used to treat cognitive deficits frequently associated with schizophrenia, according to a new study from King's College London.

New map may lead to drug development for complex brain disorders, researcher says

July 24, 2017
Just as parents are not the root of all their children's problems, a single gene mutation can't be blamed for complex brain disorders like autism, according to a Keck School of Medicine of USC neuroscientist.

Bird songs provide insight into how developing brain forms memories

July 24, 2017
Researchers at the University of Chicago have demonstrated, for the first time, that a key protein complex in the brain is linked to the ability of young animals to learn behavioral patterns from adults.

Working around spinal injuries: Rehabilitation, drug treatment lets rats recover some involuntary movement

July 24, 2017
A new study in rats shows that changes in the brain after spinal cord injury are necessary to restore at least some function to lower limbs. The work was published recently in the journal eLife.

Scientists capture first image of major brain receptor in action

July 24, 2017
Columbia University Medical Center (CUMC) researchers have captured the first three-dimensional snapshots of the AMPA-subtype glutamate receptor in action. The receptor, which regulates most electrical signaling in the brain, ...

Research identifies new brain death pathway in Alzheimer's disease

July 24, 2017
Alzheimer's disease tragically ravages the brains, memories and ultimately, personalities of its victims. Now affecting 5 million Americans, Alzheimer's disease is the sixth leading cause of death in the U.S., and a cure ...

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.