The orbitofrontal cortex, the part of the brain located above and behind the eye, is involved in even the simplest decisions (yes, I want chocolate ice cream). People with damage to this part of the brain have trouble making consistent choices.

"When we remove risk and multiple choices from the task, the most fundamental choices occur there," says Fellows.

In a recent study of patients that had had strokes, aneurysms or brain tumours, Fellows has found that different parts of the frontal lobe, where the decision-making processes are housed, are involved in different types of decision making.

Fellows created a game that used a rigged deck of cards. One deck would offer wins more than the other. After several hands, the winning deck would switch places. "You have to be on your toes and keep track of the game as it changes," she says. People with damage to the orbitofrontal cortex - the part of the brain that is important in connecting a good (deck of cards) to its subjective value (winning) - were worse at figuring out which deck was the winning deck and consistently choosing it.

She devised a similar game that asked participants to twist a joystick in one direction or another to win. Those with damage to the dorsal anterior cingulate cortex (dACC), could pick the winning deck of cards, but not make an action (twist a joystick) in the winning direction.

KEEPING FATIGUE AT BAY

It's the end of a long, challenging day, and you're surfing online shopping sites from your sofa. Can you stop a $400 online impulse purchase before it happens?

Baumeister says decision fatigue offers few clues about its pending onset. "I don't think you really notice it. Some people do report that they can't make any more decisions or they don't want to make any more decisions or that it has become difficult, but we don't know how close that feeling is to actually having decision fatigue," says Baumeister.

Such feelings are a good sign to stop making important decisions - at least for a little while. Or take the long-term view and invest some time on preventing decision fatigue in the first place, by building up your willpower.

"It's like a muscle. It gets tired when you use it, but it also gets stronger," he says.

Exercise your willpower by breaking habits. Sit up straight. Mouse with your left hand. Speak in complete sentences. "The exercises are taxing, but they have all had success because there is one stock of willpower that is used for everything," says Baumeister. "If you improve at one thing, you are more capable at everything," he says.

PICKY, PICKY

Experts offer tips on how to avoid feeling overwhelmed:

Don't make important decisions late in the day; tackle them first thing in the morning.

BlackBerry? iPhone? Android? Call a friend for a recommendation.

Don't wait for perfection. Stop looking and make the purchase once something meets your standards.

Impose a structure to your decisions. Separate the options into a few categories. If you know your schedule can't accommodate an evening course, eliminate those ones outright and don't pine over what you can't have.

HANNAH HOAG

HOW WE MAKE CHOICES: INJURED BRAINS YIELD SOME CLUES

By studying people with brain injuries, scientists can learn more about why we think and act the way we do.

The images on the left show the base of the brain and those on the right show what you would see if the brain were cut in half down the centre, from front to back.

Research done in the laboratory of Lesley Fellows, a neuroscientist at the Montreal Neurological Institute and Hospital, showed that people with damage in the orbitofrontal cortex (top two images) have trouble choosing one object over another, whereas people with damage affecting the dorsal anterior cingulate cortex had trouble choosing one action over another.

The images shown here were produced by superimposing areas of damage from several patients. The red areas show the greatest degree of overlap.

HANNAH HOAG