Hippocampal Sharp-Wave Ripples Influence Selective Activation of the Default Mode Network

Area of Science:

• Neuroscience
• Cognitive Neuroscience
• Systems Neuroscience

Background:

• The default mode network (DMN) is implicated in episodic memory, but its endogenous activity's behavioral relevance is unclear.
• Hippocampal ripples, high-frequency oscillations associated with memory consolidation, occur during sleep and wakefulness.
• Previous research suggests ripples influence brain activity, but their specific impact on resting-state networks like the DMN is unknown.

Purpose of the Study:

• To investigate whether hippocampal ripples influence endogenous fluctuations in specific resting-state networks (RSNs), particularly the DMN.
• To link circuit-level neural dynamics (hippocampal ripples) with network-level brain activity (DMN fMRI signals).

Main Methods:

• Analysis of fMRI datasets from anesthetized monkeys.
• Simultaneous hippocampal electrophysiology recordings to identify neural events like ripples.
• Correlation of hippocampal electrophysiological events with DMN and other RSN fMRI signals.

Main Results:

• A significant increase in the DMN fMRI signal was observed specifically following hippocampal ripples.
• No significant increase in DMN activity was detected following other hippocampal electrophysiological events.
• Increases in ongoing DMN activity were observed after ripples, but not in other RSNs.

Conclusions:

• Hippocampal ripples are directly related to endogenous fluctuations in the default mode network.
• This finding links network-level resting-state fMRI signals with behaviorally relevant, circuit-level neural dynamics.
• The study provides mechanistic support for the DMN's role in memory consolidation through its relationship with hippocampal ripples.