Steady or changing? Long-term monitoring of neuronal population activity

•We review in vivo chronic extracellular recording and two-photon calcium-imaging studies that address the question of the stability of neuronal population activity.•Chronic two-photon calcium imaging has become a powerful tool for analyzing the long-term stability and plasticity of neuronal activity.•Sensory cortex contains stable subnetworks of highly active neurons embedded in a sea of relatively unresponsive cells.•Throughout the cortex, neuronal activity may be adaptively stabilized by learning.

Stability and flexibility are both hallmarks of brain function that allow animals to thrive in ever-changing environments. Investigating how a balance between these opposing features is achieved with a dynamic array of cellular and molecular constituents requires long-term tracking of activity from individual neurons. Here, we review in vivo chronic extracellular recording studies and recent long-term two-photon calcium-imaging investigations that address the question of stability and plasticity of neuronal population activity in the mammalian brain. Overall, spiking activity is heterogeneously distributed among neurons in local populations and largely remains stable for individual cells over time. Tuning properties appear more flexible and may be adaptively stabilized, possibly by neuromodulators, to encode reliably and specifically salient stimuli or behaviors.