1. The kinds of early experiences on which healthy brain development depends are ubiquitous in typical early human experience—just as nature intended. This means, however, that concern should be devoted to children who, for reasons of visual impairment, auditory processing problems, major perceptual-motor delays, and other basic deficits cannot obtain these experiences on which the developing nervous system depends.

  2. Abusive or neglectful care, growing up in a dangerous or toxic environment, and related conditions are manifest risks for healthy brain development. Beyond these extremes, the nature and boundaries of the environmental conditions necessary for healthy brain growth are less well known, partly owing to the complexity and the cumulative achievements of cognitive, language, and socioemotional growth. Exploration in this area is cutting-edge research.

STUDYING THE DEVELOPING BRAIN

Neuroscience techniques have advanced significantly, rendering studies of young children's brains more feasible and informative than in the recent past. These techniques have enabled scientists to learn more about how babies' brains change with development and how vulnerable or resilient they are to environmental harm. However, the repertoire of techniques that can be used with preschool-age and even younger children is still limited. Some of the more direct methods (i.e., looking into the brain) are either invasive (e.g., positron emission tomography requires the injection of a radioactive substance) or require long periods of remaining still (e.g., functional magnetic resonance imaging). Nevertheless, by tracking the brain's activity from the outside with the electroencephalogram, eventrelated potentials, and magnetic encephalography, researchers can learn about brain functioning in very young children. For instance, scientists can record the electrical or magnetic activity of the brain while the child is presented with different stimuli (e.g., speech sounds) and identify which parts of the brain are active and how active they are when children are doing different things. This approach has been used to reveal that the neural substrate for recognizing faces and facial expressions is remarkably similar in infants and adults (de Haan and Nelson, 1997, 1999), and that babies' brains change as they learn their native language (Neville et al., 1998).

In addition, children with localized brain damage can be studied using neuropsychological tools. These entail giving young children behavioral tasks that have been shown to involve specific brain functions (e.g., working memory, spatial planning) and observing how performance varies with the particular part of the brain that is damaged (Luciana and Nelson, 1998). This approach, used in a longitudinal study of language develop-



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