Most neural connections exhibit synaptic plasticity, increases or decreases in synaptic efficacy. Several distinct forms of synaptic plasticity exist, differing in both their induction requirements and time course of expression. Synaptic plasticity allows for dynamic modification of neural circuitry that can act on time scales ranging from milliseconds to potentially lifetimes, and has been implicated in a wide range of neural and behavioral phenomena including learning and memory. A body of literature has developed demonstrating that the ability to induce synaptic plasticity is itself modifiable; that is, plasticity is plastic. This is referred to as metaplasticity, a higher-order form of synaptic plasticity. Metaplasticity's critical feature is that, once instantiated, it modifies the ability of subsequent activity to alter synaptic efficacy, fundamentally altering the rules that govern when and how changes are expressed.

Metaplasticity has been observed in a variety of brain systems and across different species, suggesting that...