To Do a Tough Job, Your Brain Conjures the Reward
Depression is marked by a lack of activition in reward circuitry.
Posted Oct 14, 2012
Yesterday at the Society for Neuroscience (SfN) conference, Michael Treadway presented research on how we decide to buckle down and complete a tough task. Based on MRI data, he suggests that the brain needs to activate reward circuitry before taking on a challenge. In a sense, you have to think about the carrot on the end of the stick before you're willing to pull the cart.
Treadway compared a group of 18 clinically depressed adults and a group of 15 adults with no history of depression on an effortful reward task while they underwent functional MRI. One of the common symptoms of depression is a loss of sensitivity to reward. Treadway hoped to explore the mechanism in the brain that accounts for the decrease in pleasure.
To measure how participants responded to reward, he designed a simple task. Basically, participants were faced with a choice between a tough task with a larger reward and an easy one that didn't pay as well. For the tough task, the subject had to press a button 100 times in 21 seconds using their non-dominant pinky. For their effort, they received an average of nearly $3. On the easy task, they needed to press the button 30 times in 7 seconds using their dominant index finger, but they only got a buck.
In terms of decisions, the depressed group was significantly more likely to opt for the easy task. The non-depressed group was more willing to take on the challenge for the extra reward. This supports the idea that loss of sensitivity to reward is a common mark of depression.
Looking at the brain imaging data, Treadway found that not only did depressed participants forego the tough task, but they processed the decision differently. Whereas the non-depressed group had increased activation in the nucleus accumbens, the depressed group did not recruit this region. The nucleus accumbens is commonly thought to be the seat of reward processing. The depressed group's lack of response here suggests that they don't account for the reward they might receive. There's no pot of gold drawing them to the end of the rainbow.
What's more, the non-depressed group had a spike in activation in the ventromedial prefrontal cortex (vmPFC), a region implicated in subjective value, when choosing the hard task. In monkeys, neurons in the vmPFC fire in proportion to the monkey's preference for a reward, so if he prefers to grape juice to orange, the activity here should reflect that. Since the non-depressed group recruits this region, it suggests that they find value in the doing the work to reach the reward. The depressed group, however, did not have increased activation here, so they may not have been able to see the worth.
Treadway's findings suggest that when we need to get work done, we compute the reward and value of finishing it before getting started. The non-depressed group chose to do the harder, but more rewarding task more often. During the decision they recruited the nucleus accumbens and vmPFC, central players in the brain's reward circuitry.
Part of the struggle with depression is finding motivation. The depressed participants chose the hard task less often and had less recruitment of reward circuitry, which may be necessary to overcome the costs to achieve gains.
So for me to get off Facebook and get my work done, I have to think of the good things that await--once I get home I've got all night to read what my friends are eating for dinner.