In the visual cortex of young mice, a single light exposure triggers a molecular cascade that shapes the boundaries of childhood learning for life. Harvard Medical School researchers have discovered that the stress hormone cortisol acts as a developmental timer, closing critical periods of brain plasticity—the precious window when our brains are most primed to absorb the world around us.
The finding matters because it reveals, for the first time, how the brain knows when to lock away its learning capacity. In infancy and early childhood, critical periods stay open, making developing brains exquisitely sensitive to experience. Sights, sounds, and sensations during these windows sculpt neural connections that echo into adulthood. But as children mature, these periods gradually close through mechanisms that neuroscientists have long puzzled over. Understanding that timing is essential to grasping not only normal development but also what goes wrong in developmental disorders and psychiatric conditions.
Bruno Gegenhuber, a research fellow in neurobiology at Harvard Medical School, was studying how vision shapes the mouse brain when he stumbled on an entirely new plasticity pathway. Working in Michael Greenberg's lab at the Blavatnik Institute, Gegenhuber and his team used single-cell sequencing to compare young mice raised in normal light with those raised in darkness. They discovered that light exposure triggers the release of corticosterone—the rodent equivalent of cortisol—from the adrenal glands into the bloodstream. Once there, cortisol binds to receptors on astrocytes, star-shaped brain cells that sit at the intersection between blood and neurons, translating signals from the body into chemical messages the brain understands.
That binding event activates a cascade of more than 100 genes within those astrocytes. This gene program orchestrates the maturation of the extracellular matrix around neurons, including specialized structures called perineuronal nets. These nets wrap around neural connections like a cage, restricting how fluidly those connections can form and reform—a physical change that explains why brain plasticity fades. In mice raised in the dark, this entire sequence never triggered, and critical-period closure stalled. Even more striking, when researchers removed glucocorticoid receptors from adult mice, they found that previously closed critical periods reopened, restoring youthful brain plasticity.
The Greenberg Lab then analyzed existing single-cell data from human brains and confirmed that the same pathway unfolds during human infancy, peaking around adolescence. This discovery opens new windows into why early-life stress derails development—chronic stress flooding the brain with cortisol could accelerate or distort this critical-period closure. It also hints at how some neurodevelopmental and psychiatric conditions linked to timing problems might arise.
Greenberg, the Nathan Marsh Pusey Professor of Neurobiology at HMS and senior author of the study, notes that the implications reach far beyond basic science. The findings, published in Nature and conducted in collaboration with Boston Children's Hospital, could eventually reshape how researchers think about brain maturation and plasticity across the lifespan. The team's next step is to catalog and characterize each of those 100-plus genes to understand precisely how they reshape the brain's learning landscape as childhood fades into adulthood.