CHARACTERISTICS OF THE SYNCHRONIZATION OF BRAIN ACTIVITY IMPOSED BY FINITE CONDUCTION VELOCITIES OF AXONS

Abstract

The electrical activity of neurons in brains fluctuates erratically both in terms of pulse trains of single neurons and the dendritic currents of populations of neurons. Obviously the neurons interact with one another in the production of intelligent behavior, so it is reasonable to expect to find evidence for varying degrees of synchronization of their pulse trains and dendritic currents in relation to behavior. However, synaptic communication between neurons depends on propagation of action potentials between neurons, often with appreciable distances between them, and the transmission delays are not compatible with synchronization in any simple way. Evidence is on hand showing that the principal form of synchrony is by establishment of a low degree of covariance among very large numbers of otherwise autonomous neurons, which allows for rapid state transitions of neural populations between successive chaotic basins of attraction along itinerant trajectories. The small fraction of covariant activity is extracted by spatial integration upon axonal transmission over divergent–convergent pathways, through which a remarkable improvement in signal-to-noise ratio is achieved. The raw traces of local activity show little evidence for synchrony, other than zero-lag correlation, which appears to be largely a statistical artifact. Brains rely less on tight phase-locking of small numbers of periodically firing neurons and more on low degrees of cooperativity achieved by order parameters influencing very large numbers of neurons. Brains appear to be indifferent to and undisturbed by widely varying time and phase relations between individual neurons and even large semi-autonomous areas of cortex comprising their mesoscopic neural masses.