The connection between the left side of the brain and the right is crucial for the formation of memories. A study by a team of scientists has shown that sleep boosts communication between the regions. Scientists from the National Institute of Health (NIH) in a project partially funded by the Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative, used the "NeuroGrid" technology to study the workings of the brain. Nick Langhals, program director at NIH's National Institute of Neurological Disorders and Stroke, in a report said, "using new technologies advanced by the BRAIN Initiative, these researchers made a fundamental discovery about how the brain creates and stores new memories." The hippocampus region of the brain is known to be the area responsible for making new memories and storing them while we sleep.
In order to remember a skill or experience, the memory needs to be strengthened through a process called memory consolidation. Although it is known that brain waves play a key role in this process, the mechanism that forms their shape and rhythm had not yet been determined – until now. Researchers have discovered that one of the brain waves needed for consolidating memories is dominated by synaptic inhibition, which they believe'could be a main factor in memory consolidation'. Although brain waves play a key role in this procedure, experts have yet to identify the mechanism that forms their shape and rhythm – until now. Researchers have discovered that one of the brain waves needed for consolidating memories.
Consolidation of declarative memories requires hippocampal-neocortical communication. Although experimental evidence supports the role of sharp-wave ripples in transferring hippocampal information to the neocortex, the exact cortical destinations and the physiological mechanisms of such transfer are not known. We used a conducting polymer-based conformable microelectrode array (NeuroGrid) to record local field potentials and neural spiking across the dorsal cortical surface of the rat brain, combined with silicon probe recordings in the hippocampus, to identify candidate physiological patterns. Parietal, midline, and prefrontal, but not primary cortical areas, displayed localized ripple (100 to 150 hertz) oscillations during sleep, concurrent with hippocampal ripples. Coupling between hippocampal and neocortical ripples was strengthened during sleep following learning.
The hippocampus is thought to initiate systems-wide mnemonic processes through the reactivation of previously acquired spatial and episodic memory traces, which can recruit the entorhinal cortex as a first stage of memory redistribution to other brain areas. Hippocampal reactivation occurs during sharp wave–ripples, in which synchronous network firing encodes sequences of places. We investigated the coordination of this replay by recording assembly activity simultaneously in the CA1 region of the hippocampus and superficial layers of the medial entorhinal cortex. We found that entorhinal cell assemblies can replay trajectories independently of the hippocampus and sharp wave–ripples. This suggests that the hippocampus is not the sole initiator of spatial and episodic memory trace reactivation.
Short-lived, high-frequency oscillations in the brain called ripples have been implicated as substrates for memory formation. Vaz et al. analyzed intracranial recordings in human subjects (see the Perspective by Gelinas). They found that ripple oscillations in the brain's medial temporal lobe were coupled with ripple oscillations in the temporal cortex. This coupling was enhanced just before successful memory retrieval. During successful retrievals with ripples, patterns of oscillations were recapitulated across multiple electrodes, consistent with the initial encoding.