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