Every day, our brains transform fleeting experiences into long-term memories. While the process remains a mystery, neuroscientists believe it involves neural replay, where neurons recreate activation sequences from original experiences. This replay can occur both before and after an experience, aiding memory storage and future planning.

In a new study, researchers at the University of California, Berkeley recorded neuron activity in freely flying bats. It marks the first time ensembles of neurons have been studied together in bats during natural behavior. The data provided insights into neural replay and theta sequences, phenomena believed to be involved in memory and planning.

“For the past 20 years, we’ve been recording single neurons in bats and asking the question, ‘When animals are doing interesting things, what do individual neurons do?’” said Michael Yartsev, associate professor of neuroscience and bioengineering at UC Berkeley. “But in the brain, there are emerging properties that you only see when you’re looking at ensembles of neurons.”

Understanding replay and theta sequences could shed light on how long-term memories form in humans, potentially leading to treatments for neurological disorders like Parkinson’s disease and Alzheimer’s.

The study was published online on July 9 in Nature and supported by grants from various scientific organizations.

Studying these phenomena requires monitoring many neurons simultaneously. Over the past decade, Yartsev's lab has pioneered wireless neural recording technologies in Egyptian fruit bats. Previously limited to small numbers of neurons, co-first authors Angelo Forli, Wudi Fan, and Kevin Qi used high-density silicon electrode arrays to record hundreds of neurons at once from flying bats.

“It’s a whole different ball game to record such large ensembles of neurons wirelessly in a flying animal,” Yartsev said.

The researchers tracked "place cells," which fire when an animal is in a specific location. “If you know that a place cell corresponds to a specific location in space...you can infer that the bat is in that location,” Forli explained.

Place cells also exhibit hippocampal replay during rest. Experiments showed replays mostly occur minutes after an experience at locations distant from where it took place. Surprisingly, all flight trajectories had replays compressed to the same length regardless of distance.

“We saw that replays for short versus long trajectories had the same duration,” Forli noted.

This constant duration may represent an elemental unit of information processing. “From a computational perspective...it’s very efficient because whatever is reading that information out knows it will arrive in these fixed sizes,” Yartsev added.

Theta sequences support replay but rely on theta oscillations absent in bats. Researchers found sequential network activity synced with wingbeats rather than oscillations as seen in rodents.

“There’s something about this frequency which is ubiquitous across species,” Yartsev said. “Our findings may provide...understanding of these behaviors not only in rats and bats but maybe also humans.”

Additional support came from several foundations including Howard Hughes Medical Institute.