Boost Your Brain (30 page)

But there’s also learned behavior around our responses to stress. Part of it we pick up from our parents, whose reactions to events in their own lives give us cues—for better or for worse—on how to respond when it’s our turn.

Much of the time, the stress we feel is our own doing. How we respond to it has to do, as you’ll recall from
chapter 6
, with our belief system. It’s all in how we view the world. A simple example is this: My co-author, Christina, might care deeply about the outcome of a game being played on a particular fall Sunday in the Baltimore Ravens’ stadium. She’s a Ravens fan, so a win matters to her. But at the very same moment that she is standing in front of the TV, biting her nails over a crucial first down, another person on the same block might feel no stress whatsoever. To him, a completed pass by Joe Flacco matters not at all. Therefore, he is utterly relaxed at game time.

This is a rather benign example, but imagine if the stressor were a poor performance review at work, or a flat tire, or a nagging relative. Whether we consider these mild annoyances or major stressors depends largely on our priorities, our life circumstances, and our belief system.

Excess Stress Is Bad

While some stress is helpful,
excess
stress can have negative effects on performance. To demonstrate this, researchers at the National Institute of Mental Health looked at the immediate effect of a stressor on healthy subjects.
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For the study, the research team recruited twenty-four volunteers and divided them into two groups. Study subjects in one group were exposed to stress by having one hand dunked into ice water; in the other group, participants had one hand dunked into warm water. Twenty minutes later, both groups were tested on working-memory performance and their heart rates and salivary cortisol levels were measured. It turns out that those exposed to short-term stress—the cold water—had better reaction times on the test but also had more false alarms and made more mistakes than the control group. In fight-or-flight mode, in other words, they were fast but not so accurate.

This was a short-term effect, of course. The real trouble with stress shows itself when we’re exposed to too much, for too long. In fact, a host of studies have shown a negative effect on the brain from prolonged exposure to excessive stress.

In one such study, researchers exposed rats to a cat over a period of five weeks and then tested the rats’ memories using a puzzle-solving maze that required them to remember the location of a platform. Non-stressed rats did well on the test; their stressed-out peers consistently failed.
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In another study, researchers at Washington University School of Medicine in St. Louis gave volunteers oral doses of cortisol that approximated the levels they’d experience under stress. The subjects were given a memory test before being given cortisol and then after one day of cortisol, after four days of cortisol, and six days after they’d stopped taking it. They were compared to a control group not given cortisol.

What happened? Although cortisol seemed to have no short-term effect on nonverbal memory, sustained or selective attention, or executive function, those given cortisol did worse on tests of verbal declarative memory. The more cortisol they’d had, the worse they did.

Longer term, excessive stress creates even bigger deficits. Why would stress have such a negative effect on brain function? For starters, “chronic stress causes at least some nerve cells in key brain regions like the hippocampus to shrink and lose their connections with other nerve cells,” explains Dr. Bruce McEwen, a professor at The Rockefeller University, who for decades has studied the effects of stress and sex hormones on the brain (and who was a post-doctoral mentor for my undergraduate professor, Dr. Meaney). The process is called synaptic degeneration, and the result is a breakdown in communication between brain cells.

Excess stress has also been shown to inhibit neurogenesis in the hippocampus. “As a result, the hippocampus becomes smaller and less effective in doing its job,” says Dr. McEwen, whose animal studies have helped shed light on the effects of stress on the hippocampus. Human studies have shown similar effects on the hippocampus.
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Over time, the effects accumulate. In one study, women who reported a history of perceived high stress over twenty years had smaller hippocampi than those who reported less stress.
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One reason for that shrinkage may be that stress lowers BDNF levels. In one animal study, rats who were intentionally stressed for six hours had reduced BDNF levels in their hippocampi. Another study, this one in humans, found reduced BDNF levels in patients who’d had childhood trauma and recent stress.
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Reduced BDNF and increased cortisol levels were also tied to a smaller hippocampus. It makes perfect sense: as BDNF is repeatedly lowered, new neurons aren’t able to survive. That, combined with synaptic degeneration, leads to hippocampal atrophy.

When it comes to chronic stress, however, the hippocampus is not all we have to worry about. In recent years evidence has emerged showing that stress does damage to other parts of the brain, most notably the prefrontal cortex—a key area for working memory, decision making, executive function, and impulse control. The prefrontal cortex also works in ways that affect behavior, for example, by helping to keep the amygdala—a key center for negative emotions—under control.

Animal studies have shown that chronic excess stress causes neurons to shrink and lose connections and functional efficacy in the prefrontal cortex, “just like in the hippocampus,” says Dr. McEwen, whose many studies included one on medical students preparing for their board exams. In that study, Dr. McEwen found that students’ perceived level of stress was tied to poor performance on a task that relied on the prefrontal cortex.
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The more stressed they felt, the worse they did. Connectivity in the prefrontal cortex, as seen on fMRI, was also reduced with high stress.

Meanwhile, though neurons in the prefrontal cortex are shrinking and showing disconnection, neurons in two other parts of the brain may actually grow with stress. One of these growth areas is the orbital frontal cortex, which responds to stimuli that predict reward or punishment. The other growth areas are in segments of the amygdala that play a role in fear, anxiety, and aggression. In animal studies, growth in these areas makes the animal “more anxious and more fearful and, in the case of the orbital frontal cortex, more sensitive to stimuli that either predict reward or punishment,” explains Dr. McEwen. Growth in these areas of the brain serves a purpose: if you’re a deer heading out across the tall grass of an open field—where predators can lurk—heightened awareness and a little anxiety might save your life. But when the danger passes, “if these changes do not reverse, then you have an anxiety disorder,” says Dr. McEwen.

Think You’re Not Tense?

Some stressors—an overdraft notice from your bank, a fight with your spouse, a mistake at work—are hard to miss because they elicit an immediate response: you feel your heart rate increase, your emotions rise.

But you may be subjected to stressors without even noticing them. One possible culprit is noise. And not just loud, jarring noises. Our brains and bodies actually respond to noise that we don’t even realize we hear. In an evolutionary way, that makes perfect sense. If your fight-or-flight response didn’t kick in when your brain picked up the almost-silent rustle of leaves in a nearby bush, you might find yourself in the jaws of a predator. In a true emergency, a keen sense of hearing—and your corresponding fight-or-flight response—still comes in handy. But in cases where there’s no real emergency, noise can simply create stress. And chronic exposure to noise can create chronic stress.

That means the workspace floor plan that’s supposed to encourage collaboration could actually be sending your stress level soaring. In 2000, environmental psychologist Gary Evans and a colleague at Cornell University reported that working in a moderately noisy open-format office results in higher levels of stress.
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That study compared stress levels of twenty workers randomly assigned to a quiet office and twenty workers subjected to three hours of noise intended to approximate the low-intensity sounds of an open-format office. Once the workers were in place, the research team performed tests to measure stress hormone levels and cognitive skills. They found that those in the noisy office had elevated stress hormones and made fewer attempts at unsolvable puzzles. That was true even though the workers didn’t report feeling higher levels of stress than their counterparts in quiet offices.

Even low-level noise like nearby traffic can cause stress, as a Cornell team found in a study of 115 Austrian fourth-graders.
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Those who lived in noisier residential areas had slightly higher resting systolic blood pressure, greater heart reactivity when presented with a stressor, and higher overnight cortisol levels, compared to children who lived in quiet residential areas.

Chronic exposure to louder noise, like that from an airport, can have a significant impact, too, as a variety of cross-sectional studies have shown in the past. In the late 1990s the closing of one airport and opening of another in Germany allowed researchers to see for the first time what happened when children switched from being in an area that is quiet to being in an area that is loud, and vice versa.

The study, conducted by Evans and counterparts in Germany and Sweden, followed 326 third- and fourth-grade children living in Munich, half near an existing airport that was slated to close and half near the site of a new airport, soon to open. The children were tested six months before the new airport opened and again one and two years after it opened.
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The researchers found that, after the new airport opened, children in the high noise areas near it showed impairment in long-term memory, reading, and speech perception. Those in the now quiet area near the old airport, meanwhile, saw improved performance in long-term memory and reading as well as in short-term memory.

The fact that scores improved once noise was removed is a promising sign that the ill effects of noise pollution are likely reversible.

Stress Nation?

When the American Psychological Association (APA) surveyed Americans about their stress levels in 2011, the picture wasn’t exactly bright. Some 22 percent of those surveyed said they were under extreme stress, ranking their stress level an 8, 9, or 10 on a scale of 1 to 10. The survey probably undercounted the highly stressed, too, since people under serious duress may be too pressed for time to answer a survey. Based on the thousands of patients I’ve seen, I’d estimate the true amount is closer to 50 percent.

Interestingly the survey also noted that 36 percent of respondents thought their stress levels had no bearing on their mental health. Only 29 percent felt they were doing an excellent or very good job at managing or reducing their stress levels.

Depression: A Brain Changer

Depression is a mood disorder characterized by prolonged feelings of sadness and other key symptoms, including disinterest or lack of pleasure in daily activities, insomnia or excessive sleepiness, excessive weight loss or weight gain, lack of energy, an inability to concentrate, feelings of worthlessness or excessive guilt, and/or recurrent thoughts of death or suicide. Depression can have many causes, including a genetic component that may make some people more prone to developing it than others.

Obviously depression isn’t merely a matter of being excessively stressed. But prolonged stress—especially the type caused by factors beyond our control—can play a part in the development of depression. And feeling perpetually stressed, or having persistent anxiety, can also be a symptom of depression. In my own medical practice I often see patients who are depressed but don’t realize it; they instead attribute their symptoms to stress. Often they’re surprised when I suggest that they may meet the criteria for clinical depression.

But they’re certainly not alone. In fact, the Centers for Disease Control (CDC) reports that 9 percent of the American adults it polled reported suffering from major depression in the two weeks before being polled. Many others may suffer from lesser depressive symptoms.

Their symptoms aren’t terribly surprising when you consider what’s going on in the brain. For starters, depressed people have lower levels of norepinephrine and serotonin, two key neurotransmitters that help pass messages in the brain, especially in areas important for attention, mood, reward, and alertness. Without the full support of these messengers, depressed brains don’t operate as well.

High cortisol levels have also long been noted in people with depression. And since excess cortisol is tied to a smaller hippocampus, it’s no surprise that depression is associated with shrinkage in the hippocampus as multiple studies have shown. Although there are some exceptions—one large study failed to find this link—there is ample evidence that the worse the depression, the smaller the hippocampus. One meta-study that looked at the results of twelve other studies found that people with major depression may have hippocampi that are, on average, 8 to 10 percent smaller than people without depression.
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Who Is Depressed?

According to the CDC, those most likely to report suffering from depression are:

• forty-five to sixty-four years of age;