makes up only about 10% of the brain, it contains more nerve cells than all the rest of the brain combined and is one of the brain's most rapidly acting mechanisms.9 It connects to the highest level of the brain, the cerebral cortex, via 40 million nerve fibers. Compare that to your optic track, which uses just 1 million to take care of seeing and reading.
The cerebral cortex is our gray matter, composing about 85% of the brain; it contains the lobes (frontal, parietal, temporal, and occipital). The densely packed neurons in the cerebral cortex work together to create neural networks, pathways of learning that constantly communicate and change.10
Are There Limits on Neurons?
There are two conflicting stories about our neurons: (1) we shouldn't kill our brain cells because they’re the only ones we've got, and (2) neurogenesis—the birth of neurons—continues throughout our lives. What's the scoop?
Until the 1960s, scientists believed that whatever neurons we had at birth were all we’d get. But about that time, experiments on rats and monkeys showed otherwise; and still other experiments on canaries showed that they developed new brain cells when they learned new songs. Researchers wanted to know if people also developed new neurons—but since it's a little tricky to dissect the brain of a learning human, they couldn't find out that way for sure.
Move ahead thirty years to the ‘90s, when scientists conducted research on terminal cancer patients who were given certain drugs labeled with fluorescent dyes in their medical treatment. After the patients died, their brains were examined. The examinations showed that the patients had generated new neurons—right up until death. They indicated that the human hippocampus, a memory center of the limbic system, retains its ability to generate neurons throughout life.11
The Myth of Limited Brain Use
Is it true that we use only 10% of our brain? Do we have 90% that's just twiddling its proverbial thumbs, wishing it had something to do already? Does that idle 90% mean we are really psychic or could move mountains if we used it?
No, that's a myth, says neuroscientist Eric Chudler, director of education and outreach at the University of Washington and developer of the informative Neuroscience for Kids website.12 Images from PET scans and MRI have shown that if someone loses 90% of her brain—or of any part of her brain—she will not just continue living as if nothing had happened. From the perspective of evolution, it does not make a whole lot of sense to build and maintain a massively underutilized organ.
Researchers still have questions about neurogenesis. New neurons in rat brains travel from the hippocampus to other parts of the brain, but researchers have not yet proven whether the same thing happens in human brains. Some camps question whether the new brain cells are neurons or glial cells and what purpose they serve.13
For us nonscientists, the important thing to know is that even though we get almost all our neurons at birth, our brains continue to change and grow, supporting our learning with new neurons, new connections, or both.
Connecting the Neurons
More important than how many main cells we have is how we use and connect the ones that stick around. These connections create maps (neural pathways) that constantly change as our moldable brain grows, learns, and matures. Those brain circuits that we actively maintain will remain and even grow stronger.
Your neural pathways constantly change as your brain grows, learns, and matures.
Researchers at Virginia University found that abilities based on accumulated knowledge keep increasing until age sixty.14 However, this study's results were based on behavior, not the biology of the brain; it also does not address the effects of practice on strengthening cognition.15
When you take up a new hobby, like playing the guitar or knitting, your brain designates more cell power to this new activity. As you stick with it, the brain accommodates this new knowledge by changing or creating neural maps and maybe even assigning extra neurons to help. You go from remembering what fingers to use for the C-minor chord to just knowing it. What if you stop for a while? If you’re a winter knitter, don't worry—those neural connections won't disappear over the summer. They just focus on something else. They’ll be there next winter, though it might take them a little time to get their knit-purl connections back.
Practice may not always make perfect, but it does help you rearrange your neurons and connections. Don't worry about feeling dumb while you're learning. You're just ushering your neurons into place.
Is Your Brain a Boy or a Girl?
Here's a stunning revelation: men and women are different. But what differences are the results of nature (the brain and biology) and what differences are the results of nurture (parenting and culture)? Male and female brains have been compared for over a century, but the excitement really heated up in the past decade, as new tools have measured brain activity, sizes of brain areas, and behavioral responses.
News about the differences between men's and women's brains popped up everywhere—from John Gray's Men Are from Mars, Women Are from Venus and Louann Brizendine's The Female Brain to Discovery Health and Newsweek. All of these sources said that brain differences, present from birth, are what make men and women distinct. Even a former president of Harvard said that, based on brain research, women didn't have the brains for math and physics.16
These new discoveries were accepted by the media and public, but they were publicized without rigorous analysis. Recent scrutiny questions the validity of the studies and the hidden assumptions beneath the results.
Brain Sex Rumors
New studies of brains use functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), both of which measure blood flow and produce three-dimensional imaging. Researchers compared male and female brains and compared the brains of those doing puzzles to those meditating, those looking at happy pictures or at sad ones, or those just listening or responding actively in a conversation. They've measured the sizes of brain components and the blood flow to areas that were active; the results were pictures of brains filled with bright blobs of color indicating which sections were active.
These studies aimed to show that male and female brains are significantly different at birth in many areas. Supposedly:
Males have better right-hemisphere skills, such as those involved in art, music, and math, due to the testosterone male embryos receive; females are better at communicating, observing, and processing emotion.
Males are better able to systematize and are more aggressive than females, also because of this infusion of testosterone in male embryos; females have more collaborative and verbal brains.
Compared to men, women have the stronger ability to “mirror” others, feeling what others are feeling or sensing what others are thinking.
Men have a larger amygdala, dubbed the “instinctual core of the brain,” than women.
Women worry more than men because their anterior cingulate cortex is larger.
Women have more neurons for language processing and comprehension in the temporal lobe cortex than men have.
The corpus callosum, a pathway of 200 to 250 million nerve fibers between the right and left hemispheres, is larger in women than in men. This greater number of cross-brain connections means women are