Memory networks are formed in the cerebral cortex by associative processes, following Hebbian principles of synaptic modulation. Sensory and motor memory networks are made of elementary representations in cell assemblies of primary sensory and motor cortex (phyletic memory). Higher order individual memories, e.g. episodic, semantic, conceptual - are represented in hierarchically organised neuronal networks of the cortex of association. Perceptual memories are organised in posterior (post-rolandic) cortex, motor (executive) memories in cortex of the frontal lobe. Memory networks overlap and interact profusely with one another, such that a cellular assembly can be part of many memories or networks. Working memory essentially consists in the temporary activation of a memory network, as needed for the execution of successive acts in a temporal structure of behaviour. That activation of the network is maintained by recurrent excitation through reentrant circuits. The recurrent reentry may occur within local circuits as well as between separate cortical areas. In either case, recurrence binds together the associated components of the network and thus of the memory it represents.
Reference: J. M. Fuster, "Cortical Dynamics of Memory," International Journal of Psychophysiology, vol. 35, pp. 155-164, 2000.