Electrophysiology of Human Memory

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Human memory function is highly variable, fluctuating between periods of high and low performance even within a given person. Neurosurgical patients with indwelling electrodes present a unique opportunity to study the neural correlates of this variability and to define both the features of neural activity at a given brain location and the functional connections between brain regions that predict variability in memory encoding and retrieval. Here, I will describe our recent efforts to characterize brain networks that support memory via correlative (passive neural recording) and causal (direct electrical stimulation) approaches. Throughout the brain, we find a spectral TAG of episodic memory function: increased low frequency (theta) power decreased alpha power and increased high-frequency activity and ripples. Furthermore, many canonical memory regions emerge as hubs of synchronous theta-frequency activity. High-frequency bands (i.e. gamma, 30+ Hz) almost exclusively exhibit desynchronization during successful memory operations. For the last several years we have extended these correlative studies and used intracranial stimulation to ask whether functional connections imply causality. We confirmed that electrical stimulation evokes increases in theta power throughout the memory network, as predicted by the strength of low-frequency functional connections. This relation was strongest when stimulation occurred in or near white matter. These findings demonstrate the importance of low-frequency connectivity to episodic memory, integrating these findings over spatial scales and through causal and correlative approaches.