1 An oscillatory hierarchy controlling cortical excitability and stimulus processing Peter Lakatos 1,2 , Ankoor S. Shah 1,4 , Kevin H. Knuth 3 , Istvan Ulbert 2 , George Karmos 2 , and Charles E. Schroeder 1,4 1 Cognitive Neuroscience and Schizophrenia Program, Nathan Kline Institute, Orangeburg, New York 10962, 2 Institute for Psychology, Hungarian Academy of Sciences, Budapest, H-1394, 3 Computational Sciences Division, Code TC, NASA Ames Research Center, Moffett Field, California 94035-1000, 4 Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York 10461 Electroencephalographic (EEG) oscillations are hypothesized to reflect cyclical variation in the excitability of neuronal ensembles [1], with particular frequency bands reflecting differing types [2-4] and spatial scales [5-7] of brain operations. Interdependence between the gamma and theta bands [5, 8] suggests an underlying structure to the EEG spectrum, and there is also evidence that ongoing activity influences sensory responses [9, 10]. However, there is no unifying theory of EEG organization and the role of the ongoing oscillatory activity in sensory processing remains controversial. This study analyzed laminar profiles of synaptic activity and action potentials, both spontaneous and stimulus-driven, in primary auditory cortex [11]. We find that - 1) The EEG is hierarchically organized; delta (1-4 Hz) phase modulates theta (4-10 Hz) amplitude, and theta phase modulates gamma (30-50 Hz) amplitude. 2) This Oscillatory Hierarchy controls baseline excitability and action potential generation, as well as stimulus-related responses in a neuronal ensemble. We propose that the hierarchical organization of ambient oscillatory activity allows auditory cortex to structure its temporal activity pattern so as to optimize the processing of rhythmic inputs.