In our exploration of neuroscience and behavior, we’ve traveled from the activities of individual neurons to the complex interaction of the billions of neurons that make up the human nervous system, most notably the brain. In the course of those travels, we presented four themes that are crucial to a scientific understanding of brain function: localization, lateralization, integration, and plasticity.
More than just a historical scientific oddity, phrenology’s incorrect interpretation of bumps on the skull helped focus scientific debate on the notion of localization—the idea that different functions are localized in different brain areas. Although rejected in the early 1800s when Franz Gall was in his heyday, localization of brain functioning is well established today. The early clinical evidence provided by Broca and Wernicke, and the later split-
The ideas of localization and lateralization are complemented by another theme evident in this chapter—integration. Although the nervous system is highly specialized, even simple behaviors involve the highly integrated interaction of trillions of synapses. Your ability to process new information and experiences, your memories of previous experiences, your sense of who you are and what you know, your actions and reactions—
The story of Asha’s stroke illustrated what can happen when that harmony is disrupted. Asha survived her stroke, but many people who suffer strokes do not. Of those who do survive a stroke, about one-
What happened to Asha? Fortunately, her story has a happy ending. Asha was luckier than many stroke victims—
Asha set a very high goal for herself: to return to teaching at the university the following fall semester. With the help of her husband, Paul, and her mother, Nalini, who traveled from India to help coach her back to full recovery, Asha made progressive and significant gains. With remarkable determination, Asha reached the goal she had set for herself. Eight months after her stroke, she returned to the classroom and her research lab.
Today, more than five years after her stroke, the average person would never know that Asha had sustained significant brain damage. Other than an occasional tendency to “block” on familiar words—
Thus, Asha’s story illustrates a final theme—
Maximizing Your Brain’s Potential
It was 1962 when a group of neuroscientists led by psychologist Mark Rosenzweig published the unexpected finding that the brains of rats raised in enriched environments were significantly different from the brains of rats raised in impoverished environments.
For lab rats, an enriched environment is spacious, houses several rats, and has assorted wheels, ladders, tunnels, and objects to explore. The environment is also regularly changed for further variety. Some enriched environments have been designed to mimic an animal’s natural environment (see Heyman, 2003). In the impoverished environment, a solitary rat lives in a small, bare laboratory cage with only a water bottle and food tray to keep it company.
Decades of research have shown that enrichment increases the number and length of dendrites and dendritic branches, increases the number of glial cells, and enlarges the size of neurons (Cohen, 2003). Enrichment produces more synaptic connections between brain neurons, while impoverishment decreases synaptic connections. With more synapses, the brain has a greater capacity to integrate and process information and to do so more quickly. In young rats, enrichment increases the number of synapses in the cortex by as much as 20 percent. But even the brains of extremely old rats respond to enriched environments. In fact, no matter what the age of the rats studied, environmental enrichment or impoverishment had a significant impact on brain structure (Kempermann & others, 1998).
Enrichment has also been shown to increase the rate of neurogenesis in many different species, from rodents to monkeys (Fan & others, 2007; Nithianantharajah & Hannan, 2006). Both the number and the survival time of new neurons increase in response to enrichment (Gould & Gross, 2002; van Praag & others, 2000). Interestingly, while enriched environments can increase neurogenesis, social isolation and a stressful environment decrease neurogenesis (Ming & Song, 2005).
Collectively, these changes result in increased processing and communication capacity in the brain. Behaviorally, enrichment has been shown to enhance performance on tasks designed to measure learning and memory, such as performance in different types of mazes (van Praag & others, 2000).
Who Moved My Exercise Wheel?
Neuroscientists have identified an additional factor that improves brain function, even in aging mammals: exercise (Hillman & others, 2008; Shors, 2014). In one study, just a month of daily exercise helped reverse cognitive declines associated with aging in previously sedentary, elderly mice (van Praag & others, 2005). After having access to an exercise wheel for 30 days, mice that were the rodent equivalent of 70 years old learned to navigate a maze much faster than mice of the same age that did not exercise. They also had better memories of maze locations. Finally, the physically active elderly mice had a greatly increased rate of neurogenesis, and the new neurons functioned as well as new neurons generated in the brains of young mice. As study coauthor Henriette van Praag (2005) points out, “Our findings show that it is never too late in life to start to exercise, and that doing so will likely delay the onset of aging-
From Animal Studies to Humans
Enrichment studies have been carried out with many other species, including monkeys, cats, birds, honeybees, and even fruit flies. In all cases, enriched environments are associated with striking changes in the brain, however primitive.
Can the conclusions drawn from studies on rats, monkeys, and other animals be applied to human brains? Obviously, researchers cannot directly study the effects of enriched or impoverished environments on human brain tissue as they can with rats.
MYTH SCIENCE
Is it true that the brain is essentially “hardwired” by adolescence?
However, consider a study conducted by Ana Pereira and her colleagues (2007). Male and female participants, aged 21 to 45, were assessed for their overall level of fitness. Using MRI scans, each participant’s brain was also mapped for the amount of blood flowing into the hippocampus. Over the next three months, the participants worked out for one hour four times a week. Finally, the same physical and brain measurements were taken again.
As you probably anticipated, all of the participants had significantly improved their overall level of aerobic fitness. More importantly, they had also substantially increased the blood flow to their hippocampuses, in some cases doubling the blood flow as measured prior to the exercise program. In general, the greater the increase in a participant’s aerobic fitness, the greater the increase in blood flow to the hippocampus.
Now for the key finding of the study: Along with the human subjects, a group of mice followed a comparable exercise program. In the mice, the exercise program resulted in increased blood flow in the same regions of the hippocampus as in humans. However, the researchers were able to directly examine brain changes in the mice. They found that the increased blood flow to the hippocampus in the mice was directly correlated to the birth of new neurons in the same region of the hippocampus. Although neuroscientists tend to be cautious in drawing conclusions, the implication of Pereira’s study is obvious: Exercise promotes neurogenesis in the adult human brain just as it does in other mammals. A footnote: The participants in Pereira’s study also improved their scores on several tests of mental abilities. Later studies have extended and confirmed these findings (Kobilo & others, 2011; Kuzumaki & others, 2011). And, other research has shown that even moderate exercise can increase brain volume in previously sedentary, older adults (Erickson & others, 2011). We describe this study in more depth in Chapter 9.
Neuroscientists have also amassed an impressive array of correlational evidence showing the human benefits from enriched, stimulating environments. For example, several studies have compared symptoms of Alzheimer’s disease in elderly individuals with different levels of education (Bennett & others, 2003). Autopsies showed that the more educated individuals had just as much damage to their brain cells as did the poorly educated individuals. However, because the better-
The results of this study echo those from earlier research on intellectual enrichment: A mentally stimulating, intellectually challenging environment is associated with enhanced cognitive functioning. Just as physical activity strengthens the heart and muscles, mental activity strengthens the brain. Even in late adulthood, remaining mentally and physically active can help prevent or lessen mental decline (Greenwood & Parasuraman, 2012; Hertzog & others, 2009; Hillman & others, 2008).
Pumping Neurons: Exercising Your Brain
So, here’s the critical question: Are you a mental athlete—
Get regular aerobic exercise, even if it’s no more than a brisk daily walk. If possible, vary your routes and try to notice something new about your surroundings on each walk.
Don’t hide in your room or apartment—
Learn to play a musical instrument. If you can’t afford music lessons, join a singing group or choir. If you already play a musical instrument, experiment with a new style or musical genre.
Take a class in a field outside your college major or in a new area. Experiment by learning something in a field completely new to you.
Read, and read widely. Buy magazines or check out library books in fields that are new to you.
Try puzzles of all kinds—
Unplug your television set for two weeks—
Better yet, take a few minutes and generate your own list of mind-