Organisation for Economic Co-operation and Development

Abstract

The concept of critical periods states that there are certain periods during early life when the brain’s capacity for adjustment in response to experience is substantially greater than it is in adulthood. This concept originated from observations by some of the pioneers in the study of animal behaviour. For example, Konrad Lorenz examined a dramatic example of a critical period: imprinting in birds. Newly hatched birds will become indelibly attached, or imprinted, to almost any prominent moving object in their environment, normally their mother. Such imprinting can only occur during a critical period soon after hatching. In some species this period lasts only a few hours. Once the attachment is formed, the birds will follow the “imprinted” object to the exclusion of all others; subsequent experience won’t alter the behaviour. Neuroscientists explain this phenomenon with neuronal correlates that underlie imprinting. In one model system, the guinea fowl, changes in neurons in the forebrain correlate with imprinting to an auditory stimulus. Another dramatic example shows that visual experience during a critical period determines how much of the visual cortex is devoted to processing input from each eye and the degree to which binocular inputs are combined (Hubel et al. 1977). If one eye of a kitten is closed during a certain critical period, the structures for this eye in the visual cortex won’t develop normally. Thus, visually guided behaviours mediated by the deprived eye are severely and permanently impaired.