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 theorized as internal representations of specific stimuli or actions. Alternatively, cell assemblies can be defined without reference to an external world, by their endogenous ability to effectively elicit specific responses in downstream (‘reader’) neurons. However, this compelling framework currently lacks experimental support. Here, we provide evidence for 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 associative learning, indicating that they were plastic and could become bound to behaviorally relevant variables. These results support cell assemblies as an endogenous mechanism for brain function.