How brain consolidates memory during deep sleep decoded

Los Angeles: Scientists have for the first time decoded how deep sleep - also called slow-wave sleep - may be promoting the consolidation of recent memories in our brain.

Research strongly suggests that sleep, which constitutes about a third of our lives, is crucial for learning and forming long-term memories. But exactly how such memory is formed is not well understood and remains, despite considerable research, a central question of enquiry in neuroscience.

The study by researchers at the University of California, Riverside provides for the first time a mechanistic explanation for how deep sleep, also called slow-wave sleep, may be promoting the consolidation of recent memories. During sleep, human and animal brains are primarily decoupled from sensory input.

The brain remains highly active, showing electrical activity in the form of sharp-wave ripples in the hippocampus (a small region of the brain that forms part of the limbic system) and large-amplitude slow oscillations in the cortex
(the outer layer of the cerebrum), reflecting alternating periods of active and silent states of cortical neurons during deep sleep.

Traces of episodic memory acquired during wakefulness and initially stored in the hippocampus are progressively transferred to the cortex as long-term memory during sleep. Using a computational model, the researchers provide a link between electrical activity in the brain during deep sleep and synaptic connections between neurons.

They show that patterns of slow oscillations in the cortex, which their model spontaneously generates, are influenced by the hippocampal sharp-wave ripples and that these patterns of slow oscillations determine synaptic changes in the cortex. The model shows that the synaptic changes, in turn, affect the patterns of slow oscillations, promoting a kind of reinforcement and replay of specific firing sequences of the cortical neurons - representing a replay of specific memory.

"These patterns of slow oscillations remain even without further input from the hippocampus," said Yina Wei, a postdoctoral researcher. "We interpret these results as a mechanistic explanation for the consolidation of specific memories during deep sleep, whereby the memory traces are formed in the cortex and become independent of the hippocampus," said Wei.

Wei explained that according to the biologically realistic network model the researchers used, input from the hippocampus reaches the cortex during deep sleep and influences how the slow oscillations are initiated and propagated in the cortical network. The study appears in the Journal of Neuroscience.