Robustness of retrieval properties against imbalance between long-term potentiation and depression of spike-timing-dependent plasticity

TL;DR

This study demonstrates that associative memory models with spike-timing-dependent plasticity (STDP) can maintain robust retrieval of patterns despite imbalances between potentiation and depression, emphasizing the importance of average balance over precise tuning.

Contribution

It shows that the stability of retrieval in STDP-based models does not require exact LTP/LTD balance, but only an average balance, using a statistical neurodynamics approach.

Findings

Storage capacity remains finite despite large fluctuations in LTD.

The basin of attraction shrinks as imbalance variance increases.

Stable retrieval is possible with average, not precise, LTP/LTD balance.

Abstract

Spike-timing-dependent plasticity (STDP) has recently been shown in some physiological studies. STDP depends on the precise temporal relationship of pre- and post-synaptic spikes. Many authors have indicated that a precise balance between long-term potentiation (LTP) and long-term depression (LTD) of STDP is significant for a stable learning. However, a situation in which the balance is maintained precisely is inconceivable in the brain. Using a method of the statistical neurodynamics, we show robust retrieval properties of spatio-temporal patterns in an associative memory model against the imbalance between LTP and LTD. When the fluctuation of LTD is assumed to obey a Gaussian distribution with mean 0 and variance $\delta^2$, the storage capacity takes a finite value even at large $\delta$. This means that the balance between LTP and LTD of STDP need not to be maintained precisely, but must be maintained on average. Furthermore, we found that a basin of attraction becomes smaller as $\delta$ increases while an initial critical overlap remains unchanged.