  The recent development of genetically encoded calcium indicators enables
monitoring in vivo the activity of neuronal populations. Most analysis of these
calcium transients relies on linear regression analysis based on the sensory
stimulus applied or the behavior observed. To estimate the basic properties of
the functional neural circuitry, we propose a network-based approach based on
calcium imaging recorded at single cell resolution. Differently from previous
analysis based on cross-correlation, we used Granger-causality estimates to
infer activity propagation between the activities of different neurons. The
resulting functional networks were then modeled as directed graphs and
characterized in terms of connectivity and node centralities. We applied our
approach to calcium transients recorded at low frequency (4 Hz) in ventral
neurons of the zebrafish spinal cord at the embryonic stage when spontaneous
coiling of the tail occurs. Our analysis on population calcium imaging data
revealed a strong ipsilateral connectivity and a characteristic hierarchical
organization of the network hubs that supported established propagation of
activity from rostral to caudal spinal cord. Our method could be used for
detecting functional defects in neuronal circuitry during development and
pathological conditions.
