But if the theta rhythm truly supports effective encoding of episodic associations, it is surprising that the brain does not always show it. Whereas electrophysiological studies in humans have replicated increases in MTL and hippocampal theta power during spatial navigation [8–12], recordings during the formation of episodic memories have revealed an inconsistent relationship between theta and successful memory encoding. In some experiments, increases in MTL and neocortical theta power are predictive of good episodic memory. But over a dozen recent studies have shown exactly the opposite: widespread decreases in theta power during successful episodic encoding and retrieval. These findings undercut the idea that the theta rhythm is a general-purpose mechanism that links episodic memory and spatial navigation under the umbrella of cognitive mapping. If the MTL is agnostic to the type of information it is acting upon, why should theta power increase during spatial navigation but decrease during the formation of event memories?
