Bat Brains Unlock Secrets of Long-Term Memory

The intricate process by which our brains convert everyday experiences into lasting memories has long puzzled neuroscientists. Now, research on bats offers fresh insights into neural replay and theta sequences, crucial elements in memory storage and future planning.

Bats Offer New Perspectives on Brain Activity

Researchers at the University of California, Berkeley, have achieved a breakthrough by recording the activity of hundreds of neurons in bats as they flew freely. This is the first time such a large group of neurons has been studied together in bats behaving naturally. The data revealed surprising information about neural replay and theta sequences, which are believed to play a key role 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?’ 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.

—Michael Yartsev, study senior author, associate professor of neuroscience and bioengineering and a Howard Hughes Medical Institute Investigator at UC Berkeley

Understanding how replay and theta sequences function in animal brains could provide valuable insights into the formation and storage of long-term memories in humans. This understanding could lead to innovative treatments for neurological conditions such as Parkinson’s and Alzheimer’s diseases. Globally, the number of people living with Alzheimer’s is projected to nearly double by 2050, reaching 13.8 million (Alzheimer’s Association).

Pioneering Technology for Brain Research

**Michael Yartsev’s** lab has been at the forefront of developing wireless neural recording technologies in Egyptian fruit bats for over a decade. These advancements have allowed researchers to observe the brains of these skilled navigators in expansive environments.

The latest study, led by co-first authors **Angelo Forli**, **Wudi Fan**, and **Kevin Qi**, utilized advanced high-density silicon electrode arrays capable of simultaneously recording from hundreds of neurons in flying bats. These electrodes also capture local field potentials, which reflect the overall electrical activity in specific brain regions.

It’s a whole different ball game to record such large ensembles of neurons wirelessly in a flying animal, said Yartsev. This was never possible before now.

How Spatial Maps are Created

The scientists focused on “place cells,” a type of neuron found in the hippocampus, to investigate neural replay and theta sequences. Each place cell activates when an animal occupies a particular location, forming an internal spatial map.

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, explained **Angelo Forli**, a postdoctoral researcher at UC Berkeley. If you can track multiple cells, you can know the path that the bat took.

Rodent studies have revealed that place cells exhibit hippocampal replay during rest, essentially replaying the firing sequence from their movements in a compressed format. Also, place cells in rodents demonstrate patterns known as theta sequences during movement, seemingly anticipating the animal’s next steps.

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? said **Forli**. We discovered a series of differences that challenge established models.

Key Findings: Replay and Theta Sequences in Bats

The research team monitored bats’ place cell activity as they freely flew around a large flight room. They identified sequences of place cells that correlated with specific flight paths. This allowed them to pinpoint replay events, or instances where these neural sequences recurred when the bats were at rest.

Unlike rodent studies conducted in artificial “sleep box” settings, the bats’ natural active and rest periods allowed for replay capture under more realistic conditions. Consequently, replays were found to primarily occur minutes after an experience and often far from the original location.

Interestingly, the duration of replay events remained constant regardless of the length of the flight trajectory. Whether the neural sequence represented a 10-meter or 20-meter flight, the replay was compressed to the same duration.

We saw that replays for short versus long trajectories had the same duration, **Forli** noted. It seems that information is cut down to the same chunk of time regardless of the length of the experience.

This consistent replay duration may represent a basic unit of information processing within the brain, the researchers suggested.

From a computational perspective, it’s incredibly advantageous to send fixed packets of information, said Yartsev. It’s very efficient because whatever is reading that information out knows it will arrive in these fixed sizes.

While rodents rely on continuous theta oscillations, bats and humans do not. The researchers discovered sequential network activity during flight in bats, which resembled rodent theta sequences. However, in bats, these fast sequences were synchronized to their 8 Hz wingbeats rather than theta oscillations.

The team hypothesizes that these theta sequences may offer a universal neural mechanism for organizing and directing various animal behaviors, given the prevalence of 8 Hz rhythms across species.

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.