Rats avoiding mental workloads offer clues to lower motivation in depression

Sometimes, hard work can be measured in sweat. Weeding a garden, painting the house or hooking golf balls out of the woods to win the Masters can leave a person physically wrung out.

But hard work isn’t only about muscles. It takes place between people’s ears, too. Mental efforts, whether dedicated to writing a flawless sentence, figuring out how to respond to an e-mail or solving a long list of algebra problems, can be just as hard as Bubba Watson’s swinging a club at Augusta National. “Difficulty is more than caloric expenditure,” says behavioral neuro­scientist John Salamone.

Some people do their best to avoid hard thinking. Think about it (or don’t, if you’re that sort of person): There’s always somebody who is happy to go pick up Chinese takeout but is loathe to decide what to order, look up the number and make the phone call.

Salamone, who studies these sorts of cost-benefit quandaries, shares another example: When his lab at the University of Connecticut in Storrs had to relocate to a different building, everyone had to help. A big, strong lab member jumped at the chance to haul boxes across campus. He would gladly provide the brawn, he said, as long as he didn’t have to pack anything up. The thought of organizing and sorting lab equipment sounded way worse than a little heavy lifting.

(A confession: On my walk to work this morning, I eyed the gardeners on N Street, thinking I would happily trade my writing deadlines for tulip-tending deadlines.)

It turns out that the dread of heavy mental workloads is not limited to people caught up in the rat race of modern life. Rats feel the crushing weight of mental work, too. And studying rodents could be a good way to figure out what’s happening when human motivation goes awry, a symptom of disorders such as depression.

Like members of their human brethren, some rats avoid hard mental work like the plague, content to skate by with the bare minimum. Other rats seem to relish mental challenges. These naturally occurring groups are completely uncharted sectors of the rat population, fascinating new subjects that could help reveal how the brain makes decisions about work.

Now that mentally lazy rats have been outed, scientists can test whether this loafing is equal opportunity. Are these same rats also slackers when the work is physical, like pushing a lever more for a greater reward? What about social interactions, or raising their pups?

By scrutinizing these slothful lab rats, researchers might begin to understand what happens when a person decides that the payout won’t be worth the effort. These results will be useful, and not just as an excuse for slackers (also known as animals that struggle with “insufficient recruitment of mental effort,” as University of British Columbia researchers politely write in a recent Neuropsychopharmacology). Understanding how people weigh the costs of a task against the potential benefits could help scientists understand and counter one of the most pernicious features of severe depression.

People in the throes of a major episode can have trouble mustering the will to handle the normal demands of a day. Going to work, getting dressed or even getting out of bed can seem like insurmountable tasks. Lab studies show that people with depression perform worse than healthy people on difficult mental tasks that require a lot of concentration.

And it’s not because they’re lazy. While the exact causes of depression aren’t yet clear (and probably vary from person to person), most scientists believe that nerve cell behavior in particular parts of the brain is out of whack. This perturbation, some scientists think, can prevent the brain from assessing cost-benefit ratios normally.

In particular, certain nerve cells that produce and detect the brain chemical dopamine seem to be involved. In rats, interfering with these nerve cells messes with an animal’s motivation to do physical work to get a reward, Salamone says. Dopamine depletion in a certain part of the brain can make formerly hard-working rats stop pushing a lever. And rats that shy away from a hard mental task might better represent what happens in depressed people.

One approach to treating depression is to encourage people to get active, even if it’s just a little bit. The technique, called behavioral activation therapy, is based on coaching a person to tackle activities by scheduling tasks and sticking to them. Salamone says the idea is to provide a manageable system that helps make tasks seem less formidable.

Scientists are just beginning to understand how the brain decides to work. But intellectually lazy rats may lend a paw. Whether they feel like it or not.

SN Prime April 23, 2012 | Vol. 2 No. 16

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

More Stories from Science News on Health & Medicine