Theta oscillations, which are implicated in spatial navigation in rodents, have been difficult to observe in humans. Using intracranial electroencephalographic (iEEG) recordings from epileptic patients, our group reported clear evidence of task-related theta during navigation of virtual, 3D environments (Kahana et al., Nature 1999).[Supported by NIH grant MH55687 and a grant from the M R Bauer Foundation.]E xamining activity from 31 to 108 electrodes per patient, we found that the raw iEEG signal contained striking episodes of theta-band (4-8 Hz) rhythmic activity lasting several cycles. These theta episodes were more prevalent in more difficult mazes; they were also more prevalent during retrieval of previously-learned spatial information than during the original learning.
Our present work further explores the hypothesis that theta oscillations play a functional role in human spatial cognition. In this report we characterize the correlational structure and phase relations of theta episodes across brain regions. We find independent as well as overlapping topographical and temporal organizations. Because theta oscillations are associated with a variety of functional task dimensions it is likely that theta plays multiple roles in human cognition.