Development of stable transgenic all-optical tools enabling read-write access to thousands of individually-defined cortical neurons reveals a functionally distinct subclass of deep layer 2/3 somatostatin-positive interneurons that exhibit general ensemble-response properties, being robustly recruited by diverse large-scale excitatory activity patterns through broad spatial integration and low activation thresholds, thereby defying conventional like-to-like connectivity rules and suggesting a specialized role in monitoring and normalizing distributed cortical excitation.

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 ∼103 neurons and simultaneous activity monitoring of ∼104 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.