Early life shifts in cortical inhibitory-excitatory balance underlies sensitive periods and skill development.

Early human development is characterized by sensitive periods which impact long-term cognitive and behavioral outcomes. While these windows of heightened plasticity are well documented, the cellular mechanisms that enable and regulate them remain incompletely understood. In this conceptual article, I propose that early-life shifts in cortical inhibitory-excitatory balance, driven by prolonged neurogenesis, migration, and maturation of GABAergic interneurons, play a central role in opening, shaping, and closing sensitive periods and thereby guide skill development. Drawing on evidence from human and animal studies, I synthesize findings showing that inhibitory interneurons are integrated into cortical circuits well into postnatal life, where they regulate intrinsic and sensory-driven activity, sculpt synaptic connectivity, coordinate interactions with glial cells, and progressively refine network dynamics. The developmental strengthening of inhibition alters excitation-inhibition ratios, drives the transition from highly synchronous early activity to decorrelated and efficient adult-like firing patterns, and gates critical period plasticity across cortical regions. I argue that these inhibitory processes are not merely stabilizing but actively facilitate learning by suppressing non-relevant activity and enabling the emergence of specialized functional networks. This framework highlights fundamental differences between infant and adult learning mechanisms and suggests that individual variability in inhibitory circuit development may underlie differences in cognitive trajectories and vulnerability to neurodevelopmental disorders. Together, this synthesis positions early inhibitory interneuron development as a key mechanistic substrate linking sensitive periods to lifelong skill acquisition and behavioral individuality.