The sleeping brain may predict future experiences

In a fascinating discovery, researchers have found that during sleep, some brain cells not only replay recent experiences but also prepare for future ones. 

The research suggests that precognitive dreams, or dreams that predict the future, may be more than just a coincidence. The brain may actually anticipate upcoming experiences.

Focus of the study

The study, conducted by researchers from Rice University and the University of Michigan, was focused on sleep and learning. It offers new insights into how the hippocampus of rats stabilizes and tunes spatial representations after they navigate a maze for the first time.

“Certain neurons fire in response to specific stimuli,” said study co-author Professor Kamran Diba. “Neurons in the visual cortex fire when presented with the appropriate visual stimulus. The neurons we’re studying show place preferences.”

New experiences and memory storage

The experts investigated how these specialized neurons form representations of the world post-experience. They focused on sharp wave ripples, a pattern of neuronal activation crucial for consolidating new memories and tagging parts of new experiences for memory storage. 

“For the first time, we have observed individual neurons stabilizing spatial representations during rest periods,” noted Rice neuroscientist Caleb Kemere.

Sleep and memory consolidation 

The connection between sleep and learning has been gradually understood over many years, with significant advancements in the 20th century. 

Research in the early 1900s began to explore sleep’s impact on cognitive functions and memory consolidation. 

However, it wasn’t until the latter half of the 20th century that more detailed studies, using EEGs and other tools, began to clearly demonstrate sleep’s critical role in consolidating memories and learning.

This link is demonstrated by improved memory test performance following sleep compared to periods of wakefulness or sleep deprivation. 

Brain replays experiences during sleep

Previous research has also shown that neurons in the brains of sleeping animals replay their exploration paths, helping to transform new experiences into stable memories. In the current study, the researchers sought to explore this phenomenon further.

“We imagined that some neurons might change their representations – reflecting the experience we’ve all had of waking up with a new understanding of a problem,” said Kemere. 

“Showing this, however, required that we track how individual neurons achieve spatial tuning, i.e., the process by which the brain learns to navigate a new route or environment.”

Activity spikes in individual neurons 

The team trained rats to run on a raised track for liquid rewards and monitored spikes of individual neurons in the hippocampus.

By analyzing the average spiking rate, they estimated each neuron’s place field – the specific area in the environment that a neuron “cared” about the most.

“The critical point here is that place fields are estimated using the behavior of the animal,” explained Kemere, noting the complexity of measuring place fields during rest.

“I’ve been thinking for a long time about how we can evaluate the preferences of neurons outside of the maze, such as during sleep,” said Diba. “We addressed this challenge by relating the activity of each individual neuron to the activity of all the other neurons.”

Tracking the preferences of brain cells 

This innovative approach led to a statistical machine learning method that predicted where the animal dreamed it was, allowing estimation of each neuron’s spatial tuning during dreams.

“The ability to track the preferences of neurons even without a stimulus was an important breakthrough for us,” said Diba.

Both Diba and Kemere praised Kourosh Maboudi, a postdoctoral researcher at Michigan and the study’s lead author, for his role in developing this method.

The machine learning technique confirmed that the spatial representations formed during new experiences are stable across several hours of post-experience sleep. However, just as the researchers expected, there was more to the story.

Predicting future experiences during sleep

“The thing that I loved the most about this research and the reason that I was so excited about it is finding that it’s not necessarily the case that during sleep the only thing these neurons do is to stabilize a memory of the experience,” said Kemere. “It turns out some neurons end up doing something else.”

“We can see these other changes occurring during sleep, and when we put the animals back in the environment a second time, we can validate that these changes really do reflect something that was learned while the animals were asleep. It’s as if the second exposure to the space actually happens while the animal is sleeping.”

This represents a direct observation of neuroplasticity occurring during sleep. Kemere noted that while most plasticity research examines waking periods, this study highlights the brain’s ability to rewire and form new representations during sleep.

The research was supported by the National Institutes of Health. The findings are published in the journal Nature.

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