Figure 4: Symmetric STDP facilitates storage and retrieval of patterns in an autoassociative network model.

(a) Schematic illustration of network topology. The model is composed of several principal neurons (filled circles) and an inhibitory interneuron (filled triangle). Principal cells are interconnected by excitatory synapses (potentiated, open circles; unpotentiated, crosses). In the schematic, there are six pyramidal cells; the real model was composed of 3,000 pyramidal neurons. The mixture of potentiated and unpotentiated synapses in the matrix was generated by prior application of three binary activity patterns (001011, 101010 and 000111). Modified from McNaughton and Morris⁴. See Supplementary Table 1. (b) Plasticity rules. Top, symmetric STDP rule, as supported by the present results. Bottom, asymmetric STDP rule^15,16,17. Δt was given in normalized units, which could correspond to one theta oscillation cycle (∼200 ms). (c) Storage of patterns in the synaptic matrix. Synaptic weight was represented as temperature map (red, maximal potentiation; blue, unpotentiated). A single test pattern in the first 300 neurons and 10 additional patterns in randomly selected neurons were applied during storage, with randomized spike time in both cases. Ordinate and abscissa represent index of pre- and postsynaptic neuron, respectively. Insets (right) show expanded views for first 600 cells. Note that the symmetric STDP rule (top) induces a higher average potentiation than the asymmetric rule (bottom). (d) Recall of patterns in the network model. Recall was triggered by partial test patterns (50% valid firings, no spurious firings in comparison to original pattern) with randomized spike timing. With the symmetric STDP rule (top; proportionality factor of inhibition g₁=0.3), the original pattern was perfectly retrieved after three recall cycles. With the asymmetric rule (bottom; g₁=0.1), retrieval was only partial, with decrease in valid and increase in spurious firing. (e,f) Top, dependence of activity correlation (left) and spike-time correlation for active cells (right) on recall cycle number for symmetric (top) and asymmetric STDP rules (bottom). Coloured lines represent pattern correlation trajectories for 10 patterns. For the symmetric STDP rule, activity correlation increases, whereas spike-time correlation is eliminated. For the asymmetric STDP rule, activity correlation declines, whereas spike-time correlation becomes inverted. Bottom, three-dimensional plot of activity correlation at the 5^th recall cycle versus pattern load m and proportionality factor of inhibition g₁ for symmetric (e) and asymmetric (f) STDP rules. For the symmetric rule, capacity was 58.1, whereas for the asymmetric rule it was 4.5. Corr., correlation.