The Developing Brain

In a now-classic series of experiments published in 1963, David Hubel and Torsten Wiesel sutured shut one eye of newborn kittens for varying periods: if reopened within the first three months vision recovered, but if reopened later the kitten remained permanently blind in that eye, even though retina, optic nerve, and visual cortex were all anatomically intact. The visual cortex's binocular columns had been taken over by the open eye and could not be reclaimed after the critical period had closed. (They shared the 1981 Nobel Prize for this and related work.) The brain is not built once but across an extended developmental schedule, and what does not happen during the schedule windows often cannot happen afterward — a principle that generalizes far beyond vision to language acquisition, attachment, and social cognition.

The human brain develops across a protracted timeline unique among mammals, with substantial maturation continuing into the mid-20s. By eight weeks gestation the basic neural tube has formed; by birth most of the brain's ~86 billion neurons are in place. Synaptogenesis in the first two postnatal years adds synapses at enormous rates, peaking around age two with densities roughly twice those of adults; synaptic pruning then removes about half across childhood and adolescence in a use-dependent manner — sensory cortex prunes earliest (largely complete by mid-childhood) while prefrontal cortex continues into the mid-20s. Myelination follows a posterior-to-anterior schedule: occipital and parietal cortex first, prefrontal last. Critical periods are windows of heightened plasticity. The visual binocular-fusion period extends to ~5–7 years (after which untreated amblyopia permanently impairs the affected eye). First-language acquisition has a period extending to puberty — the case of Genie, raised in isolation until 13, never acquiring full grammar. The Romanian orphan studies (Nelson, Zeanah, Fox) found that children placed in foster care before age 2 showed substantial recovery of cognitive and social function while those placed after 2 showed persistent deficits. The adolescent brain shows dramatic changes — the limbic reward system matures earlier than the prefrontal cortex, producing the high reward sensitivity and impaired impulse control of teenagers — and this limbic-prefrontal mismatch has been invoked to explain peak risk-taking and the onset of many psychiatric disorders (schizophrenia, depression, substance-use disorders peak in this window).