Human hippocampal theta oscillations: Distinctive features and interspecies commonalities

The hippocampus, along with its characteristic theta oscillation, has been widely implicated in various aspects of animal memory and behavior. Given the important roles that hippocampal theta oscillations have in theoretical models of brain function, it might be considered surprising that these signals have not been reported as often in humans as in animals. In this chapter we review recent research on hippocampal theta oscillations in humans, focusing on brain recordings from neurosurgical patients, which provide a key opportunity for observing hippocampal oscillations during cognition. The emerging theme of this body of work is that humans do indeed have hippocampal oscillations that are similar overall compared to the theta oscillation that is commonly found in rodents. Most notably, the human theta oscillation exhibits correlations with sensorimotor, navigation, and memory processing in the same general fashion as expected from rodents. However, some of the details of theta's relationship with behavior differ significantly compared to such signals in rodents-such as having a lower amplitude, frequency, and duration-which can make this signal less readily observable. Thus, theta oscillations are a key component of hippocampal processing in humans, but the patterns it exhibits compared to rodents point out distinctive aspects of human brain processes.