Deprivation and Disruption

An Interview with Council Member Charles A. Nelson

Are critical periods unusual?

We know that true critical periods are rare and the brain is much more flexible than many people have assumed. We know that some aspects of brain architecture, and their related behaviors, exhibit more sharply defined sensitive periods than others. We know that some areas of the brain -- for example, those involved in memory -- stay open throughout a person's lifespan. We also know that, even in brain structures that do have sensitive periods, there might be individual differences in the timing and length of plasticity.

The visual system provides an excellent example of what we mean by sensitive periods. Infants who don't get normal visual input in the first several months of life (due to congenital cataracts or other vision impairments) have serious deficits in visual perception, even if the initial problem is later corrected. The sensitive period for the visual system in the brain is very short, and if the system doesn't get normal input in that very short time, it may never fully recover normal function. The same is true for language. In the first six months of life, infants can hear the difference between all possible sounds found in all languages: the nasal French "n" sound, the difference between "r" and "l" in English, different vowel sounds in German. But when children are about a year old, their ability to hear and produce those sounds starts to narrow and focus only on those sounds from the language they're surrounded with. Their brain architecture focuses very narrowly on hearing and making the sounds they're exposed to and will need to use to communicate with those around them.

In terms of social and emotional development, however, the systems are much more flexible. A sensitive period exists for developing attachment to other people, and that extends over the first 2 to 3 years. After that, it's very difficult -- but not necessarily impossible -- for the brain architecture related to social development to change and to incorporate experience. In cognitive development - intelligence, memory, information processing -- if there is a sensitive period, it's much more broadly tuned than other areas of development, such as vision and hearing. Either the sensitive period is longer than that required in other developmental areas, or it simply doesn't exist. We don't know, for example, if there's any sensitive period for cognitive stimulation that might influence development of IQ. This means that, while early experience is certainly important to cognitive development, the door never really slams shut, and later experience may be just as important.

Your work also focuses on the effects of early biological problems on brain development. What influence do those problems have on the development of brain architecture,and what do policy makers need to know in order to offer better stewardship of the next generation?

One of the most common examples is prematurity. If a baby is born about 5 or 6 weeks premature, but is otherwise healthy and doesn't have any complications, there seem to be no consequences of being born early in terms of brain development. However, if a baby is born prematurely with specific biological or medical complications, such as low weight for gestational age or lung problems that lead to a lack of oxygen, these can damage brain architecture associated with cognitive, motor, and sensory structures. I've also studied infants of mothers who are diabetic during their pregnancy. Up to 10 percent of pregnancies are complicated by diabetes and infants born to mothers with prenataliron deficiency as a result of diabetes are at risk of developing memory impairments due to abnormal brain development.

Your studies examine the effects of early deprivation. In that regard, could you describe your work in Romanian orphanages?

Romania is one of a number of countries that has a history of institutionalizing orphaned and abandoned children at birth. In most of these institutions, children are deprived of normal stimulation, particularly in terms of sensory input and normal social interaction with consistent primary caregivers. We looked at the development of very young children (between 9 and 30 months of age at the start of the study) who were raised in institutions, and we also looked at an intervention in which we took some children who were living in institutions and placed them with highly selected and trained foster families. We also looked at a community sample of Romanian children living with their biological families. This allowed us to compare typically developing children with institutionalized and foster children. For the most part, the children in the Romanian project entered the institution at the same age, but were placed in foster care at different times, so the amount of time they spent in foster care varied widely. What we are examining is how the timing of placement in the institution or in foster care has an impact on specific aspects of development, such as brain activity, language, or motor skills.

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