Those of you who have ever taken a long road trip know how satisfying it is to finally reach your destination. There is always that long-term goal spurring you on to complete your journey. But what exactly is it that keeps us focused?
Researchers from the Massachusetts Institute of Technology (MIT) believe their new study may reveal the answer.
The study, published in the journal Nature, suggests that the neurotransmitter, dopamine, plays a part in how the brain signals the value of long-term rewards.
The study authors say that previous studies have revealed the effects of dopamine on short-term goals. Animal studies have shown how dopamine neurons show brief bursts of activity when animals receive an unexpected reward.
They add that dopamine signals play an important role in reinforcement learning. This is how animals learn to perform actions that lead to rewards.
But the researchers wanted to investigate how animals focus on longer-term goals as they do in real life, such as hunting for food.
Increased levels of dopamine
The researchers trained rats to navigate a maze toward a milk chocolate reward. The aim of the study was to see how the dopamine system changes to adapt to "delayed gratification."
A series of tests through the mazes were run, during which the rats would hear a tone instructing them to either turn left or right at intersections.
The amount of dopamine that entered the striatum - a brain structure important for reinforcement learning, was measured through a technology called fast-scan cyclic voltammetry (FSCV). This is a series of small, implanted, carbon-fibre electrodes that can measure dopamine concentration based on its electrochemical fingerprint.
The results of the research revealed that the level of dopamine increased steadily throughout each trial, and peaked as the rat reached the chocolate, suggesting that the rat anticipated the reward, the researchers say.
They add that the dopamine signal did not vary with running speed or trial duration and, compared with suggestions from previous studies, the level of dopamine did not depend on the probability of the rat getting a reward.
Professor Ann Graybiel, investigator at McGovern's Institute for Brain Research at MIT, says:
"Instead, the dopamine signal seems to reflect how far away the rat is from its goal. The closer it gets, the stronger the signal becomes."
Professor Graybiel adds that the size of the signal was also related to the size of the reward. It was found the bigger the piece of chocolate the rat would receive, the dopamine signal increased to a higher concentration.
Research could explain Parkinson's symptoms
The trials were also conducted in a more complex-shaped maze, meaning the rats had to make extra turns and run further before reaching the chocolate.
Professor Graybiel says:
"It's as if the animal were adjusting its expectations, knowing that it had further to go.
This means that dopamine levels could be used to help an animal make choices on the way to the goal and to estimate the distance to the goal.
"This 'internal guidance system' could also be useful for humans, who also have to make choices along the way to what may be a distant goal."
Professor Graybiel says that she would be shocked if something similar were not happening in human brains, and that this may explain why people suffering from Parkinson's disease have difficulty in sustaining long tasks.
"Maybe that's because they can't produce this slow ramping dopamine signal," she says.
Future research will involve looking at how the dopamine signal rises in the brain, Prof. Graybiel says, as many animals have "place cells" which determine their spatial environment. She adds:
"As our rats run the maze repeatedly, we suspect they learn to associate each point in the maze with its distance from the reward that they experienced on previous runs."