Secret Life of the Grown-Up Brain (12 page)

But other, more complex types of memory get a bit dodgy. Take a short break from a book you’re reading, even for a day, and you’ll forget not only what you’ve read on the last few pages but that you’ve
read that book at all
. A friend, Michael, told me that on a recent plane ride, he settled in to finish a book he’d started earlier that same week. But after he picked up the book, he found he “couldn’t remember ever reading any of it.” Unwilling to admit he’d forgotten what he
knew
he’d just read, he decided to start the book in the middle anyhow. “I just started reading halfway through,” he told me. “I have my pride.”

Such recollections for recent events—books we’ve just read, breakfasts we just ate—are called episodic memories. And our talent in this area generally does not blossom with age.

Why? How can some forms of memory parts stay put while others go missing? Do we, by middle age, simply have so many meals and movies and books in our heads that we have to get rid of some—a storage issue? It’s true that our brains have to jettison something or we’d explode. In fact, the few people who throughout history have been incapable of forgetting anything have been driven crazy as a result. Our brains are set up to set priorities, to weed out the irrelevant.

Still, you’d think the basic outline of a book you’re enjoying would stay put. Could we simply have too many weighty matters on our minds in general? Maybe we just can’t be bothered using up valuable brain space to remember what was on pages 1 through 67?

Marilyn Albert, a neuroscientist at Johns Hopkins University who has been studying the aging brain for decades, says that some difficulties in the normal healthy brain are not imaginary—and not a simple issue of overload. “We used to think it was because we had too much on our minds or because we have been away from school for so long,” Albert said recently. “But the declines are real and they begin in middle age.”

In fact, our increasing problems with some complex types of memory can be tied to how our brain changes its functions as it ages. And researchers are now able to see how this happens.

Cheryl Grady, a brain scientist at the University of Toronto, for instance, has actually watched the middle-aged brain take a few detours into distraction. Using a brain scanner, she has caught it daydreaming.

In a recent study, Grady found that the key part of the brain that we use to concentrate—the dorsolateral prefrontal cortex, part of that crucial frontal lobe region—lights up red-hot, as expected, in young adults when they’re asked to recall words or pictures they’ve just seen—a kind of difficult-to-do episodic memory.

But by middle age, she finds, such focused thoughts can be shoved aside by just about anything. As she scanned the brains of study participants, Grady was surprised to find that many older people trying to recall more complex information used their key frontal brain areas a bit less and a lower section of the brain more. And this second area is not helping. In fact, this fascinating brain region, called the default area—a region whose recent identification is one of the major discoveries in how the brain operates—is a key to why middle-aged brains can sometimes find themselves drawing a blank.

“This is the region we use when we’re thinking about ourselves, our internal monologues,” Grady told me as she explained her recent findings. “For instance, if you’re in a brain scanner and you aren’t doing anything, you might be thinking, ‘Gee, I’m kinda uncomfortable. ’ Or you might be thinking that you should get some milk at the store later on that day. This is the part of the brain that we call the default mode. It’s what the brain uses to daydream.”

Starting in middle age, the brain’s ability to switch
off
the default mode starts to wane. Faced with the task of remembering we’re boiling water, our brains veer off into their own internal worlds, thinking about those great boots we’d like to buy or that football game we’re planning to watch, none of it pertinent to the task at hand. And while we muse, all thoughts of boiling water disappear.

“This is one of the areas in which the aging brain does not do so well,” Grady told me. “Our ability to tune out irrelevant material is reduced. In middle age, we seem to be in transition from the patterns in youth to those of older age in this area. And it might be one of the reasons we become more distractible.”

In fact, the ability to focus is one of our most crucial brain functions. It’s a skill we acquire as babies and hone throughout adolescence. And it depends, to a large degree, on the development of our frontal lobes, which are not fully matured until we’re twenty-five years old. This area helps us to focus, in part by blocking out—inhibiting—irrelevant details.

In a recent study using functional MRI, which can observe activity in the brain, Adam Gazzaley has also watched older people have more trouble keeping their brains focused. Shown both faces and scenes and told to focus only on faces, they had more activity in the area of their brain that registers faces—appropriately. But the area that registers scenes, which should have been suppressed or inhibited, also became active. And the older adults who had the most trouble focusing also had the most trouble remembering what they saw.

As we age, our frontal lobes don’t block out irrelevant details that interfere as well, perhaps because they switch into default mode, or because of declines in connections or in the brain’s chemical messengers, creating what’s called an “inhibitory deficit.” Explaining their own recent findings, published in the journal
Nature Neuroscience
in 2005, Gazzaley and coauthor Mark D’Esposito, a professor of neuroscience at the University of California at Berkeley, concluded: “older individuals are able to focus on pertinent information but are overwhelmed by interference from failing to ignore distracting information.”

When I spoke with Gazzaley, he had just finished another scanning study that tried to pinpoint exactly when this happens as we attempt to pay attention. Not only are we increasingly lured into our daydreaming default mode, but our frontal lobes may fail to perform their top-down enforcement job of blocking out distractions. Shown faces and scenes and told to concentrate only on faces, older brains—for just a millisecond—let distracting and irrelevant scene information sneak in. The older brains then quickly adjusted and began to block out such distractions. But in that tiny moment the floodgates were opened and focus was lost.

This may be how a slower processing speed interferes with our memories as we age. Our frontal lobes may take too much time to tamp down interference, so we get too much neural “noise” at the start. And studies show that those who have the most initial interference also seem to have the most trouble forming solid memories or staying focused on what they are doing or saying.

“If, in the first second, you don’t suppress some of the incoming information, that means you get too much information in at once and that’s bad because once that information is in there, it’s in there,” Gazzaley explained. “With some older brains the suppressing machinery of the prefrontal cortex [part of the frontal lobes] is not coming on line fast enough and it’s letting irrelevant information in.”

And while most of Gazzaley’s studies were done with adults past the age of sixty, there’s ample evidence that such difficulties can begin much earlier, in middle age, a time when our brains can begin to be more tempted to take a rest and space out in our default modes while too much useless information rushes in. “We see this at age forty being kind of an intermediate problem,” said Gazzaley.

But here we have to stop, because while these difficulties arise in many brains at middle age, they do not occur in
all
brains. Nearly every study that spans ages from the forties to the early or mid-seventies—and sometimes later—shows astonishing variability. Brains are obviously varied at any age, but in middle age, that range of variability starts to increase. Some brains still operate with a knife-edged clarity, others have grown duller—most are somewhere in between. And that means that huge declines are not inevitable. As Marilyn Albert, the longtime neuroscientist at Johns Hopkins, said recently, the “true hallmark” of the brain at midlife is “variability.”

“So now we have developed two categories: the age-impaired and the age-unimpaired,” Albert said. “The question is, what is the explanation for that? Do those who are doing well have no age-related brain structure decline or, more likely, have they developed adaptive strategies?”

In fact, this is
the
key question. Why do some brains age well while others don’t? And can we more accurately define normal aging as opposed to true pathology, such as Alzheimer’s? Can we find out what makes the difference? Is it inborn or will adaptive strategies work? Over and over, scientists have been struck by the fact that it is in middle age when brains start to show not only slight declines but larger differences among one another. And it’s not just human brains. While mental scores are scattered at any age, studies in a range of animals have found that the rate of variability in those scores starts to rise markedly in middle age. This is when paths begin to diverge in earnest.

“There is enormous variability and we see this variability across species,” Albert said.

Indeed, a close look at one of our closest relatives—the rhesus monkey—is now confirming this, too. At a lab in Boston, an intriguing study of the middle-aged brain is still ongoing. And while it, too, is finding some downward trajectories, it shows a surprisingly wide spectrum—some doing okay, others not.

Not long ago, to see all this, I spent an afternoon at the lab of Mark Moss at Boston University School of Medicine, and, more specifically, with Bojangles, a rhesus monkey that was putting on a pretty good show with his own monkey frontal lobes when I caught up with him.

Through the years, one of the best tests of a human’s frontal lobes and their ability to focus our attention has been what’s called the Wisconsin Card Sorting Test, which has been around since the 1940s. The test takers, shown a group of cards, first sort them by suit—hearts, say. Then they switch and sort by number, all nines and fives, for instance. The idea is that the brain first learns the first task, then switches to another task.

In general, our brains are set up to keep doing what they’ve just been doing—a brain likes a good rut. So someone taking this card-sorting test is naturally tempted to keep picking hearts. To switch, the brain must
inhibit
its urge to stay in that rut and instead move over to its new mission. One of the key roles of the frontal lobes is to inhibit urges. And if our frontal-lobe inhibitory machinery is faulty, switching from sorting playing cards by suits to numbers becomes tougher. Without a strong push to stop what we’ve been doing, we keep doing it. We pick the hearts when we’re supposed to pick nines and fives.

It’s a classic test, still being used. And no one ever thought it could be used on monkeys. But it turns out monkeys do this pretty well, too. A few years ago, Moss, chairman of the neurobiology department at Boston University School of Medicine, a gregarious, out-of-the-box sort, was studying the aging brain of the monkey and decided to see if he could teach a version of the card test to monkeys. And, as he told me when I went to see him at his office, still surprised, “Lo and behold, we could.”

To show how this works, Moss took me to the lab near his office to see Bojangles put on his show. Kept in a large box to limit distractions, the monkey was shown three things over and over on a computer screen: a red triangle, a blue star, and a green square.

Bojangles first had to learn that he would get rewarded only if he picked the red triangle. And through a process of trial and error, he figured it out and got an M&M. Then the game switched and Bojangles was rewarded only if he selected the blue star. The idea was to find out how long it took Bojangles to catch on and switch from red triangles to blue stars. Would his frontal lobes kick in and suppress his urge to keep picking red triangles? Could he figure out how to keep getting those M&M’s?

He did. After a few false starts, Bojangles picked blue stars and got his M&M’s. But there was a catch. Bojangles was a young adult at age six, which is equivalent to about age eighteen in humans. He was still a teenager. And Moss has found that this task, generally, has not been as easy for older monkeys. In fact, after studying forty-one monkeys, Moss found that difficulties clearly begin in middle age. It was the first time anyone had managed to do such a large test on the middle-aged monkey’s brain. And the news was not all good.

The findings “showed that middle-aged monkeys, like those of advanced age, were significantly impaired on the conceptual set shifting task,” Moss wrote in his groundbreaking 2006 study, which was published in the journal
Neurobiology of Aging.

The test with the monkeys generally mirrored what has been suggested in studies with humans. But with humans, there was always a nagging question: Did any difficulties in middle age stem from a brewing case of preclinical Alzheimer’s or vascular disease—or were they simply a part of normal aging?

Through the years, it’s been notoriously difficult to tease out the difference, especially now that we know that dementia probably begins much earlier than anyone ever realized—and long before it’s evident in behavior.

But monkeys don’t get Alzheimer’s. So, if monkeys are screened for other vascular problems, they can be a fairly good model for what happens in normal, healthy human brains as they age—and not everything, it seems, always goes right.

Continuing his research, Moss has since scanned the brains of monkeys as well as examining their brain tissue. Aging is no simple process, but Moss is convinced that one of the biggest culprits in the aging brain may be selective declines in white matter—the same white matter whose overall growth helps us to get so smart to begin with.