Modeling binocular competition through Hebbian plasticity and constrained connectivity

In the mammalian brain, inputs from the two eyes compete with each other during development for synaptic contacts with postsynaptic targets. Traditionally, binocular competition has been modeled using a Hebbian-like plasticity rule together with a fixed-sum constraint on total connectivity. Previous work has shown the certain, but not all, formulations of the constrained-connectivity model lead to binocular competition. These differences between model formulations had been analyzed using a correlation-based approach, particularly relevant for the developmental period after eye opening, when the two eyes are normally active together but at varying levels of inter-ocular correlation. More recently, experimental work has established a role for Hebbian plasticity in binocular competition also prior to eye opening. In contrast to later developmental stages, at this pre-vision stage, activity of the two eyes is largely de-correlated. The goal of the present study is twofold. First, a simplified analysis of the constrained-competition model is proposed for the case of de-correlated binocular activity as observed prior to eye opening. This analysis provides a more intuitive explanation for the sensitivity of the constrained-connectivity model to formulation details. Second, based on recent experimental findings, this work proposes a new, modified model in which binocular competition increases the total connectivity instead of operating under a fixed-sum constraint. Simulations and analysis show that this form of “growth-promoting” competition is less sensitive to formulation details and may therefore be biologically advantageous at early stages of visual development.