Sub-threshold neuronal activity and the dynamical regime of cerebral cortex

Cortical neurons exhibit temporally irregular spiking patterns and heterogeneous firing rates. These features arise in model circuits operating in a ‘fluctuation-driven regime’, in which fluctuations in membrane potentials emerge from the network dynamics. However, it is still unclear whether the cortex operates in this regime. We evaluated the fluctuation-driven hypothesis by analyzing spiking and sub-threshold membrane potentials of neurons in the sensory and frontal cortex recorded during a decision-making task. Standard fluctuation-driven models account for spiking statistics but fail to capture the heterogeneity in sub-threshold activity. We address this issue by effectively incorporating dendritic conductances into the standard models. Our model suggests that the frontal cortex operates in a fluctuation-driven regime. In contrast, excitatory neurons in layer 4 of the barrel cortex are not fluctuation-driven; they spike in response to occasional synchronous inputs. Our work reveals fundamental differences between cortical areas, suggesting that they operate in different dynamical regimes. ### Competing Interest Statement The authors have declared no competing interest.