Studies like those described above suggest that it is difficult, but not impossible, to acquire new language skills well into adulthood. For example, we all know from our own experience that it is possible to improve our vocabulary in our native language throughout our lives. Similarly, while it is unquestionably easier to learn to play a musical instrument during childhood, it is certainly possible for adults to learn to play with additional lessons and extra practice.
These examples indicated that critical periods apply to the development of certain abilities, but not others. The brain plasticity that occurs during critical periods – enabling the development of abilities such as vision, hearing, and the capacity for language – has been called “experience-expectant,” because it is responsive to stimuli that are so common in human life that they are practically guaranteed to be available (Greenough, 1987). Yet, because of experience-expectant development, when health problems such as cataracts occur during the critical period for the development of vision, or when chronic ear infections occur during the critical period for the development of hearing, the child may not develop normal sensory abilities. The critical timing issues associated with experience-expectant development of the brain are one of the most important reasons that children require early, prompt and timely access to health services when developmental problems are detected.
For other abilities – such as the ability to learn a new language, to improve our native language vocabulary, or to learn a musical instrument – the window of opportunity appears to remain open for a longer period of time if not throughout a person’s life. This type of brain plasticity has been called “experience dependent.” It is responsive to experiences that are not necessarily present in everyday life (Greenough, 1987), but that instead depend on an individual’s unique life circumstances. From an evolutionary standpoint, it is helpful for an individual to be able to acquire new abilities throughout life and remain amenable to the influence of unexpected opportunities to learn more.
From a policy perspective, many of the most important brain based capacities of children are not experience-expectant, but experience-dependent. For example, research suggests that literacy is a complex set of skills that can be encouraged by experiences that may not be available to everyone, such as being read to daily or being enrolled in early childhood education (Whitehurst and Lonigan, 1998). Thus, from a policy standpoint, the goal of early childhood brain development is not only to ensure that all children develop functional sensory and motor skills, but that they are exposed to the experiences and social interactions that are thought to encourage the underlying experience-dependent neural foundation upon which literacy can be built.
What do the last three decades of brain research tell us about what constitutes an appropriate experience at any given point in a child’s life? On the one hand, research suggest that there are serious and potentially irreversible consequences when animals are deprived of the kinds of stimulation they would have expected to receive under normal conditions. On the other hand, we know that mother cats do not go to unusual lengths to provide their offspring with visual stimulation. Rather, most kittens develop normal visual abilities from watching their brothers and sisters wrestle, or from chasing a mouse. Similarly, most rats will develop normal brains from running through sewers or fields in search of food while learning to steer clear of predators. If we were to glean a lesson for early childhood brain development from these studies, it would be that what children need is not necessarily a cutting-edge mobile hanging above their cribs, or a classical music tape playing as they lie down for their afternoon nap. What they do need are interesting and stimulating experiences that occur everyday, and that may or may not include these high-tech extras. Such experiences include talking, playing, singing, looking at books – all the things that most parents can do with their young children, if they have sufficient time and an intuitive or formal understanding of their importance. To the extent possible, children need these experiences to be provided in ways attuned to their emotional and other needs, and in accord with their developmental age.