Noninvasive studies of episodic memory generally support the theta hypothesis. Since the 1990s, scalp EEG and MEG studies have uncovered increases in theta during successful memory (see Fig. 2). With the assumption that hippocampal projections to the neocortex drive theta responses detectable by scalp EEG and MEG (see Box 3), authors have generally concluded that these results support the hypothesis of theta unifying spatial navigation and episodic memory. However, noninvasive measures of neural activity are limited in their capacity to precisely localize the source of a neural oscillation and capture blurred neural signals as they are filtered through skull, muscle, and skin.
