Simulating Hebb cell assemblies: the necessity for partitioned dendritic trees and a post-not-pre LTD rule

Abstract

Cell assemblies are groups of interconnected neurons which become capable of self-sustaining, reverberatory activity through repetitive activation. The mechanisms proposed by Hebb are underconstrained and may not be sufficient to allow for the formation of cell assemblies. The authors goal was to arrive at the minimal requirements for the formation, persistence and reliable activation of simulated cell assemblies. They demonstrate that: (1) an anti-Hebb rule is needed to decrease the saturation of cell assembly activity, (2) a synaptic modification rule which decreases synaptic weights when post-synaptic activation occurs in the absence of pre-synaptic activity (post-not-pre) is necessary, but not sufficient, for stable assemblies, and (3) simulated dendritic trees must 'be partitioned into independent regions of activation. Furthermore, neither a synaptic modification rule for inhibitory synapses, nor a rule specifying a decrease in synaptic weight when pre-synaptic activation occurs in the absence of post-synaptic activation (pre-not-post) are necessary for the formation of persisting and reliable cell assemblies. The simulations are simple, but they point to interactions between network and unit architecture, synaptic modification rules, and cell assembly behaviour.