Egyptian Fruit Bat (Rousettus aegyptiacus) in flight. Taken at Rothschild Boulevard, Tel Aviv, Israel. Credit score: Zoharby/Wikipedia
Day-after-day, our mind takes numerous fleeting experiences—from walks on the seashore to displays at work—and transforms them into long-term recollections. How precisely this works stays a thriller, however neuroscientists imagine that it includes a phenomenon known as neural replay, through which neurons quickly recreate the identical activation sequences that occurred throughout the unique expertise. Surprisingly, neural replays can occur each earlier than and after an expertise, suggesting they assist in each reminiscence storage and in addition future planning.
In a brand new examine showing in Nature, neuroscientists on the College of California, Berkeley, recorded the exercise of a whole lot of neurons concurrently in freely flying bats. It’s the first time that an ensemble of neurons—reasonably than simply particular person neurons—has been studied in live performance in bats as they fly round and behave naturally.
The info supplied shocking new insights into neural replay and theta sequences, one other phenomenon which is believed to be concerned in reminiscence 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?'” stated examine senior writer Michael Yartsev, an affiliate professor of neuroscience and bioengineering and a Howard Hughes Medical Institute Investigator at UC Berkeley.
“But in the brain, there are emerging properties that you only see when you’re looking at ensembles of neurons. In this study, we looked at these two phenomena—replay and theta sequences—that are only visible when you track many neurons at the same time.”
A clearer understanding of the position of replay and theta sequences within the brains of animals might make clear how long-term recollections are shaped and saved in people, probably resulting in new remedies for neurological issues like Parkinson’s illness and Alzheimer’s.
‘An entire completely different ball recreation’
Finding out neural replay and theta sequences is difficult as a result of it requires listening in on tens or a whole lot of neurons within the mind concurrently. Over the previous decade, Michael Yartsev’s lab has pioneered wi-fi neural recording applied sciences in Egyptian fruit bats, giving his group an unprecedented view contained in the brains of those navigational specialists as they forage in massive environments.
Beforehand, wi-fi recording units have been solely in a position to detect indicators from small numbers of neurons at a time. Within the new examine, co-first authors Angelo Forli, Wudi Fan and Kevin Qi efficiently utilized high-density silicon electrode arrays that may file a whole lot of neurons directly from flying bats. These electrodes also can file native discipline potentials, a measure of the general electrical exercise in a area of the mind.
“It’s a whole different ball game to record such large ensembles of neurons wirelessly in a flying animal,” Yartsev stated. “This was never possible before now.”
To check neural replay and theta sequences, the researchers tracked the exercise of “place cells,” a sort of neuron that’s discovered within the hippocampus of many species. Particular person place cells hearth when an animal is in a particular location in area, creating an inside spatial map of their atmosphere.
“If you know that a place cell corresponds to a specific location in space, and the cell is active, then you can infer that the bat is in that location,” stated Angelo Forli, who’s a postdoctoral researcher at UC Berkeley. “If you can track multiple cells, you can know the path that the bat took.”
However place cells aren’t solely energetic when an animal is transferring round. Experiments in rodents have proven that they exhibit hippocampal replay throughout relaxation, basically refiring in the identical sequence as they did throughout the motion however in a shorter, time-compressed format.
Place cells in rodents additionally exhibit patterns known as theta sequences, which occur throughout motion, and are believed to signify the animal “looking ahead” only a few steps from its present location.
“Previously, these phenomena were exclusively investigated in rodents, because that’s what the technology allowed. We wanted to find out if they also exist in bats, and if they do, are they any different from what we see in rodents?” stated Forli. “We discovered a series of differences that challenge established models.”
A basic unit of knowledge processing
Within the experiment, the researchers recorded the exercise of bats’ place cells as they flew freely round a big flight room and recognized which sequences of place cells corresponded with particular trajectories. They have been then in a position to determine replay occasions, or moments when these similar neural sequences occurred when the bats have been at relaxation.
Most of what we learn about replay has been gleaned from experiments on rodents in unnatural settings, equivalent to a “sleep box,” to file replay occasions following behavioral runs. This introduces synthetic boundaries between energetic and inactive states.
In distinction, bats have many pure energetic durations and relaxation durations throughout the similar experimental session, permitting for the seize of replay below much less restrictive circumstances. This led to the invention that replays principally happen minutes after the expertise, and infrequently at areas distant from the place the expertise passed off.
Surprisingly, the researchers additionally discovered that the size of those replay occasions was the identical for all flight trajectories, regardless of how lengthy the flight was. Basically, if one neural sequence corresponded to a 10-meter flight, and one other neural sequence corresponded to a 20-meter flight, the replays of each of these sequences have been time-compressed to the identical size.
“We saw that replays for short versus long trajectories had the same duration,” Forli stated. “It seems that information is cut down to the same chunk of time regardless of the length of the experience.”
The researchers hypothesize that this fixed replay length could signify an elemental unit of knowledge processing within the mind.
“From a computational perspective, it’s incredibly advantageous to send fixed packets of information,” Yartsev stated. “It’s very efficient because whatever is reading that information out knows it will arrive in these fixed sizes.”
The group’s subsequent query involved theta sequences, a sort of ensemble phenomenon that’s believed to assist replay and to depend on theta oscillations within the hippocampus. Nonetheless, not like rodents, bats and people each lack steady theta oscillations, which happen at a frequency of roughly 8 Hertz, or eight wingbeats per second.
Apparently, the researchers discovered sequential community exercise throughout flight in bats, akin to theta sequences in rodents, however with one main distinction: not like rodents, the quick sequences in bats had no relationship to theta oscillations, however have been, as an alternative, synced to the bats’ 8 Hz wingbeats.
From the quivering of a mouse’s whiskers to the rhythms of human speech, there are numerous different animal behaviors that happen at frequencies round 8 Hz. The researchers hypothesize that these theta sequences may present a common neural mechanism for the way these behaviors are organized and directed within the animal mind.
“There’s something about this frequency which is ubiquitous across species, particularly mammalian species,” Yartsev stated. “Our findings may provide the beginning of a mechanistic understanding of the neural basis of these behaviors, not only in rats and bats, but maybe also in other species like humans.”
Extra info:
Replay and illustration dynamics within the hippocampus of freely flying bats, Nature (2025). DOI: 10.1038/s41586-025-09341-z. www.nature.com/articles/s41586-025-09341-z
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