Throughout life, each person's future and ultimately their survival depends upon choices made each day, whether it be accepting that new job offer, continuing to pursue a particular relationship, or deciding what to eat for lunch. In order to choose the best option, it’s important to weigh the pros and cons of each possibility and predict whether the outcome might be worth the effort it will cost to obtain it.
As a concrete example, consider a person who is deciding what to eat for lunch. They need to think about the type of food they want to eat, how far away they will need to travel in order to obtain it (can they make it there and back during their lunchtime?), how much money the food will cost, and whether the food will adequately nourish their body. On paper, this type of calculation may require pen and paper, but the brain is able to handle this algorithm quite well. The ultimate reward is to eat a meal that will provide the maximum amount of nutrients and energy for the body but requires a minimal amount of energy to obtain it.
Scientists at the University of Washington in Seattle and the University of Oxford are studying the processes that underlie our brain's analysis of these situations. Each aspect must be taken into account to ensure that the net effect is beneficial for us. These researchers recently reported in Nature Neuroscience that dopamine released from the nucleus accumbens encodes information regarding the value of the reward itself but not the costs associated with obtaining that reward unless the cost is particularly low.
At the heart of reward processing is the mesolimbic dopamine pathway, which lies in the midbrain and contains neurons that release dopamine to the nucleus accumbens. This circuit analyzes how likely it is to receive the reward, how much of that reward is possible to obtain, and how long it will take to receive the reward.
In order to achieve these results, the group studied the quantitative amounts of dopamine released in rats at the nucleus accumbens when the value of a reward or the cost of a reward was changed. To begin, the rats were trained to choose between two options:
1. Pressing a lever 16 times, which is the cost, to receive 1 pellet of food, which is the reward.
2. A similar protocol in which the cost was manipulated by increasing to 32 lever presses or decreasing to 2 lever presses OR a similar protocol in which the reward was manipulated by increasing to 4 food pellets or decreasing to no food pellets at all.
A significantly greater amount of dopamine was released when the rat chose 4 food pellets over 1 food pellet; likewise, more dopamine was released when the rat chose 1 food pellet over no food pellets. Thus, the higher levels of dopamine were released when the 'better' of the two possible rewards was chosen.
When the cost of the reward was manipulated, more dopamine was released when the rats chose to press the lever 2 times instead of 16 times. However, there was no difference in the amount of dopamine released when the rat chose 16 lever presses over having to press the lever 32 times. It was concluded, then, that the nucleus accumbens is important for determining the value of the reward but unless the cost is particularly low (if you're getting a real good deal!), evaluating the cost is processed elsewhere.