Imprinting ducklings, kittens with one eye covered, children learning languages and orphans in Romania: the evidence for sensitive windows when experience shapes the brain most powerfully.
Development & Plasticity
Intermediate
Critical Periods: Windows of Opportunity
The Editors
May 13, 2026
3 min read
In plain English
A critical period is a window in development when the brain is especially sensitive to certain experiences. If the right input arrives, the circuit develops normally; if it's missing, recovery later can be hard. Vision and language have well-known windows.
Intermediate level: Builds on the basics. Introduces the real vocabulary and the mechanisms behind it.
Concepts to know first
Some brain circuits need particular experiences at particular times to develop normally. These windows are called critical periods (if the effects are permanent) or sensitive periods (if they're strong but not absolute).
Imprinting
In the 1930s the Austrian zoologist Konrad Lorenz found that newly hatched greylag geese follow the first moving object they see — normally their mother, but sometimes Lorenz himself — and form a lasting bond with it. This imprinting happens only in a short window after hatching.
Vision
Hubel and Wiesel showed that covering one eye of a kitten for a few months during an early window left it permanently unable to use that eye properly, even though the eye was healthy — the cortex had rewired to favour the open eye. The same deprivation in adult cats had little effect. In children, this is why congenital cataracts are removed early and why amblyopia ("lazy eye") is treated by patching the stronger eye in early childhood; the older the child, the less effective treatment tends to be (though some adult improvement is possible).
Language
Children learn languages, especially their sounds and grammar, with an ease adults rarely match. People who learn a second language after childhood usually retain an accent. Rare and tragic cases of children deprived of language exposure, and studies of deaf children who receive sign language or cochlear implants at different ages, suggest a sensitive period for first-language acquisition, with early exposure giving the best outcomes.
Social and emotional development
The Bucharest Early Intervention Project, begun in 2000, randomly assigned some young children in Romanian institutions to foster care. Children placed earlier — especially before about two years of age — showed better outcomes in IQ, attachment and some brain activity measures, pointing to sensitive periods for social and cognitive development.
Can windows be reopened?
Critical periods close partly because inhibitory circuits mature and molecular "brakes" — such as perineuronal nets around certain neurons — form. In animals, removing some of these brakes can reopen plasticity in adulthood, a promising line of research for recovery from injury.
Everyday examples & things to try
Analogy
Wet cement
In a sensitive period, a circuit is like wet cement: easy to shape, and the impression lasts. Later the cement sets. You can still carve it, but it takes far more effort.
Real world
Why toddlers get eye checks
Many health systems screen young children's vision because problems like a misaligned eye can be corrected early, while the visual cortex is still plastic. Wait too long and the brain may permanently ignore input from the weaker eye.
Key takeaways
- Critical/sensitive periods are windows when experience strongly shapes specific circuits.
- Examples: imprinting, binocular vision (kittens, amblyopia), language and early social care.
- Windows close as inhibition matures and molecular brakes form; research aims to reopen them.
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