Phase transitions, cortical gamma, and the selection and read-out of information stored in synapses

Abstract

simulation of electro-cortical dynamics is extrapolated to outline a mechanism for selection and release of information stored in cortical synapses. The simulations accord with Freeman’s experimental and theoretical findings on gamma synchrony, phase transition, phase cones, and null spikes. Patches of cortex excited to gamma oscillation mutually synchronize into larger fields, are controlled, and organized into sequences, by traveling waves in the cortex and by cortical/subcortical interactions. At a critical level of cortical excitation, transition from damped gamma resonance to autonomous gamma oscillation occurs, and is roughly analogous to a phase transition in the thermodynamic sense. As transition is approached, patches of cortex exhibit selective sensitivity to action-potential pulse trains modulated in the gamma band, while autonomous gamma releases pulse trains modulated in the same band, implying coupling of input and output modes. Retrograde propagation of action potentials into the dendritic tree, coupled with a sharp change at the transition in the timing of firing of closely situated excitatory and inhibitory cells, implies a mechanism for the selective access and release of information from synapses with functionally different roles, in the proximal and distal pyramidal dendritic trees. In turn, this explanation is logically consistent with recent work on physiological modifications of synapses, and information-theoretic accounts of learning.