So far, we have focused on theta power. But at the heart of the study of oscillations is the idea that rhythmic activity coordinates the activity of single cells or populations of cells within and across different brain regions. The presence of oscillations (be it inferred from changes in power or via oscillation detection) is a prerequisite for these phenomena, but we can also investigate them more directly. Studies have mostly addressed (1) the synchronization of single-unit firing and faster gamma oscillations (30–100 Hz) with theta phase (see Box 5), as well as (2) the phase-phase synchronization of theta oscillations measured in different brain regions. Measures of neural synchrony may be less susceptible to concurring changes in broad-band power. If theta synchrony is enhanced during memory formation and retrieval, this finding would therefore strengthen the theta hypothesis while also allowing more direct insight into the mechanisms by which theta oscillations support memory.
