Fig. 5: Postsynaptic neurons with a common input are more functionally similar to each other than expected from a pairwise like-to-like rule.

a, Left, schematic illustrating the null hypothesis that postsynaptic neurons (postsyns, dark grey circles) of a common presynaptic neuron (presyn, yellow circle) have no additional feature similarity with each other beyond their like-to-like similarity with their common presynaptic neuron. Neurons are shown embedded in functional space, where units with similar functional properties are closer together. In this scenario, postsynaptic neurons are distributed uniformly around the presyn in the like-to-like region of functional space (large grey circle). Right, schematic illustrating the alternative hypothesis that the postsynaptic neurons are closer in functional space than predicted from a pairwise like-to-like rule, equivalent to being clustered non-uniformly within the like-to-like region. b, Schematic illustrating the functional connectivity model used to simulate the null hypothesis in a. Left, pairwise functional measurements—including signal correlations, feature weight similarity and RF location distance—were passed through a function relating functional similarity to connection probability. Right, then, within this modelled network, we computed the pairwise similarity of all postsynaptic neurons downstream of a common presynaptic neuron. c, Observed mean pairwise signal correlations between postsynaptic neurons compared with those expected from model predictions across projection types (n = 52 presynaptic neurons for HVA→HVA, n = 38 for HVA→V1, n = 17 for V1→HVA, n = 35 for V1→V1). Box plots show median, interquartile range (box) and 1.5× interquartile range (whiskers); points indicate outliers. Three of four projection types showed significantly higher similarities than predicted. Two-sided Wilcoxon signed-rank test; exact P values are presented in Supplementary Table 21.