Retention of cell assemblies by weak STD.

(A) A first external input activates 20% of excitatory neurons (ca1, blue shaded area), and then a second input successively activates other 20% of excitatory neurons (ca2, green area). Neurons not stimulated by the external inputs are regarded as background (bg). (B) Time evolution of relative synaptic weight w₂. Blue shade indicates the interval of the first stimulus, and the green shade denotes the second one. We defined the retention time of a cell assembly as the time at which w₂ crosses threshold from above (w₂ = 0.015: dotted line). (C) Time evolution of the average synaptic weight for three values of u_sd. The weights were separately averaged over synapses within and between different cell assemblies and background neurons. In the left and middle panels, black lines for bg-to-bg connections are hidden behind purple lines. (D) Raster plots of spiking activity corresponding to the three cases shown in C. Color codes are the same as in Figure 2C. First 500 neurons belong to the first assembly and the second 500 neurons to the second assembly. (E) Synaptic weight matrices of excitatory connections are shown for the above three cases. (F), (G) The relative synaptic weight w₂ and the retention time of ca2 are shown as functions of the release probability u_sd. (H) Relationship between the input duration to ca1 and the relative synaptic weight w₂ at t = 30 min.