Glossary
Theta waves
What are theta waves?
Theta waves are neural oscillations commonly described in the approximate 4–8 Hz range. Hippocampal theta has been studied extensively in relation to exploration and memory.
How they work
Theta rhythms are proposed to organize the timing of neuronal activity and synaptic change. Their generators and functions depend on context. Theta measured at the scalp is not interchangeable with theta recorded in the hippocampus.
A navigation experiment
Ekstrom and colleagues (2005) analyzed recordings from depth electrodes in six patients with epilepsy as they played a virtual taxi game. The patients searched for passengers and delivered them to stores. Theta activity increased during virtual movement in both the hippocampus and neocortex.
This was a measured relationship between activity and movement within a navigation task. It did not show that theta alone caused learning or that the experience was later replayed during sleep.
Hippocampal and scalp theta
A depth electrode samples activity near a brain location. Scalp EEG measures the combined electrical activity of much larger populations of neurons. Kahana's 2006 review describes increased theta in scalp recordings during waking verbal working-memory tasks, as well as intracranial theta during navigation. Consequently, a theta increase recorded while someone holds words in mind is not interchangeable with a local hippocampal signal during virtual movement.
The frequency label identifies a rhythm; its interpretation also requires the recording location, method, and task. It is not a universal marker of a single mental state.
Why it matters
Theta gives researchers a way to investigate timing in memory-related activity.