Figure 3. Semantization of episodic memory traces. A, Schematic of the Item (green) and Context (blue) networks. Attractor projections are long-range connections across HCs in the same network and learned associative projections are connections between networks. B, Spike raster of pyramidal neurons in HC1 of both the Item and Context networks. Each context/item memory pattern corresponds to the activation of a unique set of MCs in its network. Items and their corresponding context representations are simultaneously cued in their respective networks (compare Fig. 2A). Each item is drawn with a unique color, while contexts inherit their coactivated item’s color in the raster (i.e., the yellow pattern in the Item network is repeated over four different contexts, forming four separate associations marked with the same color). The testing phase occurs 1 s after the encoding. Brief 50-ms cues of already studied items trigger their activation. Following item activation, we detect evoked attractor activation in the Context network. C, Average cued recall performance in the Context network (20 trials). The bar diagram reveals progressive loss of episodic context information (i.e., semantization) over the number of context associations made by individual cued items (compare Fig. 2A). D, Distribution of plastic connection weights between the Item and Context networks (NMDA component shown here). Weights are noticeably weaker for items which participate in multiple associations. The distributions of synaptic weights exhibit a broader range for the items with multiple context associations, as the sample size is larger. The inset displays the distribution of EPSPs for the binding between Item and Context networks. The EPSP distributions follow the trend of the associative weights. The amplitudes (<1 mV) are lower for higher context variability. E, The distribution of intrinsic excitability currents of pyramidal cells coding for specific context representations. The intrinsic excitability features similar distributions because each context is activated exactly once, regardless of whether the associated item forms multiple associations or not. F, Average cued recall performance in the Item network (20 trials). Decontextualization over the number of associations is also observed when we briefly cue episodic contexts instead (compare Fig. 2B). G, Distribution of strength of plastic connections from the contexts to their associated items. Analogously to D, synapses weaken once an item is encoded in another context. H, Intrinsic plasticity distribution of cells in the Item network. Intrinsic excitability distributions are higher for pyramidal cells coding for repeatedly activated items; ***p < 0.001 (Mann–Whitney, N = 20 in C, F). Error bars in C, F represent SDs of Bernoulli distributions. Distributions of one, two, three, and four associations in D, G, H show significant statistical difference (p < 0.001, Mann–Whitney, N = 2000).