When my dad was in his 80s, he was able to remember everything from decades earlier, but little about events of previous weeks. For some of us, it is the opposite — we have good short-term memory but remember little of long ago.
What causes this difference? New research has discovered how the brain "replays" certain memories while we sleep.
Long-term memory is usually divided into two types: declarative (explicit) memory and nondeclarative (implicit) memory. Explicit memories include all of the memories that are available in consciousness. Explicit memory can be further divided into episodic memory (specific events) and semantic memory (knowledge about the world).
Implicit memories are those that are mostly unconscious. This type of memory includes procedural memory, which involves memories of body movement and how to use objects in the environment, such as driving a car or using a computer.
Research strongly suggests sleep, which constitutes about a third of our lives, is crucial for learning and forming long-term memories. But exactly how long-term memory is formed is not well understood and remains a central question of inquiry in neuroscience.
In a recent study, neuroscientists at the University of California, Riverside report that they now may have an answer to this question. And the answer has a lot to do with deep sleep, which makes up at least 20 percent of our total sleep time, occurring mostly in the first third of the night.
Using a computational model, researchers explain how the hippocampus influences synaptic connections in the cortex. Their study suggests 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 somehow decoupled from sensory input, yet the brain remains highly active. It shows electrical activity in the form of sharp-wave ripples in the hippocampus and large-amplitude slow oscillations in the cortex, 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.
However, the underlying mechanisms of how sleep rhythms contribute to consolidating memories acquired during wakefulness remain unclear. In this study, researchers studied the role of slow oscillations — 0.2-1 Hz rhythmic transitions between "up" and "down" states during stage 3/4 sleep — on dynamics of synaptic connectivity in the thalamocortical network model implementing spike-timing-dependent synaptic plasticity.
They found the spatiotemporal pattern of up-state propagation determines the changes of synaptic strengths between neurons. Furthermore, an external input, mimicking hippocampal ripples, delivered to the cortical network results in input-specific changes of synaptic weights, which persisted after stimulation was removed.
These synaptic changes promoted replay of specific firing sequences of the cortical neurons. The study proposes a neuronal mechanism on how an interaction between hippocampal input — such as mediated by sharp wave-ripple events, cortical slow oscillations and synaptic plasticity — may lead to consolidation of memories through preferential replay of cortical cell spike sequences during slow-wave sleep.
According to Yina Wei, a postdoctoral researcher and the first author of the research paper, sleep is critical for regulation of synaptic efficacy, memories and learning. Input from the hippocampus — the sharp-wave ripples — determines the spatial and temporal pattern of these slow oscillations.
By influencing the nature of these oscillations, this hippocampal input activates selective memories during deep sleep and causes a replay of specific memories. During such memory replay, the corresponding synapses are strengthened for long-term storage in the cortex. These results suggest the importance of the hippocampal sharp-wave ripple events in transferring memory information to the cortex.
