a Propagation across neuron populations synchronized spike volleys due to repeated input volley presentations. The 15 input axons were activated with 20 spike volleys (top line) that were propagated across neuron populations one (line 2), two (line 3), and three (line 4). For each neuron group, spikes of the 15 excitatory neurons are shown in 15 successive lines. Spike volleys became progressively synchronized for successive groups and for successive volley presentations. Since all time constants of the network are much smaller than the intervolley interval, this synchronization appears to be caused by temporally asymmetric Hebbian learning. b Connection strength after 20 volley presentations depends on connection types and connection delays. Synaptic conductances of feedforward connections (mean 2.1 nS) became much larger than those of feedback (mean 0.26 nS) and intragroup connections (mean 0.76 nS). Furthermore, conductances of feedforward connections with short delays became larger than those with long delays (correlation = −0.57, t(673) = 18, p < 0.001)