Researchers at Michigan Institute of Technology found that the neurotransmitter dopamine plays a significant role in helping the brain stay focused on its long-term goals. Their findings may also help explain why people with Parkinson’s disease, in which dopamine signalling is impaired, often have problems sustaining the motivation needed to complete long tasks.
Past studies have linked dopamine with reward, suggesting that the dopamine neurons show bursts of activity when animals receive an unexpected reward. These findings were important for reinforcement learning – the process by which an animal learns to perform a task or action that leads to a reward. In most studies however, the reward is given almost immediately. But this is not what happens in real life, especially in humans. In most cases, people have to work hard and wait for a certain period of time to get their reward (for instance, employees have to patiently wait for a couple of weeks before they receive their salaries).
The brain on long-term goals
The MIT team, headed by Professor Ann Graybiel, decided to study the changes in the dopamine system when the reward or gratification is delayed. For their study, the researchers trained lab mice to navigate a maze to reach a reward. During each trial, the subjects would hear a tone instructing them to navigate either to the left or right to find a chocolate milk reward.
Graybiel and her colleagues wanted to measure how much dopamine was released in the striatum – the part of the brain involved in reinforcement learning. To do this, they teamed up with Paul Phillips of the University of Washington who developed a technology called fast-scan cyclic voltammetry (FSCV). The devise uses tiny, implanted, carbon-fibre electrodes to continually measure dopamine concentration based on its electrochemical fingerprint.
Mark Howe, the study co-author and a former student of Graybiel who is now a postdoc in the Department of Neurobiology at Northwestern University, said they used the FSCV method to be able to measure the amount of dopamine released at up to four different sites in the brain simultaneously, as the animals travel through the maze. "Each probe measures the concentration of extracellular dopamine within a tiny volume of brain tissue, and probably reflects the activity of thousands of nerve terminals." he said.
Researchers expected to see pulses of dopamine at different times in the trial, but they were surprised to see that it increased steadily as the rats get nearer towards their goal. And despite the variation in their behaviour, the dopamine signals remained the same. What’s more, it didn’t depend on the probability of getting a reward, which previous studies suggest.
"Instead, the dopamine signal seems to reflect how far away the rat is from its goal," explained Prof Graybiel. "The closer it gets, the stronger the signal becomes."
They also found that the size of the signal was related to the size of the expected reward. When the rats were trained to anticipate a larger amount of chocolate milk, their dopamine signals rose more steeply to a higher final concentration.
Same works for humans
"I’d be shocked if something similar were not happening in our own brains." said Graybiel. In the case of Parkinson’s for instance, in which dopamine signalling in the brain is impaired, patients have difficulty sustaining their motivation to finish a long task. The lead researcher suggests that it’s probably because these people can’t produce this slow ramping dopamine signal.
Their work was published in the journal Nature.
Source of this article:
Prolonged dopamine signalling in striatum signals proximity and value of distant rewards