A MODEL OF NEOCORTEX

Prompted by considerations about (i) the compositionality of cognitive functions, (ii) the physiology of individual cortical neurons, (iii) the role of accurately timed spike patterns in cortex, and (iv) the re gulation of global cortical activity, we suggest that the dynamics of cortex on the 1-ms time scale may be described as the activation of ci rcuits of the synfirechain type (Abeles 1982, 1991). We suggest that t he fundamental computational unit in cortex may be a wave-like spatio- temporal pattern of synfire type, and that the binding mechanism under lying compositionality in cognition may be the accurate synchronizatio n of synfire waves that propagate simultaneously on distinct, weakly c oupled, synfire chains. We propose that Hebbian synaptic plasticity ma y result in a superposition of synfire chains in cortical connectivity , whereby a given neuron participates in many distinct chains. We inve stigate the behaviour of a much-simplified model of cortical dynamics devised along these principles. Calculations and numerical experiments are performed based on an assumption of randomness of stored chains, in the style of statistical physics. It is demonstrated that: (i) ther e exists a critical value for the total length of stored chains; (ii) this storage capacity is linear in the network's size; (iii) the behav iour of the network around the critical point is characterized by the self-regulation of the number of synfire waves coactive in the network at any given time.