UC Berkeley

The aim of this study was to measure and classify spatial patterns in sensory cortical EEGs relating to conditioned stimuli (CSs) in order to test the hypothesis, based on clinical reports, that cortical dynamics is not continuous but operates in steps that resemble frames in a cinema. Recent advances in the application of the Hilbert Transform to intracranial recordings of the EEG in animals have revealed markers for repetitive phase transitions in neocortex at frame rates in the theta band. The frames were sought in multichannel EEGs that had been recorded from 8 x 8 high-density arrays that were fixed on primary sensory cortices of rabbits trained to discriminate visual, auditory or somatic conditioned stimuli with reinforcement (CS+) or without (CS-). Localization of frames in EEGs was by use of a new index, H-e(t), called "pragmatic information". Each spatial pattern was represented by a feature vector from the 64 analytic amplitudes at a maximal value of HQ) from the Hilbert transform and expressed as a 64 x 1 feature vector specifying a point in 64-space. Classification with respect to CS+/- was by calculation of Euclidean distances of points from centers of gravity of clusters after preprocessing by nonlinear mapping. Stable spatial patterns were found in the form of amplitude modulation (AM) of aperiodic waveforms that included all channels. The impact of a CS on a sensory neocortex reorganized background EEG into two types of sequential patterns of coordinated activity, initially local and modality-specific, later global. The initial stage of phase transitions required 3-7 ms. Large-scale cortical activity then reorganized itself repeatedly and reliably over relatively immense cortical distances within the cycle duration of the center frequency of oscillation. The size, texture, timing, and duration of the AM patterns support the hypothesis that these frames may provide the basis for multisensory percepts (Gestalts). (c) 2006 Published by Elsevier B.V.