Topological analysis of neuronal assemblies reveals low-rank structure modulated by cholinergic activity.

Neuronal assemblies are fundamental building blocks of brain function (e.g. memory, spatial navigation, eye fixation, etc.). However, the principles underlying the connectivity patterns that support their function have remained elusive. The optic tectum of the zebrafish larva is organized into distinct functional neuronal assemblies. These assemblies display all-or-none preferred activation states and inhibitory competition, mechanisms that improve the decoding of visual information. Here, we combined light-sheet microscopy to capture the dynamics of large neuronal networks (~2,000 neurons) in the optic tectum; genetic cell-type markers for studying the physiological and functional properties of tectal assemblies; and techniques from topological data analysis to study the dynamic connectivity patterns that enable the emergence and functional role of the neuronal assemblies. We found that during spontaneous activations, tectal assemblies maintain a tight and stable ratio of E-I activity despite the large increase in activity. Topological analysis of the spontaneous activations indicated a low-rank organization of the assemblies and a discrete number of temporal activation patterns. Finally, we observed that the cholinergic system can modulate the topological features of the assemblies to alter their functional role.