A neurobiological theory of meaning in perception. Part 5. Multicortical patterns of phase modulation in gamma EEG
- Author(s): Freeman, Walter J, III
- Rogers, Linda J
- et al.
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.
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