Opposite and complementary roles of the two calcium thresholds for inducing LTP and LTD in models of striatal projection neurons

Synaptic plasticity has been shown to occur when calcium, flowing into the synapse due to incoming stimuli, surpasses a threshold level. This threshold level is modifiable through a process called metaplasticity. Some neurons, such as the striatal projection neurons, use different sources of calcium as the signal for synaptic strengthening (long-term potentiation, LTP) or weakening (long-term depression, LTD), resulting in them having two thresholds for inducing plasticity. In this study, we show opposite and complementary roles of metaplasticity in these two thresholds for inducing LTP and LTD on learning how to solve the linear and nonlinear feature binding problem (FBP and NFBP). In short, metaplasticity in one threshold (e.g. LTD) allows synaptic plasticity of the opposite type (e.g. LTP) to be properly expressed. This happens because metaplasticity in the LTD threshold protects strengthened synapses from weakening, thus allowing them to persistently increase during learning (and encode learned patterns). Similarly, metaplasticity in the LTP threhsold prevents weakened synapses from strengthening, thus allowing them to persistently decrease. Metaplasticity in both thresholds is necessary when synapses are clustered and the neuron needs to rely on supralinear dendritic integration for learning.