The aim of this study was to find evidence for repetitive global phase transitions occurring simultaneously over multiple areas of cortex during normal behavior. EEGs were recorded from multiple high-density arrays of 14-16 electrodes surgically fixed on the visual, auditory, somatomotor, and entorhinal cortices of trained cats and rabbits, and from a linear array of 64 electrodes on the scalp of volunteers. Analytic phase relations between gamma EEG signals from multiple cortices were examined with high temporal resolution provided by the Hilbert transform. An index of synchronization was applied to intercortical pairs of signals to detect and display epochs of engagement between pairs. The measure was adapted to derive an index of global synchronization among all 4 cortices that was calculated as a t-value. Global epochs of phase stabilization ('locking') were found to involve all cortices under observation. The phase values were not clustered at zero but were in distributions about nonzero means. Episodic destabilization (decoherence) occurred aperiodically at intervals corresponding to rates in the delta range, with equal likelihood before the onset of the conditioned stimuli (CSs) and in post-stimulus test periods including performance of conditioned responses (CRs). Preferential pairwise phase stabilization was sought but not found between the sensory cortex receiving the auditory or visual CSs and the entorhinal or somatomotor cortex at times of CSs or CRs. The cospectrum from cross-correlating the global synchronization index with the global spatial ensemble average of the unfiltered EEG peaked in the delta range (1-3 Hz) near 2.5 Hz in cat and below 2 Hz in rabbit. The cospectrum of the EEG with the derivative of the analytic phase in humans peaked in the alpha range (7-12 Hz) The results indicated that macroscopic states of synchronized neural activity related to Gestalts formed during perception, that included the primary sensory and limbic areas and perhaps the entire neocortex of each cerebral hemisphere.
The analytic signal given by the Hilbert transform applied to an electroencephalographic (EEG) trace is a vector of instantaneous amplitude and phase at the temporal resolution of the digitizing interval (here 2 ms). The transform was applied after band-pass filtering for extracting the gamma band (20-80 Hz in rabbits) to time series from up to 64 EEG channels recorded simultaneously from high-density arrays giving spatial "windows" of 4 x 4 to 6 x 6 mm onto the visual, auditory, or somatosensory cortical surface. The time series of the analytic phase revealed phase locking for brief time segments in spatial patterns of nonzero phase values from multiple EEG that was punctuated by episodic phase decoherence. The derivative of the analytic phase revealed spikes occurring not quite simultaneously (within +/- 4 ms) across arrays aperiodically at mean rates in and below the theta range (3-7 Hz). Two measures of global synchronization over a group of channels were derived from analytic phase differences between pairs of channels on the same area of cortex. One was a synchronization index expressing phase locking. The other was a decoherence index estimating the variance in phase among multiple channels. Spectral analyses of the indices indicated that decoherence events recurred aperiodically at rates in and below the theta range of the EEGs. The results provide support for the hypothesis that neurons in mesoscopic neighborhoods in sensory cortices self-organize their activity by synaptic interactions into wave packets that have spatial patterns of amplitude (AM) and phase (PM) modulation of their spatially coherent carrier waves in the gamma range and that form and dissolve aperiodically at rates in and below the theta range. Each AM pattern is formed by a nonlinear state transition in the cortical dynamics, as shown by spikes in the derivative. Phase locking within each PM pattern is not at zero phase lag but over a fixed distribution of phase values that is consistent with the radially symmetric phase gradients already reported called "phase cones" detected by Fourier-based methods. The insight is suggested that sensory cortices are bistable comparably to cardiac dynamics, with a diastolic state that accepts sensory input and an abrupt transition to a systolic state that transmits perceptual output. Further support for this inference will require improvements in methods for temporal resolution of the times of onset of spatial patterns of phase modulation.
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