Flexible communication between cell assemblies and ‘reader’ neurons

Cell assemblies are considered fundamental units of brain activity, underlying diverse functions ranging from perception to memory and decision-making. Cell assemblies have historically been conceived of as internal representations of specific stimuli or actions. Alternatively, cell assemblies can be endogenously defined by their ability to effectively elicit specific responses in downstream (‘reader’) neurons. Yet, whether cell assemblies are selectively detected by downstream neurons remains unknown. Here, we provide evidence for such assembly–reader communication. Reader activation was genuinely collective, functionally selective, yet flexible, implementing both pattern separation and completion. These processes occurred at the time scale of membrane integration, synaptic plasticity and gamma oscillations. Finally, assembly–reader couplings were selectively modified upon learning, indicating that they were plastic and reflected behaviorally relevant variables. These results support cell assemblies as an endogenous mechanism for brain function