My Plastic Brain Page 3
It takes two to four years to acquire “the Knowledge,” and it's a fiendishly difficult test to pass. Maguire found that, to cope with the challenge, the brain has to invest more resources in spatial memory, sprouting more gray matter in the hippocampus. And since there is only so much space in this crowded inner part of the brain, a neighboring area, the anterior hippocampus, had to shrink to make room. This, the same study suggested, makes taxi drivers perform worse on certain visual memory tasks.
There are other examples of brains growing and shrinking in response to learning, too. Studies with musicians showed that they have larger areas of the brain associated with fine movements and sound processing than non-musicians. These changes match up with the amount of time someone has been practicing, showing that it is practice—rather than any innate advantage—that makes the difference. Years of practice don’t seem to be strictly necessary, either. Novice jugglers show an increase in size in areas of the brain that process fast-moving objects after just a few weeks.22
All of these studies are well known and are done by respected scientists—who know their stuff and aren’t trying to sell you anything. But when anyone tries to put what Hebb said and what these brain-scanning studies are saying together, and pretend that he or she knows what is going on, the whole argument very quickly starts to unravel. So far, there is no way to watch firing and wiring going on at the same time as watching blobs on brain scans getting bigger, in living human brains. Which means there is no way of knowing whether the increase in brain volume seen in brain-imaging studies comes from growth of new cells, a rush of new connections, or something else, like new blood vessels sprouting up to service a busier bit of brain. All in all, it makes the “rewire your brain” spiel a little more difficult to swallow.
This was all becoming a bit complicated, so I got in touch with Heidi Johansen-Berg, a professor of cognitive neuroscience and the head of Oxford University's functional brain-imaging center. We’d spoken a few times over the years for various things I’d been writing. She definitely doesn’t come across as prone to exaggeration, and so, to my mind, was the perfect person to cut through the hype and tell it like it is. I asked her to talk me through what we know for sure about brain plasticity on the phone, and, despite the fact that I’d been pretty much stalking her around that time for comments on various articles, she agreed.
What Johansen-Berg told me was that it's actually pretty unlikely that new connections—the “firing together and wiring together” bit—would account for a bigger blob showing on a brain scan: “It sounds attractive, as something that would go on when you are learning something, but, because the size of those connection sites is absolutely tiny, it's very unlikely that an increase in spines would give you something that you could detect with MRI.”
So if new connections aren’t likely to be bulking out the gray matter, what is? Johansen-Berg wanted to know the same thing, so she did a trawl through the research in this area, later publishing a review article in the journal Nature Neuroscience.23 In this overview of current research, she concluded that brain change involves lots of things, but at the moment it's not possible to say which of them is behind the appearance of bigger blobs on brain scans, or (more likely) whether those growing bits of brain come down to a combination of all of them. In a nutshell, she told me that the popular idea of “rewiring a brain” could involve any or all of the following.
MORE NEURONS
In some parts of the brain, including the hippocampus, new neurons are created to cope with the demands of learning and memory. So the birth of neurons (aka neurogenesis) is probably behind at least some of the change in the London cabbies’ brains. But neurogenesis hasn’t been conclusively found to happen outside of a few specific areas of the brain, and so can’t explain every growing blob found on every before-and-after brain scan.
MORE “GLUE”
Neurons are what most of us think of as “brain cells”; they are the ones that carry out the fancy-pants electrical processing that (somehow) turns into our thoughts, desires, and memories. But they are definitely not the only brain cells that make up gray matter. People argue about the exact numbers, but what we do know is that neurons are at least equaled and may be outnumbered by another family of cells called glia.
Glia comes from the Greek word for glue—a name these cells were given because they make up a kind of sticky scaffolding system that holds the neurons in place. And for a long time that is all they were thought to do. But, recently, there have been a few tantalizing clues that they might have something to do with learning as well.
One type of glial cell, called an astrocyte, in particular has attracted researchers’ interest. In animal studies, where you can teach the animal something, and dissect its brain afterward to see what changed, astrocytes have been found to become larger after an animal learns. So this might account for measurable changes in human brains, too. “This might well be something you could see on a brain scan,” says Johansen-Berg.
It might be that, when we learn, the astrocytes make sure that a particular circuit is better serviced, and so we can get on with the job of thinking more easily. Or it could be that the astrocytes themselves are doing something more directly linked to the thinking process. So far, nobody knows. Whichever it is, astrocytes are clearly important for the job of thinking, and human astrocytes are particularly good at the job. In 2013, a group of scientists put human astrocytes into a mouse brain to see what would happen to its navigation skills. As a result, the mice became a lot better at navigating a maze and remembering where objects were hidden compared to the control mice, which only had their own astrocytes to work with.24
More intriguingly still, studies of Einstein's brain have shown that he had far more astrocytes in brain areas to do with abstract thinking than you would expect. So, even though astrocytes don’t communicate at the lightning speed of neurons, they might be doing something that helps us think. Or, as Johansen-Berg put it in her fabulously understated way, “There is a growing sense that we might have been missing something quite important about the astrocytes.”
MORE PIPES
While astrocytes are busy doing whatever it is that they are doing, animal studies have also shown that the blood vessels that link them to the neurons also sprout new branches. When a region of brain is being worked particularly hard, more blood means more energy, oxygen, and all the other things that an active cell needs to keep running efficiently. As an explanation for a changing brain, new blood vessels sound less exciting than new neurons or connections. Still, blood vessels make up around 5 percent of gray matter, so if they start to branch out, then it might add enough bulk to show up in a scan. If that's the case, what is often sold as “rewiring the brain” might actually be more of a replumbing job.
MORE CABLES
Rewiring certainly does happen, though, whenever we learn something new. New spindly branches between neighboring neurons almost certainly contribute to the growth of the brain blobs. For example, studies back in the ’90s showed that people who have had more education throughout their lives have more dendritic branches, the small, localized connections between neighboring neurons.25
What most of us probably think of as “rewiring,” though, comes down to the white matter—the long-range cables connecting one region of the brain to another part, which might be several centimeters away. Almost any bit of thinking you do needs input from more than one brain region, so how well connected these different bits are, and how fast the long-range wires can conduct electricity, can make a big difference to how efficiently the brain can process information. Wires that have become connected in unhelpful ways might be behind some of our less desirable habits, like eating to excess or getting a kick out of gambling.
White matter is named for the white fatty covering, called myelin, which coats the axons of neurons, insulating them and allowing electrical signals to pass along the axon ten times faster. More electricity passing along the wires as we repeat thoughts and behaviors is wh
at gives the brain the impetus to upgrade a normal connection to a super-fast one. For those interested in the details, it seems to work like this: electrical activity stimulates the release of a chemical called glutamate, which attracts glial cells called oligodendrocytes. These cells get to work, adding a spiral of fatty insulation, made from the neuron's cell membrane. More activity in a circuit can also lead to changes in the wiring by making the wires themselves longer, fatter, or more densely packed.
Once a coat of myelin has been added to the axons of the neurons, the extra layer of insulation inhibits branching, protecting well-used highways from being diverted or broken down. This is another reason why it is so difficult to unlearn bad habits.
This particular mechanism could cause a bit of a problem with any attempt to rewire my brain. If the pathways I want to change have been there long enough to become properly entrenched, with a nice thick coating of myelin, will it still be possible to change them? To make things worse, these wires apparently don’t just snake randomly around the brain, making and breaking at a moment's notice—they are bundled up into thick bunches of cables, called fasciculi, which keep them all neatly together and running in the right direction (see figure I.1). Imagine the effort it would take to unravel that lot and then make a few tweaks! It just doesn’t seem feasible.
Heidi Johansen-Berg tells me, though, that adding new branches to the existing long-range cables running between brain regions might be possible: “There is much less evidence for that but there is some evidence.” In one study, from 2006, where macaques were taught to use a rake to bring food toward them, they sprouted new connections from visual areas of the brain to the areas involved with knowing where their limbs are in space.26 The scientists who conducted the study don’t claim that a new wire was added to the bundle, though. Instead, they say that a new branch probably shot out from a wire somewhere nearby.
Figure I.1. The fasciculi of the brain. (Courtesy of Nedzad Gluhbegovic, The Human Brain: Photographic Guide [Hagerstown: Harper and Row, 1980])
A less drastic option for the brain is to tweak what is already there—not so much changing the wiring as the way in which the wires are used. Neuroscientists distinguish between structural changes—actual physical changes to the system itself—and functional changes, differences in the way that that structure is used, electrically or chemically, or in the strength of the connection at the synapse, where two neurons meet. Both structural and functional changes can make a big difference to the way the brain works in the real world, and one kind of change may lead to the other.
All in all, it's fair to say that neuroplasticity is fascinating but complicated, and even the experts don’t know exactly what is going on inside our heads when we learn something new. There are good grounds to be confident that neuroplasticity is a real thing and hasn’t been made up by unscrupulous marketing people. But, on the other hand, if people try to tell you that doing any one thing will make your neurons fire together, wire together, and, hey presto, will rewire your brain, they are being about as honest as those web ads that promise to erase your belly fat if you follow their “one weird trick.”
I must admit, when I learned this, I was a bit worried. How were changes in my brain to be measured, in the labs of the great and good, when not one of them really knows what is happening to it? Short of having bits of my skull replaced with glass and strapping a microscope to my head, there is no way of knowing for sure. On the other hand, if I can feel the changes, and the scientists can measure by other means—like before-and-after cognitive tests, or changes in electrical activity—then it's a fair bet that something physical has changed or is in the process of changing; I just won’t be able to put my hand on my heart and tell you what. Which sounds like a cop-out, but it's really not; others couldn’t tell you either—and if they say they can then, at best, they’re exaggerating.
So, this much we know: the brain's circuits are a bit like muscles, in that if you give them more to do, they will get stronger and work better. So brain training should work, right? This, too, is a contentious area, with scientific analyses flying back and forth between academics, and the media trumpeting that brain training does or does not work, at every turn. It's no wonder that everyone is so confused.
One thing that bothers me is why it's so difficult to tell if brain training works or not. If my mission were to tone up my forty-something body, it would be easy. If I wanted rock-hard abs, I’d take up Pilates or commit to doing sit-ups every morning before breakfast; if I managed to keep it up for a month, I’d begin to see a much less spongy belly. If I wanted to improve my general fitness, I’d go running every morning instead—and, after a month, I’d know I was improving if I felt significantly less like death while doing it.
In the brain, what with it being locked in a box, success is harder to measure. The best anyone can do to measure such changes is to point at differences on a brain scan or at before-and-after measurements on standardized tests—which sounds simple enough, until you consider that most cognitive abilities involve widely distributed networks in the brain, and there may or may not be small changes all over the place. Add this to the fact that the brain is changing all the time anyway, and it's no wonder there isn’t a simple answer.
To be fair to the neuroscientists who are working in the field, the challenge is huge. Anyone who wants to prove that a particular brain game does anything useful needs to show that any changes in score are down to the training and not due to a cunning new strategy devised between the initial and follow-up tests, or because something totally unrelated has changed in a person's brain during the same period. Anything like finding a new love, learning a new chess strategy, or having some kind of unexpected life stress might make as many, if not more, changes as a few minutes of training each day.
In children, it's even more difficult to say what drives change because they are growing and changing even more than the rest of us. Cognitive neuroscientist Dorothy Bishop, of University College London, who has a healthy skepticism about brain training in general, points out that, if you measure children's feet before and after a course of brain training, they will probably have grown. Does that mean that brain training makes children's feet grow? Probably not.
Then there is the not insignificant “Hawthorne effect”—a kind of variation of the placebo effect, which says that if you give people a lot of attention and they know that they’re being observed, they will get better at whatever it is that you are observing. So it could be the warm, fuzzy glow of being the center of attention that causes an improvement rather than anything magical about the task itself. (Like if you spend a year thinking almost exclusively about your own brain, for example. Hmm.)
Finally, there is the question of transfer: does the training make you better at just this specific game or does it help with other things, too? This is the real nub of the issue because while it's fun to get good at a brain game on the computer, most people aren’t doing it for just that reason: they want a better memory or reaction time or reasoning skills in real life. And whether transfer is occurring is not easy to prove.
Going back to the body-improvement analogy, transfer is a simple enough thing to spot following physical exercise. You wouldn’t do sit-ups and expect to get toned biceps or a pert butt because it's obvious that sit-ups mainly work the abs. The benefits don’t transfer to any other bit, especially not the bit you are lying on at the time. You might expect, though, to get toned arms and stronger abs if you did one hundred push-ups instead, because you can’t do push-ups without keeping your body straight, and that works the abs as well as the arms.
Something like running covers even more bases—because that works not only several major muscle groups but also the heart and lungs, so not only will it make climbing the stairs easier, but swimming and playing football with the kids will get easier too. Likewise, swimming or playing football will make it easier to run. Cardiovascular fitness training transfers further than working one or two areas of the body; it impro
ves the underlying basis of health.
Studies like Maguire's (on taxi drivers taking “the Knowledge,” mentioned earlier) have shown that by taxing particular parts of the brain, you can tone up specific parts of the circuitry. As long as you keep up the exercise, the circuits will become stronger, work better, and you will be able to do more with them. But at no point has Maguire found that taxi drivers get cleverer in general. The changes to the taxi drivers’ brains are the equivalent of having rock-hard abs after doing a lot of sit-ups (not something many taxi drivers can lay claim to). What brain-training programs aim for is the brain equivalent of cardio training—some way to work key, underlying brain skills that will make everything work better.
This cardio-fitness equivalent for the brain is proving difficult to come by. One of the best-studied candidates is working memory: the ability to hold information in mind while you manipulate it to work out what to think, say, or do next. It's a skill that has a hand in pretty much every complex task that the brain has to do, which is probably why differences in scores on working-memory tests predict general intelligence and reasoning ability. If it were possible to improve how working memory functions—the thinking goes—it might just make the whole system work better and the person who owns that brain a little bit smarter.
It does make sense, which is why there has been a huge amount of research into whether working memory can be trained and, if so, whether that makes people smarter in general. The trouble is while some studies have found that it works great—improving other measures of intelligence and brain function—others, even exact reruns of previous studies, have concluded that it does nothing at all. All of which has led some psychologists, like Charles Hulme of University College London, to suggest that it's all a bit of a waste of time.
Hulme and his colleague Monica Melby-Lervåg, of the University of Oslo in Norway, recently analyzed the results of over eighty studies of working-memory training, many of which claimed to have shown improvements in general intelligence after just a few weeks of training. They concluded that, when you put all of these studies together and analyze them as if they were one big study (this is called a meta-analysis), the effects on general intelligence disappear. All you are left with is a small, and very temporary, improvement on playing the game used for training.27 Which would suggest that working-memory training is not the cardio-fitness-for-the-brain kind of exercise that we’ve been hoping for, or indeed been sold by brain-training companies.