How balance between LTP and LTD can be controlled in spike-timing-dependent learning rule

Spike-timing-dependent plasticity (STDP) has been suggested to play a role in developing functional cortical circuits. However, for STDP to contribute to the organization of synapses, the STDP learning curve should satisfy a requirement that the magnitude of long-term potentiation (LTP) is slightly smaller than that of long-term depression (LTD). In the absence of this approximate balance between LTP and LTD, all the synapses are potentiated toward the upper limit or depressed toward the lower limit. Therefore, in this study, we explore the conditions under which the LTP/LTD balance in STDP can be controlled adequately. We examine a plasticity model that incorporates the activity-dependent feedback (ADFB) mechanism where LTP induction is suppressed by higher postsynaptic activity. In this model, increasing the strength of ADFB function gradually decreases the temporal average of the ratio of the magnitude of LTP to that of LTD, whereas enhancing background inhibition level augments this ratio. Additionally, we demonstrate that the changes in LTP/LTD balance is accompanied by the alteration in the variability of output firing as well as the synaptic pattern obtained by STDP.