In developmental psychology, there is a distinction between sensitive and critical periods. Both are hypothetical periods of time, typically characteristic of childhood. They are not set standards; just guidelines that usually work.
A sensitive period is a relatively long period of time when the developing cells are easily damaged. A developing fetus, for example, is highly sensitive to alcohol; even minimal drinking can cause damage. The developing nervous system is sensitive during the entire pregnancy. Exposure to rubella (German measles) just before birth or just after birth, if left untreated, can be devastating to the infant.
A sensitive period can also imply additional plasticity of neurons, aiding in learning. In general, if you want to be expert, start when you are young. Children seem naturally more sensitive to learning stimuli than adults. Language acquisition seems particularly sensitive to age. It is difficult, if not impossible, to be a native speaker of a language that you learn after about 12 years old. In humans, the sensitive period for developing a fully functional visual system seems to be limited to the first three years.
All mammals have limits during development. There are things that have to occur in order for something else to happen. The period of time for these basic building blocks to develop is shorter than a sensitive periods. These time frames are called critical periods.
Critical Periods For Processing
Two studies help illustrate critical periods: imprinting and the visual system of cats.
Konrad Lorenz classic studies in the 1930s with baby geese showed the value of imprinting. Gosling follow the first moving object they see. Usually, this object is their mother but when Lorenz was the first thing they saw, the little geese followed him around as if he was their mother. The timing of imprinting is limited to only a few hours. This is a critical period.
Critical periods are shorter than sensitive periods. In humans, the sensitive period for binocular vision is about three years. But the critical period is the first three to six months. Deprivation at an early stage causes a cascade of problems in later stage.
Cats have the same problem. In the 1960s, Hubel & Wiesel imposed visual deprivation on new born kittens. They sewed one eye shut, allow monocular vision (one eye) but not binocular vision (two eyes). After three months, they opened the eye but the developmental damage was done. The cats never developed the brain activity needed for binocular vision. The neurons needed for using both eyes were not active. Their visual system was slow and had difficulty finding the edges of objects.
In humans, surgery for child born with cataract (a cloudy lens) must be operated on within the first few months or their brains will never be able to process binocular vision. The inputs will work but not the processing.
Critical Periods For Learning
If the sensitive period for learning language is before age 12, the critical period is much shorter. At birth, any child can learn any language. There is no genetic code for English versus Greek versus Chinese. Any language will do. Newborns are open to anything.
By six months of age, infants begin parsing (getting rid of) sounds not used in their language. If your language doesn’t require rolling certain sounds or producing gutturals, those elements are eliminated. The brain is very efficient. It only wants to keep things it will use. Similarly, by nine months, infants will babble in an accent. Southern babies have a drawl, New England infants clip their endings.
It has been hypothesized that there is a critical period for the acquisition of all skills. It is true that experts often started when they were young. The opposite, that starting young will make you an expert, is less tenable and untested. There is a critical-sensitive period for language acquisition; learning is easiest between preschool and puberty. It is probably best to teach children a foreign language during this period but it doesn’t mean that they will reach native speaker levels of proficiency. People vary.
One of the most frustrating things theory builders face is that people vary. Nothing screws up a perfectly designed theory than actual human behavior. People are smarter and dumber than the model predicts. They can solve things not included in the model, and not solve things the model says they should be able to do.
Critical and sensitive periods provide guidelines but your actual mileage may vary. You baby may be faster or slower than expected. Your infant might be fascinated by small objects sooner or longer than other babies. You child might begin speaking when developmental theories say should be babbling.
If you’re reading this, you are probably not in a sensitive or critical period. You brain is fully grown. You have lived long enough that your neurons are already established, your processing centers are fully operational, and your synapses optimized. You’re at the height of your development.
Being at your optimal best, of course, means that the next phase of your life is decelerating. Now you’re going downhill. Enjoy.