Fruit fly study reveals how neural gates switch on short-term memory

A study in fruit flies led by researchers at NYU Langone Health has confirmed a long-standing suspicion about how the brain encodes short-term memories.

Unlike the human brain, which has tens of billions of neurons, the fly brain has fewer than 200,000. Despite its relative simplicity, though, the fruit fly brain bears a striking resemblance to its human counterpart, especially in terms of how it is organized and how the brain cells called neurons communicate with each other.

The fruit fly is a valuable model for neuroscientists because all the connections between the neurons—together called the “connectome“—have been fully mapped, meaning that researchers can directly study how different neuron types interact to cause certain behaviors.

“Our study shows how a neuronal circuit forms a short-term memory in response to a fly sensing an odor, enabling it to remember a direction and travel toward a smell it wants to remember,” said study senior investigator Katherine Nagel, Ph.D., an associate professor in the Department of Neuroscience at NYU Grossman School of Medicine. “Scientists have long assumed an arrangement like this powers working memory, but our experiments confirm this arrangement exists and show how it works in a specific context.”

Working memory needs stability and flexibility

In the new study, published online Oct. 7 in the journal Nature, the researchers examined how neurons interact to encode information into working memory—more specifically, how neural interactions can be both stable (that is, capable of being maintained over a period of time) and quickly turned on or off.

People need to be able to quickly activate working memory, such as by temporarily remembering the few digits of a security code. But it doesn’t make sense for people to waste energy by holding on to unnecessary information, such as by memorizing every set of numbers they come across during the day.

An odor unlocks a memory circuit

When the researchers exposed fruit flies to a heady whiff of apple cider vinegar, the flies traveled toward the odor, even for a few seconds after the smell disappeared. While monitoring the flies’ brains during this process, the researchers found that two different types of neurons responded to the smell with similar patterns of electrical activity, leading them to believe that the cells were working together to control the flies’ movement in response to the odor.

The researchers found that the two types of neurons, called PFG and hΔK, form what is known as an attractor network, a type of neural circuit in which a set of neurons “talks” to itself until a stable signal emerges. In this case, though, there is a twist.

PFG and hΔK are not continuously communicating with each other. Most of the time, hΔK activity is blocked. When communication between the PFG and hΔK neurons is blocked, the PFG neurons track the fly’s orientation in space by receiving information from the fly’s “compass” system.

But when the block is lifted and the hΔK and PFG neurons can communicate with each other, the fly is able to lock in on a particular origin, such as the source of a particular odor, so it can then move toward it.

Researchers call this system a “split attractor network,” in which PFG receives the content of a memory, hΔK controls the timing of the memory formation, and the communication block acts as a gate. This combination provides the flexibility and stability of the signal that working memory requires.

Mapping the circuit’s wider role

“Right now, one of the frontiers in neuroscience is understanding what specific networks are doing, and the fruit fly is one of the best models to study that,” said Nagel. “The fly has an amazing track record for revealing how human biology works in a clear and simple way. My hope is that it gives us insight into processes like working memory that we have not yet had the tools to study in depth.”

Moving forward, Nagel said her laboratory wants to learn how this circuit is controlled across different time frames and to characterize the kinds of information other types of neurons are tracking. The team also wants to understand how and why different regions in the brain are capable of controlling similar functions simultaneously.

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

Full profile →