Provided by the National Institute of Mental Health
It is well known that the early months and years
of life are critical for brain development. But the question remains: just how
do early influences act on the brain to promote or challenge the developmental
process? Research has suggested that many both positive and negative
experiences, chronic stressors, and various other environmental factors may
affect a young child's developing brain. And now, studies involving animals are
revealing in greater detail how this may occur.
One important line of research has focused on brain systems that control stress
hormones—cortisol, for example.1,2 Cortisol and other stress hormones play an
important role in emergencies: they help our bodies make energy available to
enable effective responses, temporarily suppress the immune response, and
sharpen attention. However, a number of studies conducted in people with
depression indicate that excess cortisol released over a long time span may have
many negative consequences for health.3,4,5 Excess cortisol may cause shrinking
of the hippocampus, a brain structure required for the formation of certain
types of memory.
In experiments with animals, scientists have shown that a well-defined period of
early postnatal development may be an important determinant of the capacity to
handle stress throughout life.2 In one set of studies, rat pups were removed
each day from their mothers for a period as brief as 15 minutes and then
returned. The natural maternal response of intensively licking and grooming the
returned pup was shown to alter the brain chemistry of the pup in a positive
way, making the animal less reactive to stressful stimuli. While these pups are
able to mount an appropriate stress response in the face of threat, their
response does not become excessive or inappropriate. Rat mothers who
spontaneously lick and groom their pups with the same intensity even without
human handling of the pups also produce pups that have a similarly stable
reaction, including an appropriate stress hormone response.6
Striking differences were seen in rat pups removed from their mothers for
periods of 3 hours a day, a model of neglect compared to pups that were not
separated. After 3 hours, the mother rats tended to ignore the pups, at least
initially, upon their return. In sharp contrast to those pups that were greeted
attentively by their mothers after a short absence, the "neglected" pups were
shown to have a more profound and excessive stress response in subsequent tests.
This response appeared to last into adulthood.7,8
The implications of these animal studies are worrisome. However, research is in
progress to determine the extent to which the hypersensitive or dysregulated
stress response of "neglected" rat pups can be reversed if, for example, foster
mothers are provided who will groom the pups more intensely, or if the animals
are raised in an "enriched" environment following their separation. An enriched
setting may include, for example, a diverse and varied diet, a running wheel,
mazes, and changes of toys.
Animal investigators are well aware of another kind of long-term change, again
rooted in the first days of life. Laboratory rats are often raised in shoebox
cages with few sources of stimulation. Scientists have compared these animals to
rats raised in an enriched environment and found that the "privileged" rats
consistently have a thicker cerebral cortex and denser networks of nerve cells
than the "deprived" rats.9,10
Another study recently reported that infant monkeys raised by mothers who
experienced unpredictable conditions in obtaining food showed markedly high
levels of cortiocotropin releasing factor (CRF) in their cerebrospinal fluid
and, as adults, abnormally low levels of cerebrospinal fluid cortisol.11 This is
a pattern often seen in humans with post-traumatic stress disorder and
depression.5 The distressed monkey mothers, uncertain about finding food,
behaved inconsistently and sometimes neglectfully toward their offspring. The
affected young monkeys were abnormally anxious when confronted with separations
or new environments. They were also less social and more subordinate as adult
animals.
It is far too early to draw firm conclusions from these animal studies about the
extent to which early life experience produces a long-lived or permanent set
point for stress responses, or influences the development of the cerebral cortex
in humans. However, animal models that show the interactive effect of stress and
brain development deserve serious consideration and continued study.
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For More Information
Please visit the following link for more information about organizations that
focus on the human brain.
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All material in this fact sheet is in the public domain and may be copied or
reproduced without permission from the Institute. Citation of the source is
appreciated.
NIH Publication No. 01-4603
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