Widespread ripples synchronize human cortical activity during sleep, waking, and memory recall

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Memory Research

Background:

• Declarative memory relies on integrating information from distributed brain regions.
• The mechanism for binding these distributed elements into unified memories remains unclear.
• The binding-by-synchrony theory suggests synchronous neural oscillations facilitate this process.

Purpose of the Study:

• To investigate the role of high-frequency oscillations (ripples) in binding distributed memory representations.
• To examine cortico-cortical and hippocampo-cortical synchrony during memory processes.

Main Methods:

• Intracranial recordings in humans during sleep and waking states.
• Analysis of high-frequency oscillations (ripples) for co-occurrence, overlap, and phase-locking between cortical areas and the hippocampus.
• Correlation of ripple activity with successful delayed memory recall.

Main Results:

• Widespread cortical ripples (∼70 ms, 90 Hz) frequently couple, co-occur, and phase-lock between distant cortical locations, even across hemispheres.
• Cortico-cortical ripple synchrony increases with more participating cortical sites and is mediated by direct cortico-cortical connections.
• Hippocampo-cortical ripples co-occur but do not phase-lock, and their incidence increases before successful memory recall, suggesting a role in binding.

Conclusions:

• Cortico-cortical synchrony via ripples provides a mechanism for binding distributed neural information during memory retrieval.
• Hippocampo-cortico-cortical ripples possess properties supporting binding-by-synchrony, crucial for memory recall and potentially broader cognitive functions.
• Synchronized neural activity via ripples facilitates information exchange and memory integration across long distances in the brain.