A continuing debate in language acquisition research is whether there are critical periods (CPs) in development during which the system is most responsive to environmental input. Recent advances in neurobiology provide a mechanistic explanation of CPs, with the balance between excitatory and inhibitory processes establishing the onset and molecular brakes establishing the offset of windows of plasticity. In this article, we review the literature on human speech perception development within the context of this CP model, highlighting research that reveals the interplay of maturational and experiential influences at key junctures in development and presenting paradigmatic examples testing CP models in human subjects. We conclude with a discussion of how a mechanistic understanding of CP processes changes the nature of the debate: The question no longer is, “Are there CPs?” but rather what processes open them, keep them open, close them, and allow them to be reopened.

Brain plasticity is dynamically regulated across the life span, peaking during windows of early life. Typically assessed in the physiological range of milliseconds (real time), these trajectories are also influenced on the longer timescales of developmental time (nurture) and evolutionary time (nature), which shape neural architectures that support plasticity. Properly sequenced critical periods of circuit refinement build up complex cognitive functions, such as language, from more primary modalities. Here, we consider recent progress in the biological basis of critical periods as a unifying rubric for understanding plasticity across multiple timescales. Notably, the maturation of parvalbumin-positive (PV) inhibitory neurons is pivotal. These fast-spiking cells generate gamma oscillations associated with critical period plasticity, are sensitive to circadian gene manipulation, emerge at different rates across brain regions, acquire perineuronal nets with age, and may be influenced by epigenetic factors over generations. These features provide further novel insight into the impact of early adversity and neurodevelopmental risk factors for mental disorders.