Before revision:   Most unsupervised SNNs are trained through competitive learning with Spike-Timing-Dependent Plasticity (STDP).
After revision:  Among existing SNNs, unsupervised SNNs are based on synaptic plasticity and considered to have more potential in imitating the learning process of biological brain. Most unsupervised SNNs are trained through competitive learning with Spike-Timing-Dependent Plasticity (STDP).