Geometric eigenmodes beat connectome eigenmodes for FC in fly, mouse and human but not in worm; simulated connectome-driven FC with spatial blur gives the same result

Question: Pang et al. (2023) reported that eigenmodes of cortical geometry represent human fMRI better than connectome eigenmodes. Does this occur in other animals, and can the comparison identify what makes functional connectivity (FC)? Data: campaign-derived structure and FC for worm (40 animals, calcium imaging, Cook connectome), fly (49 flies, light-field imaging, FlyWire), mouse (group fMRI of 79 mice, Allen tracer model) and human (group fMRI of 69 persons, DSI). Method: normalised-Laplacian eigenmodes of the log-weight connectome and of a distance kernel; share of spatially centred FC variance in the first N/4 modes (AUC); distance-bin shuffle nulls; linear-dynamics simulations with spatial blur; a matched generative fit. Results: geometric modes captured more variance in fly (+0.085, 44 of 49 flies), mouse (+0.055) and human (+0.043), also with the best of six variants per graph type. In worm the sign changed with trace processing (raw -0.010, 27 of 40 animals favoured the connectome; first differences +0.006). In simulation, FC made only by the connectome plus a blur of 0.5 to 1 nearest-neighbour distance gave geometric advantages of +0.00 to +0.19, so the eigenmode rule called the generator at chance level. The distance-bin shuffle null also gave p <= 0.05 for 43 of 80 FC sets made only by geometry. A two-parameter generative fit identified the generator in 95% of simulated cases; on real data its connectome and geometric fits differed by less than 0.01 in fly (all frames), mouse and human, favoured the connectome in worm raw traces and geometry in fly after global-signal regression. Conclusion: geometric eigenmodes are a better compact FC basis in three animals, but this does not show that geometry rather than the connectome makes FC. Limitations: region-level graphs, group FC only for mouse and human, Euclidean distance, a simple linear model, and post-hoc controls.

eigenmodes · geometric eigenmodes · connectome · functional connectivity · structure-function · C. elegans · Drosophila · FlyWire · mouse fMRI · Allen mouse connectivity · human connectome · spatial autocorrelation · null models · cross-species