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Theta Oscillations in Human Cortex During a Working-Memory Task: Evidence for Local Generators

¹Volen Center for Complex Systems, Brandeis University, Waltham; ²Department of Neurosurgery, Children's Hospital and ³Department of Neurology, Brigham and Women's Hospital, Boston, Massachusetts; and ⁴Department of Psychology, University of Pennsylvania, Philadelphia, Pennsylvania

Submitted 20 April 2005; accepted in final form 28 September 2005

Cortical theta appears important in sensory processing and memory. Intracanial electrode recordings provide a high spatial resolution method for studying such oscillations during cognitive tasks. Recent work revealed sites at which oscillations in the theta range (4–12 Hz) could be gated by a working-memory task: theta power was increased at task onset and continued until task offset. Using a large data set that has now been collected (10 participants/619 recording sites), we have sufficient sampling to determine how these gated sites are distributed in the cortex and how they are synchronized. A substantial fraction of sites in occipital/parietal (45/157) and temporal (23/280) cortices were gated by the task. Surprisingly, this aspect of working-memory function was virtually absent in frontal cortex (2/182). Coherence measures were used to analyze the synchronization of oscillations. We suspected that because of their coordinate regulation by the working-memory task, gated sites would have synchronized theta oscillations. We found that, whereas nearby gated sites (<20 mm) were often but not always coherent, distant gated sites were almost never coherent. Our results imply that there are local mechanisms for the generation of cortical theta.

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