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"The stimulation provided by these mothers is certainly a large
part of what causes the brain to develop more extensively," said
neuroendocrinologist Bruce McEwen of Rockefeller University in New
York City.
However, some scientists cautioned against comparing rat and humans
mothers too closely, or attributing infants' intelligence to relatively
small differences in parenting styles.
"I don't want to put any more pressure on mothers," said
Rebecca Burwell, a psychologist at Brown University. "The rat mothers
showed differences in skills, but they all were in the normal range. So it
doesn't really speak to parental abuse. Some individuals may be very
sensitive to subtle variations in parenting."
In the experiment, the McGill team divided 32 female rats into two
groups. One group provided a high level of care to their offspring,
including stroking, grooming, licking and attentive nursing.
The other group included mothers that were more indifferent. When the
baby rats matured, they were tested in a swimming maze in which they had
to find a small platform submerged in a shallow tank. The rats were tested
15 times over three days.
The rats scored about the same on the first day of testing. However,
the offspring of the attentive mothers scored higher on days two and
three. The smarter rats also scored high on other tests throughout their
lives, he said.
The researchers then examined cells extracted from the hippocampus, a
brain region vital to memory and learning. In rats, cells in the
hippocampus are furiously connecting in the first days of life.
They found extra synapses, or connections between nerve cells, in
samples from the nurtured offspring. They also found more receptors for
growth hormones and the NMDA neurotransmitter that is crucial to learning.
The cells of the neglected offspring did not show similar enhancements.
"There is evidence for a direct relationship between maternal care
and hippocampal development, and spatial learning in adulthood,"
Meaney said. "This is experience-dependent development: Use it and it
grows. Don't, and it disappears."
In a second round of experiments, the researchers switched the
offspring between the good and neglectful mothers. Rats born to neglectful
mothers but raised by good mothers also scored well on the maze test and
showed more neural connections.
But rats born to attentive mothers but raised by indifferent mothers
scored well, too.
Meaney said the findings suggest that proper nurturing can strongly
influence how the body carries out genetic instructions.
Social scientists said the experiments help to explain the biological
underpinnings of the mother-child relationship. "Neglect is a form of
abuse," said Bruce Hershfield of the Child
Welfare League of America in Washington. "Young babies respond to the
holding, the hugging, the touching. Their brains grow, and it is important
in the development of a trusting relationships."
Copyright 2000 The Associated Press.
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