July 27, 2022 – Feeling forgetful? Struggling to match names to faces lately? It doesn't necessarily mean you aren't well or that your thinking skills are fading. You may simply not be getting enough sleep.
Researchers have long known that sleep is vital to relational memory, the ability of the brain to make connections between objects, places, people, and events. What they haven't identified is what happens during sleep to help the memory make the right connections.
To find out, a pair of investigators from the University of California, San Diego built computer models of the brain's thalamus and cortex and studied activity in these regions during an artificial version of an awake state and a deep sleep.
During the computerized modeling exercise, the researchers either strengthened or weakened the connections between neurons, depending on how active they were. First, they trained the modeled network during the awake mode to directly associate one thing with another, such as A+B and B+C.
Then during deep sleep, they observed how the network made indirect associations, such as A+C, on its own. They published their findings this month in The Journal of Neuroscience.
Maxim Bazhenov, PhD, one of the researchers, explained that the indirect links occur because the neurons related to A, B, and C all fire in close order, called sleep replay, which creates a connection between all three neurons.
"Therefore, after sleep, activating any one group, such as A, activated all other related groups, such as B and C," he said in a prepared statement.
The thalamus is the part of the brain that picks up on sensory cues and makes sense of them, and the cortex is essential for memory, learning, and decision-making. Our neurons are taking in sensory input when awake, but it's during deep sleep that the cortex makes sense of the day's input. While that's happening, the brain repeats electrical activity called slow waves.
Sleep replay triggers synaptic plasticity, the activity among neurons that allows them to communicate with one another and the primary way the brain creates, changes, or deletes memories.
The computer-based model mainly helped the researchers understand how relational memory in the brain works – how the brain connects seemingly unrelated pieces of information. But it also sheds light on what might not be working in people with certain neurological or psychiatric conditions that affect memory, such as schizophrenia or autism spectrum disorder.
Their results suggest that finding ways to improve slow-wave sleep in people with these conditions may help their brains make those links and improve their overall challenges with memory and making associations.