Research shows how a fly’s brain learns to focus on more important odors

Quantitative biologists at Cold Spring Harbor Laboratory (CSH) have discovered how the brain of a fly learns to focus on more significant odors by ignoring tremendously prevalent, ordinary ones.

The study represents a major step towards interpreting the functioning of human senses and how computer sensing could be made to function better.

A dog with his nose buried in flowers is still able to notice odors from other plants and animals. To explain how this is possible, CSHL researchers studied fruit fly brains. Flies use a small cluster of neurons to process multiple odors, yet can still pay attention to the most important ones. Image Credit: Courtesy Pond5 (Sergej Razvodovskij).

Imagine a dog playing in a garden and smelling all of these different flowery smells, and then somewhere in the distance a predator appears, like a fox. The raw input coming into the dog’s nose is a smell that consists of something like 90% flowers and only five or 10% of this predator.”

Saket Navlakha, Associate Professor, Cold Spring Harbor Laboratory

Navlakha continued, “The question that we sought out to study here is, ‘how does this dog suppress this uninformative signal of all these flowers… and amplify the significant part, which is the predator, so that it then reacts appropriately?”

Dogs are known to have intricate brains, and hence the CSHL scientists resorted to fruit flies to better understand it.

How does the modest brain of a fly learn to focus on newer yet rarer odorants by ignoring prevalent, mundane odors? Such findings may be relevant to humans to dogs, and can potentially be used for training artificially intelligent machines.

Postdoctoral researcher Yang Shen and Navlakha demonstrated the way this amazingly intricate neural task, known as odor habituation, occurs in the brain of the fly and how it can be converted into a computer code. The study was published in the May 11^th, 2020 issue of the scientific journal, PNAS.

Shen informed that fruit flies often found themselves in situations where their small brains have to differentiate between a large number of “usual” smells and something that is likely to be more significant.

A fruit fly “has about 100,000 neurons in its brain and the actual connectivity of many of these neurons has now been mapped out,” informed Navlakha.

“This allows us to understand what are the mechanisms that the brain uses to solve this habituation problem from an algorithmic perspective,” Navlakha added. “So that became the launching point for this project.”

The scientists discovered that a crucial signal filtering process drives odor habituation. When an odor is detected by a fly, a few select neurons (known as Kenyon cells) in the insect react to it. The firing pattern in these responding neurons constitutes the so-called “tag” for the odor.

If an odor is continuously present but does not send any urgent message to the fly, then the neurons that constitute the odor’s tag will begin to reduce their activity over a period of time. This is the brain familiarizing with a background odor.

An odor tag that has fewer active neurons may not trigger a response in an organism when compared to an odor that has plenty of neuron activity.

Consequently, odors that are constantly around can be overlooked in favor of a new kind of odor that is weak but still significant.

Although a fly’s brain inhibits responses to the repeated odors, the process can also be reversed if the odor turns out to be more significant or rarer to the fly.

By framing this computationally, we can better our understanding of the whole process of how signals are being processed in the brain.”

Yang Shen, Postdoctoral Researcher, Cold Spring Harbor Laboratory

Navlakha is aiming to close this gap between neuroscience and computational research.

One of the goals of this entire research direction is to try and understand the brain as a kind of library of algorithms that has been evolved to solve basic information processing problems. This odor habituation process is something that has not been extensively used in, for example, robotics applications where you have a lot of streaming data and you want to filter unnecessary data.”

Saket Navlakha, Associate Professor, Cold Spring Harbor Laboratory

“It wasn’t our goal to say that this algorithm from neuroscience is better than anything else that anyone has developed. but it has been an opportunity to understand how biology solves this problem and how habituation affects our ability to perceive and discriminate odors,” Navlakha concluded.

When a fly is first exposed to a new smell (odor A) about 5% of the brain’s specialized odor neurons (Kenyon cells) become active, creating a unique activity tag. After habituation, the number of Kenyon cells that make up the tag for odor A are significantly fewer in number. A new, non-habituated odor (odor B) excites a large number of Kenyon cells in a new pattern related to the new odor. The fly pays the most attention to the odor that elicits the most activity—in this case, at first odor A and later, odor B. Video Credit: Cold Spring Harbor Laboratory.