Learning with filopodia and spines: Complementary strong and weak competition lead to specialized, graded, and protected receptive fields.

Abstract

Filopodia are thin synaptic protrusions that have been long known to play an important role in early development. It has recently been found that they are more abundant in the adult cortex than previously thought, and more plastic than spines (button-shaped mature synapses). Inspired by these findings, we introduce a new model of synaptic plasticity that jointly describes learning of filopodia and spines. The model assumes that filopodia exhibit strongly competitive learning dynamics -similarly to additive spike-timing-dependent plasticity (STDP). At the same time it proposes that, if filopodia undergo sufficient potentiation, they consolidate into spines. Spines follow weakly competitive learning, classically associated with multiplicative, soft-bounded models of STDP. This makes spines more stable and sensitive to the fine structure of input correlations. We show that our learning rule has a selectivity comparable to additive STDP and represents input correlations as well as multiplicative models of STDP. We also show how it can protect previously formed memories and act as a synaptic consolidation mechanism. Overall, our results can be seen as a phenomenological description of how filopodia and spines could cooperate to overcome the difficulties that strong and weak competition each have.