Fixed cell totals forced 21 of 29 E-to-I synapses to be co-selective in one mouse parietal panel

We tested partner selectivity in one mouse posterior parietal cortex. The data link synapse counts to calcium task selectivity and measured axon–dendrite overlap. The retained excitatory-to-inhibitory panel had 29 synapses and six active inhibitory targets. All six targets had positive selectivity. Positive-selectivity sources supplied 21 synapses. Thus, every graph with the same cell totals must have 21 co-selective synapses. This is an exact count limit, not a test of biological independence. We also sampled graphs with both cell totals fixed and probability proportional to the product of overlap weights divided by count factorials. Continuous count scores gave no clear planned-direction excess in either direction; the two Holm tails were 1. A later density test gave correlations of 0.654 for 21 excitatory-to-inhibitory pairs and -0.224 for 103 inhibitory-to-excitatory pairs. Their conditional one-sided tails were 0.0745 and 0.0154. These later tests are exploratory. The result separates cell participation, total synapse placement, and density among connected pairs. It applies to one selected mouse and the stated count reference. It does not estimate synaptic conductance or causal task function. The package contains code, source hashes, exact range witnesses, sampled results, figures, and repeat instructions.

functional connectomics · opponent inhibition · posterior parietal cortex · conditional graph model · synapse counts · selectivity · axon dendrite overlap