CRITICAL PERIOD   [ back to Glossary Index ]
Critical Period is a neurologic term for the narrow windows of time in which certain types of brain development will take place in the presence of appropriate stimuli. It has been studied extensively in mammals, including cats, rats, monkeys and humans. The classic study was done with kittens. Those kittens which had their eyelids sewn together during a certain period of weeks after birth, never developed the brain circuitry in their occipital lobes essential for processing visual information. Even though their eye structures were completely normal, they remained "cortically blind" their entire lives.

Cognitive development research on human children done over the past few decades showed that enormous and highly accelerated brain growth occurs from birth until age 3, and that the brain was largely complete in terms of size and wiring patterns by age 5. An important part of this wiring process became known as neural "pruning." The human infant is born with a great excess of brain cells. During his early years, neurons which get stimulated grow and make connections with other neurons, because their services are needed and they have a purpose; but neurons which are not stimulated by the child's environment die out and get "pruned" from the neural vine. Home and pre-school environments can be classified as "enriched" or "impoverished" in conformity with the quantity and quality of brain stimulation they present.  This research spawned a whole literature for pediatricians, educators and parents encouraging them to provide maximum stimulation to their infants, toddlers and pre-schoolers to boost their brain development during the critical period when the number, the thickness and the complexity of synaptic connections was determined. It was believed by many people during the 1980s and 1990s that adult IQ was shaped and fixed by the time the child reached age 5.

New research has confirmed that the adult's capacity for intelligence,  creativity and normal social interaction is shaped to a great extent by how his parents care for him and stimulate him before age 5. However, it has also shown the existence of other spurts of brain growth later in life. In March 2000 neurologist Arthur Toga of the UCLA School of Medicine announced the findings of his research into the maturing of the human brain in the Journal of COgnitive Neuroscience. It showed that frontal lobe development has a major peak during age 3-6 and that from age 6 to puberty, gray matter development shifted to the temporal lobe (the language area) and the parietal lobe (where spatial relationships are processed).

At age 11 in girls and age 12 in boys there was another spike in frontal lobe development accompanied by a sharp drop in temporal lobe development (which may explain why it is so hard to learn a new language after age 12). From the mid-teens to the early 20s there was a substantial loss of gray matter in the frontal lobes.  Dr. Toga was heartened by the data, because it means pubescent children can shape and improve their brains through their own choice of school and home activities, and their diligence in pursuing those activities. The data also explains why the 15-24 year old age group has far more traumatic brain injuries than any other, most of them associated with alcohol consumption and reckless driving, i.e. because loss of frontal lobe circuits leads to impaired judgment and extravagant risk-taking.