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. 2017 Dec 13;37(50):12153–12166. doi: 10.1523/JNEUROSCI.0588-17.2017

Figure 2.

Figure 2.

The spatial organization of the GCL restricts combinatorial expansion. A, We constructed spatially realistic models of the GCL, shown here in a simplified rendering. Large spheres represent simulated MFRs, with colors representing ID numbers. Gray spheres represent GrCs. Gray lines indicate GrC dendrites. A 247 MFR system is depicted here, with all synaptically connected GrCs per MFR rendered. Scale bar, ∼25 μm. B, Diagrams illustrating low- and high-diversity systems. Bottom rows, MFRs with ID indicated by color. GrCs (middle) combine local MFR inputs to assume a unique combinatorial identity (blended colors). Throughout the remainder of the study, we examine how MFR diversification and clustering in a spatial model of the GCL influence GrC combinatorial diversity, testing, for example, how spatial segregation of inputs favors particular combinations (e.g., the purple GrC, indicating that a theoretical combination of red and blue MFRs cannot exist because red and blue MFRs are too far apart). C, Effect of GrC dendrite length on access to unique MFRs (mean ± SEM; n = 10 systems with 10 trials each, for each MFR diversity level). An anatomically realistic dendritic range between 12 and 20 μm restricts access to ∼6 unique inputs, regardless of the diversity of the system. D, Populations of combinatorially unique GrCs as defined by their k-input combinations (quartets, triplets, etc.) as a function of increasing numbers of unique MFR identities uniformly distributed across the population. Dashed line indicates the total number of GrCs in the system. E, A function plotting the relative contribution of each additional MFR ID to the total number of MFR combinations made by GrCs (Eq. 1). As the model diversifies with increasing numbers of unique MFRs, fewer new combinations are produced per additional MFR ID number, peaking at 5%–20% diversity (i.e., between 5 and 20 MFRs/100 share the same ID) for quartet and triplet systems. This peak indicates the point beyond which increasing diversity contributes relatively less to the number of unique MFR combinations made by GrCs. F, The effect of MF LTP on combinatorial representation. With a GrC firing threshold of 4 active inputs, a full quartet of active MFRs is required to drive activity. As the strengths of inputs increase, the mean number of potential combinations that can drive a given GrC increase.