The neurotransmitter dopamine may indicate the value of long-term rewards in the brain, says a new study from MIT on how the brain maintains focus on long term goals.
The study results could also clarify why patients suffering from Parkinson’s disease, in which dopamine signaling is impaired, typically have trouble keeping motivated to finish tasks.
In previous studies, have dopamine has been linked to rewards, and dopamine neurons showed brief bursts of activity when animals get an unforeseen reward. These dopamine signals are believed to be important for reinforcement learning, in which an animal learns to perform actions that lead to reward.
For most previous studies, the reward has been delivered within a few seconds. Real life gratification, though, is not always immediate. Animals need to travel while hunting for food, and have to keep their motivation for a distant goal while also responding to more immediate cues.
Likewise for humans, for example on a long road trip the driver has to remain focused on reaching a final destination while also reacting to traffic, stopping for snacks, and entertaining children in the back seat.
The research team at MIT chose to study how dopamine changes during a maze task similar to working for delayed gratification. Rats were trained to navigate a maze to reach a reward. During each trial a rat would hear a tone cueing it to turn either right or left at an intersection to find a chocolate milk reward.
Instead of only measuring activity of dopamine-containing neurons, the researchers wanted to measure how much dopamine was released in the striatum. The striatum is a brain area involved with reinforcement learning.
They teamed up with University of Washington’s Paul Phillips, who developed a technology called fast-scan cyclic voltammetry (FSCV) in which small, implanted, carbon-fiber electrodes allow for continuous measurements of dopamine concentration based on its electrochemical fingerprint.
Thousands of Nerve Terminals
“We adapted the FSCV method so that we could measure dopamine at up to four different sites in the brain simultaneously, as animals moved freely through the maze,” says Mark Howe, first author. “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."
The researchers expected, from previous work, to perhaps see pulses of dopamine released at different times in the trial, “but in fact we found something much more surprising,” said team leader Ann Graybiel. The level of dopamine increased steadily throughout each trial, peaking as the animal approached its goal, as if in anticipation of a reward.
“The dopamine signal seems to reflect how far away the rat is from its goal,” Graybiel continues. “The closer it gets, the stronger the signal becomes."
Researchers also found that the size of the signal was related to the size of the expected reward. When rats were trained to anticipate a larger chocolate milk drink, the dopamine signal rose more steeply to a higher final concentration.
Slow Ramping Dopamine Signal
“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,” says the Salk Institute’s Terrence Sejnowski. “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.”
How relevant is this research to humans? “I’d be shocked if something similar were not happening in our own brains,” Graybiel says. It is known that Parkinson’s patients, in whom dopamine signaling is impaired, often seem apathetic, and have difficulty in sustaining motivation to complete a long task. “Maybe that’s because they can’t produce this slow ramping dopamine signal.”
Mark W. Howe, Patrick L. Tierney, Stefan G. Sandberg, Paul E. M. Phillips, Ann M. Graybiel. Prolonged dopamine signalling in striatum signals proximity and value of distant rewards. Nature, 2013; DOI: 10.1038/nature12475