Hebbian plasticity. Classic Hebbian plasticity includes homosynaptic long-term synaptic potentiation (LTP) and long-term synaptic depression (LTD). Coincident activation in pre-synaptic neuronal inputs and the post-synaptic neuron strengthens specific synapses (shown in red). Inactive pre-synaptic inputs (or inputs out of phase with post-synaptic depolarization) are not potentiated and/or are depressed (shown in blue). The term “homosynaptic” refers to the fact that plasticity only occurs at the stimulated synapse.