Directional coupling of slow and fast hippocampal gamma with neocortical alpha/beta oscillations in human episodic memory

Abstract

Episodic memories hinge upon our ability to process a wide range of multisensory information and bind this information into a coherent, memorable representation. On a neural level, these two processes are thought to be supported by neocortical alpha/beta desynchronisation and hippocampal theta/gamma synchronisation, respectively. Intuitively, these two processes should couple to successfully create and retrieve episodic memories, yet this hypothesis has not been tested empirically. We address this by analysing human intracranial EEG data recorded during two associative memory tasks. We find that neocortical alpha/beta (8-20Hz) power decreases reliably precede and predict hippocampal "fast" gamma (60-80Hz) power increases during episodic memory formation; during episodic memory retrieval however, hippocampal "slow" gamma (40-50Hz) power increases reliably precede and predict later neocortical alpha/beta power decreases. We speculate that this coupling reflects the flow of information from neocortex to hippocampus during memory formation, and hippocampal pattern completion inducing information reinstatement in the neocortex during memory retrieval. Significance StatementEpisodic memories detail our personally-experienced past. The formation and retrieval of these memories has long been thought to be supported by a division of labour between the neocortex and the hippocampus, where the former processes event-related information and the latter binds this information together. However, it remains unclear how the two regions interact. We uncover directional coupling between these regions, with power decreases in the neocortex that precede and predict power increases in the hippocampus during memory formation. Fascinatingly, this process reverses during memory retrieval, with hippocampal power increases preceding and predicting neocortical power decreases. These results suggest a bidirectional flow of information between the neocortex and hippocampus is fundamental to the formation and retrieval of episodic memories. \body

Classic Psychedelic Drugs: Update on Biological Mechanisms

“…Changes in gamma synchronization would then reflect changes in information processing, including endogenously-generated states. Hence, alterations in gamma synchronization may well contribute to changes in processes like autobiographical memory retrieval 193 and awareness of one’s own internal state during the psychedelic experience 194 195 .…”

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“…Probing rat hippocampal neurons in vitro, Bi and Poo () showed that the postsynaptic neuron must fire 20 ms (that is, 1 cycle of a gamma oscillation) after the presynaptic neuron to induce LTP. Given that these hippocampal neurons have been shown to lock gamma‐band activity (Jutras, Fries, & Buffalo, ) and hippocampal gamma‐band activity is predictive of memory formation (e.g., Griffiths et al, ; Long & Kahana, ), one could speculate that increases in the amplitude of hippocampal gamma oscillations reflect increases in STDP.…”

Alpha/beta power decreases track the fidelity of stimulus-specific information

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