Human intracranial high-frequency activity during memory processing: neural oscillations or stochastic volatility?

•Increases in intracranial high frequency activity (HFA) mark memory processing.•Interpreted as gamma oscillations, HFA is assumed to mechanistically mediate memory.•Recent evidence challenges this traditional view.•HFA is more consistent with neural noise, or stochastic volatility.

Intracranial high-frequency activity (HFA), which refers to fast fluctuations in electrophysiological recordings, increases during memory processing. Two views have emerged to explain this effect: (1) HFA reflects a synchronous signal, related to underlying gamma oscillations, that plays a mechanistic role in human memory and (2) HFA reflects an asynchronous signal that is a non-specific marker of brain activation. We review recent data supporting each of these views and conclude that HFA during memory processing is more consistent with an asynchronous signal. Memory-related HFA is therefore best conceptualized as a biomarker of neural activation that can functionally map memory with high spatial and temporal precision.