What motivates some people to get off the sofa and run, walk, ride or Zumba regularly?
If paying for a fitness center membership, buying the latest gear or saying, “I’m going to make myself do it,” was enough to motivate us to exercise, we’d be a much healthier nation.
Rutgers neuroscientist Joan Morrell, whose research focuses on studying the brain and motivation, believes the answer lies in what she calls “my favorite part of the brain,” the prefrontal cortex. That’s where reasoning happens and choices are made in response to stimuli.
The prefrontal cortex plays a key role in learning and decision-making. It can be trained, untrained and retrained by actions, creating habits that lead to motivation. “Our motivational habits have been falsely wired (in our brains) by our cultural training,” says Morrell, professor in the Center for Molecular and Behavioral Neuroscience at Rutgers-Newark.
When we drive four blocks to buy a quart of milk instead of walking, we help establish a pattern of non-exercise, she explains. “Before you know it, we’re not working to make habits of activity because it’s easier not to. This all starts in the prefrontal areas.” Through the prefrontal cortex, individuals select actions to seek stimuli that have what scientists call “high positive incentive salience” – or are rewarding to them, as non-scientists might think of it.
Simply knowing that exercise is good for us is not enough reward. Regular exercisers have built habit over time, based on positive incentive salience in either the activity itself, the aftereffects, or both.
“I believe it’s trained out of us in the first world, but it could be trained into us,” Morrell says, distinguishing between developed nations’ need for exercise and less developed cultures where physical activity is interwoven into daily survival.
Morrell started thinking about the brain processes behind exercise motivation while working on her longstanding research with rats on motivation in maternal caregiving and cocaine-seeking.
She noted that the animals showed a relentless quality in tending their pups and searching for the drug. Rats became similarly relentless about spontaneously running when given access to a running wheel.
In their nightly activity, male rats ran about three miles and females ran four-and-a-half miles. It took only three weeks for the rats to voluntarily run at their greatest capacity. When they had free access to a wheel, they ran, groomed and ate. If they were given only 30 minutes or two hours with a wheel each night, the rats would “run like crazy” with few breaks, Morrell says.
Using scientific tools, Morrell and graduate student Julia C. Basso established that the rats ran because they “liked” the experience of running, “liked” being with the wheel and “liked” the aftereffects of the exercise. The tests showed that the wheel was a rewarding stimulus – had positive incentive salience – for them.
“Our working hypothesis was that the brain circuits that lead to motivation for drugs, caring for young and exercising all have certain limited overlapping elements” in the prefrontal cortex, Morrell says. “We think those parts interact in some way that’s right on the edge of what we understand in the field.”
By temporarily shutting off parts of the rats’ brains, the team discovered that the prefrontal cortex area needed for the motivation to run is the same one that fosters rats’ drug-seeking behavior. Maternal motivation maps to an adjacent “sister” area.
This finding means the prefrontal cortex is not one unit. “It’s broken into at least two major parts that are differentially necessary for the motivation to seek a particular stimulus,” says Morrell. “The infralimbic part is necessary for pup-tending and the prelimbic for cocaine and exercise.”
Although the human brain has far greater cognitive and cultural capacity than the rat brain, the basic neurobiological system is similar. The biological roots shown in Morrell’s research point to a path for building exercise motivation and reducing the ills of inactivity – obesity, chronic illness and depression, to name a few.
“My postulate is we can learn that exercise and its aftereffects are rewarding. That’s what’s going to get us out of this pickle we’re in,” Morrell says.
Morrell and Basso will present their findings at the annual meeting of the Society for Neuroscience in October. The study is Basso’s thesis work and she is lead author.