Abstract

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.

Highlights

  • Synchrony of firing of widely distributed neurons in large numbers is necessary for emergence of spatial structure in cortical activity by reorganization of unpatterned background activity

  • The firing is grouped in time by oscillations in dendritic current in the beta (12-30 Hz) and gamma (30-80 Hz) ranges that arise from negative feedback among excitatory and inhibitory neurons [1, 2]

  • The synchrony among multiple EEG records has been evaluated by measuring the phase of the EEG record from each channel with respect to the phase of the spatial average EEG over all channels and calculating the standard deviation (SDX) of the spatial distribution of phase values [3]

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Summary

Introduction

Synchrony of firing of widely distributed neurons in large numbers is necessary for emergence of spatial structure in cortical activity by reorganization of unpatterned background activity. The same currents are largely responsible for local field potentials and EEG. The firing is grouped in time by oscillations in dendritic current in the beta (12-30 Hz) and gamma (30-80 Hz) ranges that arise from negative feedback among excitatory and inhibitory neurons [1, 2]. EEG signals from arrays up to 1 cm in width were highly correlated but with varying patterns of amplitude modulation. The spatial patterns of phase modulation across the array showed epochs of low SDX that were bracketed by brief episodes of high variance [4] showing that high synchrony was episodic

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