UC Berkeley

EEG patterns correlated with conditioned stimuli were sought in amplitude modulation of synchronous beta-gamma oscillations (12-80 Hz). EEG signals were recorded from high-density 8 x 8 (5.6 x 5.6 mm) arrays fixed on the surfaces of primary sensory areas in rabbits trained to discriminate visual, auditory, or tactile conditioned stimuli. EEG preprocessing was by (i) band pass filtering to extract the beta-gamma range (deleting theta-alpha); (H) low-pass spatial filtering (not high-pass Laplacians used for localization), (iii) spatial averaging (not time averaging used for evoked potentials); (iv) close spacing of electrodes for simultaneous recording in each area (not sampling single signals from several areas); (v) calculating variances among patterns in 64-space derived from the 8 x 8 arrays (not by fitting equivalent dipoles). These methodological differences were essential to reveal discontinuities in cortical activity: "state transitions". Each transition began with an abrupt phase re-setting, followed sequentially by resynchronization, stabilization of a spatial pattern of amplitude, and dramatic increase in global pattern amplitude. State transitions recurred at irregular intervals in the theta range. An estimate of perceptual information in the beta-gamma EEG disclosed 2 to 3 patterns with high information content in each trial that began with a state transition, lasted approximately 0.1 s, and recurred at theta rates.