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Extremely Dilute Modular Neuronal Networks: Neocortical Memory Retrieval Dynamics

A model of the columnar functional organization of neocortical association areas is studied. The neuronal network is composed of many Hebbian autoassociators, or modules, each of which interacts with a relatively small number of the others. Every module encodes and stores a number of elementary percepts, or features. Memory items, or patterns, are peculiar combinations of features sparsely distributed over the multi-modular network. Any feature stored in any module can be involved in several of the stored patterns; feature-sharing is in fact source of local ambiguities and, consequently, a potential cause of erroneous memory retrieval activity spreading through the model network. The memory retrieval dynamics of the large multi-modular autoassociator is investigated by means of quantitative analysis and numerical simulations. An oscillatory retrieval process is found to be very efficient in overcoming feature-sharing drawbacks; it requires a mechanism that modulates the robustness of local attractors to noise, and neuronal activity sparseness such that quiescent and active modules are about equally noisy. Correlated activation of interconnected modules and extramodular neuronal contacts more effective than the intramodular ones seem to be general requirements in order to efficiently achieve satisfactory quality of memory retrieval. It is also shown that, even in ideal conditions, some spots of the network cannot be reached by retrieval activity spread. The locations of these activity isles depend on the pattern to retrieve and on the cue, while their extension only depends on architecture of the graph and statistics of the stored patterns. The existence of these isles determines an upper-bound to retrieval quality that does not depend on the specific retrieval dynamics adopted, nor on whether feature-sharing is permitted. The oscillatory retrieval process nearly saturates this bound.