Models of cell assembly decay

Hebb considered the cell assembly to represent a concept in the brain and thus to be an underlying construct of human thought. He proposed that the cell assembly is a connected group of neurons whose pattern of firing is such that a reverberatory activity persists after the removal of a stimulus. Once a cell assembly is activated something must eventually cause it to decay. Clearly thoughts have to be extinguished to make way for others, the question is how. Various suggestions have been made concerning mechanisms that could cause an assembly to decay in the long term including inhibition by other assemblies and passive fatigue. In this paper two classes of models are used to implement this decay, the first is based on building cell assemblies with specific weights and connections that have a linear decay. The second class is based on manipulating variables within a cell assembly model, creating long term fatigue or activation decay. This class of models may be more biologically plausible than the first, and can produce the expected temporal dynamics in the presence of an ambiguous stimulus. However neither class can yet produce the correct prolongation of activation when the stimulus is re-presented.