Studying neural interactions in the living brain at cellular resolution is a major challenge in neuroscience. In the fi rst part, I will present a novel ‘all-optical’ method for mapping neural connectivity in vivo. This method, involving 2-photon volumetric calcium imaging of neural activity and targeted 2-photon optogenetic perturbations, allows large-scale and rapid mapping of neuronal interactions in the living brain (500,000 neuronal pairs in 30 mins). Combining connectivity mapping with a novel naturalistic behavior in mice revealed functional connectivity motifs underlying a cognitive map for reward positions and outcomes in the frontal cortex. In the second part, I will show that analyses of interactions between 1,000,000 neurons, that we imaged simultaneously across multiple
cortical areas during the same behavior, revealed an intricate organization of cortical population dynamics and inter-areal communication patterns during action-selection. These results, which are based on recent advances in brain imaging technology, pave the way to study how neuronal interactions on different spatial scales give rise to behavior.