Brain Development

Methods for Studying Brain Development

The scientific methods for studying human brain development have changed dramatically since the mid-20th century. Some of the earliest insights came from research with laboratory animals. In the 1960s, for example, rat pups were raised either individually in laboratory cages (an impoverished environment) or with litter mates in laboratory enclosures with frequent toy changes (an enriched environment). The rat pups raised in solitude performed more poorly on tests of learning and memory than did those raised in an enriched environment.

Rat brains were examined after testing and researchers reported that the cerebral cortex of the rat pups raised in solitude was thinner. The effect only was found prior to weaning, however, demonstrating a sensitive period for the effects of environmental enrichment on the brain of laboratory rats. “Sensitive period” refers to the presence or absence of a specific experience (e.g., environmental enrichment) at a particular time of development (e.g., prior to weaning). Importantly, sensitive periods typically have a long-lasting influence on development.

Beginning in the late 1960s through the early 1980s, researchers studying the brains of human cadavers reported age-related differences in myelination and number of synaptic connections. Myelination is the accumulation of fats and proteins around nerve cells (neurons) that enables faster transmission of information within a cell. Synapses are the small gaps between brain neurons; more synapses yield more interconnections among neurons. Synaptic connections show a progression (“blooming”) and then a regression (“pruning”) at different ages depending on the particular brain. After discovering these age-related differences in human brain tissues, scientists reasoned that the human brain must also have sensitive periods of development, as had been shown with laboratory rat pups.

By the early 21st century, researchers presented scientific evidence that sensitive periods are indeed associated with human brain development in conditions of deprivation. Due to circumstances associated with an extreme political regime, some children in Romania were reared in highly impoverished orphanages. Researchers showed that children living in orphanages and community children living with their families showed very different brain activity.

Furthermore, children who were removed from the dire orphanage environment and placed in foster care with community families had a pattern of brain activity measured via electroencephalogram (EEG, a measure of neural electrical activity) that was more similar to that of the community children than the children who remained in orphanages. This difference was especially noticeable for children removed from orphanages and placed in foster care before 24 months of age, suggesting a sensitive period of human brain development.

The most current and detailed information about brain development across the lifespan comes from the brain-imaging technique called magnetic resonance imaging (MRI). Whereas EEG measures [Page 283]actual neural activity, MRI infers neural activity. Neuronal activity is associated with oxygenated blood, and the differences in proportion of oxygenated and deoxygenated blood can be detected by MRI. This method is used to study both structural and functional brain development. How scientists interpret what they see using both MRI and EEG techniques has its roots in the laboratory work with animals.

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