11 October 2019
Neocortical networks must generate and maintain stable activity patterns despite perturbations induced by learning and experience-dependent plasticity, and this stability must be maintained across distinct behavioral states with very different sensory drive and modulatory tone. There is abundant theoretical and experimental evidence that network stability is achieved through homeostatic plasticity mechanisms that adjust synaptic and neuronal properties to stabilize some measure of average activity, and this process has been extensively studied in primary visual cortex (V1), where chronic visual deprivation induces an initial drop in activity and ensemble average firing rates (FRs), but over time activity is restored to baseline despite continued deprivation. Here I discuss recent work from the lab in which we follow bidirectional FR homeostasis in individual V1 neurons in freely behaving animals, as they cycle between natural periods of sleep and wake. We find that – when FRs are perturbed by visual deprivation or eye re-opening – over time they return precisely to a cell-autonomous set-point. Intriguingly, this firing rate homeostasis is gated by sleep/wake states in a manner that depends on the direction of homeostatic regulation: upward firing rate homeostasis occurs selectively during periods of active wake, while downward firing rate homeostasis occurs selectively during periods of sleep. These data indicate that neocortical plasticity is regulated in a complex manner by vigilance state, and raise the possibility that temporal segregation of distinct plasticity mechanisms is important for proper circuit refinement.
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