Functionally similar presynaptic neurons put synapses slightly closer on shared dendrites in the MICrONS mouse, but presynaptic soma position explains this; no branch-scale clustering found

Question: do excitatory neurons with similar visual responses put their synapses near each other (10 um or less) on the dendrite of a shared target cell, as functional synaptic clustering predicts? Data: one mouse, MICrONS visual cortex. We used the v1300 synapse export with positions, v661 dendrite skeletons of 63,348 target cells, and the response properties of 12,894 coregistered neurons (Ding et al. 2025). Method: for each target cell, we measured the path distance along the dendrite between the closest synapses of each pair of functional presynaptic neurons. We compared the response similarity of close pairs with other pairs on the same cell, against a within-cell permutation null that keeps presynaptic layer, area and synapse count, with a jackknife over presynaptic neurons. Results: with all functional neurons (31.5 million pairs, 305,200 close pairs), close pairs had a higher signal correlation by +0.0036 (95% CI 0.0030 to 0.0043), about 17% of the like-to-like wiring effect. The excess was the same from 0 to 40 um and decreased to zero near 100-160 um, so it is a broad gradient, not a branch-scale cluster. Close pairs compared only with pairs 10-40 um apart gave +0.0004 (-0.0004 to 0.0013). Presynaptic pairs with close synapses had nearer somata; after adjustment for soma distance, layer and area, the excess was +0.0001 (-0.0005 to 0.0008), at most about 4-6% of the like-to-like effect. Feature similarity and receptive-field proximity behaved the same; orientation gave only a 0.4-degree coarse effect. In vivo signal correlation kept a small residual (+0.0012) only for neurons imaged in the same scan, which suggests shared imaging signal. The 148 proofread axons gave wide intervals that agree. Limitations: one animal; most axons not proofread (a local-synapse subset agreed); calcium and digital-twin tuning only; about 1% of inputs are characterised.

functional synaptic clustering · dendrites · connectomics · MICrONS · mouse visual cortex · signal correlation · like-to-like wiring · permutation test · null result