Large-scale cellular-resolution read/write of activity enables discovery of cell types defined by complex circuit properties

The complexity of the mammalian brain’s vast population of interconnected neurons poses a formidable challenge to elucidate its underlying mechanisms of coordination and computation. A key step forward will be technologies that can perform large-scale, cellular-resolution monitoring and interrogation of distributed brain circuit activity in behaving animals. Here, we present an all-optical strategy for precise optogenetic activity control of ∼10 ³ neurons and simultaneous activity monitoring of ∼10 ⁴ neurons within and across areas of mouse cortex—an order-of-magnitude leap beyond previous capabilities. Tracking population responses following delivery of precisely-defined widely-distributed activity patterns to the visual cortex of awake mice, we were surprised to identify neurons robustly responsive to stimulation of diverse ensembles, defying conventional like-to-like wiring rules. These cells were primarily deep L2/3 somatostatin-positive (SST) interneurons with functional properties distinct from other SST neurons, and appeared to play a role in brain dynamics that could only have been identified through broad cellular-resolution circuit interrogation. Our work reveals the value of measuring large-scale circuit-dynamical properties of functionally-resolved single cells, beyond genetic and anatomical classification, to define and explore the roles of cell types in brain function.