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'Milk brain." It's a casual, common slur. Even women use it to describe how disorganized they feel in the first frantic days after giving birth. Yet milk brain is just a temporary effect, brought on by sleep deprivation, plus the need to learn (or relearn) the details of child care.

The lasting effect of being a mother, neuroscientists are finding, is the exact opposite of milk brain."It's interesting to consider what contributed to the myth of the less-than-intelligent maternal brain," says Kelly Lambert, a professor of behavioural neuroscience and psychology at Randolph-Macon College in Virginia.

"A few studies have shown that, even though the moms thought they were impaired or not up to speed, tests failed to show any impairment. So maybe the mom's awareness of any mistakes or shortcomings is enhanced during this maternal period. . . . She is probably doing better than the non-maternal person in a similar situation."

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It's true. Blood tests, laboratory experiments with animals and humans, and magnetic-resonance images of working brains reveal that from pregnancy on, female mammals are brighter, bolder and better able to cope with life than their childless counterparts.

These brain improvements are permanent, lasting from the childbearing years into senescence. Nature automatically turbo-charges the brains of mothers. As all kids know, there's a shorthand name for superhuman: mom.

Mothers don't necessarily outshine non-mothers in every possible way, says Craig Kinsley, a professor of neuroscience at Virginia's University of Richmond, and one of Dr. Lambert's senior collaborators. All the same, the behaviours that do improve are so central that they give a new mom an immense advantage over her childless sister. From curiosity to self-confidence to sensory acuteness, the maternal brain shines in a host of ways.

The triggers for maternal brain enhancement involve the same signalling apparatus that governs the whole cascade of life. Puberty, mating, conception, pregnancy and birth are all controlled by chemical messengers called hormones.

Hormones perform a bewildering variety of functions in all mammals, but particularly in females. Estrogens, including a powerful variety called estradiol, are produced in a pregnant woman's ovaries and placenta.

When they travel to the brain, they change it -- a process that until recently was thought to be impossible. Under hormonal direction, brain cells are enlarged in the hypothalamus, a region that strongly affects maternal behaviour.

Hormones increase neuronal branches in a nearby brain area called the hippocampus. The hippocampus isn't directly involved in how mothers behave, but it does play a key role in their general learning and memory. From conception onward, both of these key functions are intensified in moms.

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Pregnancy hormones also beef up the brain's amygdala and prefrontal cortices. The amygdala, part of an ancient core brain called the limbic system, regulates intense emotions, including maternal love. The prefrontal areas process sensory input, empathy and conscience.

The upshot of pregnancy, Dr. Kinsley says, is a maternal brain changed forever in critical ways -- not just different, but vastly improved.

In his group's animal experiments, mother rats proved two times better than virgin rats at finding food in mazes, and up to three times bolder in exploring unknown situations.

And in competitions involving multitasking, mother rats beat the tails off their virgin cousins. In every aspect of evolutionary success -- cognition, exploration, adventurousness, general intelligence -- the moms were tops.

Some critics say rats are rats and humans are humans, and never the twain shall meet. Not so, Dr. Kinsley says. All mammals seem to share a common machinery for hormonal brain enhancement. "Mapping the brain circuitry of rats onto that of humans provides strong support for animal models," he says.

For all mammals, it seems, the same maternal machinery has stayed around for millions of years. It works. So why mess with a good thing?

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At the Medical University of South Carolina, Jeffrey Lorberbaum, an assistant psychiatry professor, is using magnetic resonance imaging to examine the brains of human mothers listening to the cries of newborns. The mothers, it turns out, can detect their own babies with virtually unfailing accuracy. And they use the same traditional maternal circuits that have been confirmed in rats.

Closer to home, at the University of Toronto, recent experiments by psychology professor Alison Fleming have cast light on another hormone's role in mother-child bonding. Cortisol is known to many neurologists as a stress hormone -- in most people, male or female, it accompanies depression. But during the first postpartum week, cortisol seems to correlate with "attuned" (sympathetic) motherhood.

It's as if cortisol has a variety of roles -- a conclusion perfectly consistent with the rest of human neurochemistry, whose molecules constantly "moonlight" in wildly different jobs. Insulin, for example, acts as both a hormone and an enzyme to metabolize blood sugar.

Dr. Fleming asked new mothers, most so recently delivered that they were still in hospital, to inhale a variety of odours. The women were not told what they were being exposed to; the samples they smelled were in containers that held no clue to the odours' source.

"In fact, we put our samples in Baskin-Robbins ice-cream containers," Dr. Fleming says with a laugh.

Besides a variety of control substances, she says, "we put in garments from each mom's baby. We then recorded the 'hedonic rating' of the smells -- the pleasure they gave to those inhaling them. Women with higher cortisol levels were most attracted to the infant body odours. Also, women who had the best ability to detect their own children by smell also had the highest levels of cortisol in their blood."

Dr. Fleming suggests that the cortisol found in new mothers may arise from the physical stress of delivery. Then, by improving maternal care, it solidifies the fledgling bond between mother and child. Cortisol "tunes up the female brain's responses to the baby's cues," she concludes.

(Similar stress infusions of cortisol may benefit students and writers, whose cognitive abilities are in desperate need of help at deadline time.)

What about dad in all this? Dr. Lorberbaum's group reports that human fathers are far less responsive than the mothers to the sounds made by kids, even their own. In MRI scans, maternal brains "lit up" more than paternal ones.

Dr. Kinsley says these data agree with his personal observations. "Here's something you can try yourself," he suggests. "Next time you're in a restaurant and a child or baby cries, check what happens. In my experience, all the men in the place keep talking without interruption, while the women instantly orient themselves toward the cry. Granted it's just anecdotal, and not a controlled experiment. But I find the odds to be strong that this is indeed what will occur."

Looking at facts such as this, Dr. Kinsley jokes that you would almost think that men are irrelevant to reproduction -- that is, until you looked at the evolutionary forces that created the maternal and paternal brains. "Once you do this, you won't see the gender differences among human brains as an indictment of all males," he says. "In the vast majority of mammalian species, fathers have little or nothing to do with their progeny. We humans have a lot of evolution to get past."

Fortunately, Dr. Kinsley says, "the brain has an almost infinite capacity for plasticity." By "plasticity" he means that the brain, recovering from injury or facing a novel situation, has an amazing capacity to adapt.

Brains damaged by a lack of oxygen caused by a stroke may steadily shift control of certain functions to undamaged cells. Epileptics with large parts of their brains removed to forestall life-threatening seizures live normal lives, as if their brains were still complete. And men who love their wives and children often exhibit behaviour as "maternal" as that of their wives.

"I recently started doing research with a paternal rodent model focusing on the California deer mouse, a monogamous, biparental species," Dr. Lambert wrote in an e-mail message. "I'm amazed at the nurturing responses displayed by those mice -- even when the pups are 'alien pups' that are not their own. We've found that these mouse fathers show more efficient foraging and less anxiety, just as we have observed in the rat moms."

Other tests show elevated levels of non-hormonal regulators in the brains of new human fathers. These molecules, including the neuropeptides oxytocin and vasopresin, which act like endorphins, the "feel-good" chemicals that the brain secretes, are also involved in mother-baby bonding.

"We view the maternal experience as an enriching experience -- new sights, sounds, smells and the like," Dr. Lambert says.

She adds that "most research suggests that being exposed to novel situations enhances the complexity of the brain."

Does the obverse apply? Might childless people, especially men, have atrophied brains? "It may be that the crusty old bachelor is living an impoverished life in solitude and predictability, and this may not be great for his brain and mental health," Dr. Lambert admits.

Yet people can change. Brain plasticity might explain the amazing ability of some childless men to turn from curmudgeonly behaviour and learn to love kids, even at an advanced age, Dr. Kinsley says. As renowned psychologist R.D. Laing wrote more than 30 years ago, children may be as necessary to the full development of the adult as the other way around.

The dynamic brain

Less than a generation ago, the neurological dogma was that brains don't change. After the 40th week of gestation, the brain had all the neurons it would ever have. From that point on, brain cells might die from a lack of oxygen, alcohol poisoning or old age. But individual neurons would never change internally, and were never replaced.

This belief was first challenged by animal researchers. Before and during mating season, areas in the brains of songbirds that produced and recognized songs might double or triple. When mating season finished, these beefed-up areas slimmed down to normal.

Science now realizes that human brains, like those of the songbirds, also self-redesign at various points.

The first big renovation takes place in toddlers, readying them for language.

Another brain change at five to six years old gives kids basic logic, which is why cultures the world over start formal schooling at this age.

A third brain reno takes place throughout adolescence. This change is truly massive, less a paint job than a gut and rebuild. The teenaged brain automatically rewires its prefrontal cortex, the area just behind the forehead that handles rational thought. This delicate, unbelievably complicated transformation, involving billions of neurons and trillions of inter-neuron linkages, prepares teens for the adult world.

For women, there is another great mental change in store: motherhood. "What was once a largely self-directed organism devoted to its own needs and survival becomes one focused on the care and well-being of its offspring," researchers Craig Kinsley and Kelly Lambert wrote recently in Scientific American. "Although scientists have marvelled at this transition, only now are they beginning to understand what causes it."

-- William Illsey Atkinson

William Illsey Atkinson, a frequent contributor to The Globe and Mail, is writing a book on the effect of hyper-violent video games on adolescent brains.

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