Figure 1. Possible synaptic modifications of two interconnected neurons fire nearly inphase in the presence of dendritic and axonal propagation delays.

(A) Two representative identical neurons which are reciprocally coupled by initially symmetric excitatory synapses. t₁ and t₂ are the exact spiking times of neurons 1 and 2, respectively. g_ij, τ_dij and τ_aij, i, j = 1, 2, j ≠ i denote synaptic strength, dendritic and axonal propagation delays of the synapse j → i, respectively. (B–D) Assuming dendritic and axonal delays are identical in both directions i.e. τ_d21 = τ_d12 = τ_d and τ_a21 = τ_a12 = τ_a, is the effective time lag for which STDP causes synaptic potentiation (upper)/depression (lower sign) of the corresponding synapse (horizontal dotted lines). T is the period of the spiking of the neurons. The vertical bars in the middle triple lines indicate spiking of the neurons. Green and red arrows represent the time of the transmission of the spike of the presynaptic neuron and the backpropagated potential of the postsynaptic neuron to the synaptic site, respectively. (B) Potentiation of both synapses in case τ_a < |Δt − τ_d|. (C) Formation of unidirectional connection when |Δt − τ_d| < τ_a < |Δt + τ_d|. (D) Depression of both synapses in case that |Δt + τ_d| < τ_a. (E–G) Illustration of corresponding synaptic modifications based on a balanced STDP profile and the schematic Gaussian distribution of pre (green)- and postsynaptic (red curve) spike times for different exemplary values of dendritic and axonal propagation delays used in our simulations.