The finding may also explain why patients with Parkinson’s disease often have difficulty in sustaining motivation to finish tasks. Previous studies have linked dopamine to rewards, and have shown that dopamine neurons show brief bursts of activity when animals receive an unexpected reward. (Agencies)
These dopamine signals are believed to be important for reinforcement learning, the process by which an animal learns to perform actions that lead to reward.
The MIT team, led by Institute Professor Ann Graybiel trained rats to navigate a maze to reach a reward. During each trial a rat would hear a tone instructing it to turn either right or left at an intersection to find a chocolate milk reward. From previous work, the researchers expected that they might see pulses of dopamine released at different times in the trial, but in fact they found something much more surprising. The level of dopamine increased steadily throughout each trial, peaking as the animal approached its goal, as if in anticipation of a reward.
The rats' behaviour varied from trial to trial. Some were running faster than others, and some would stop briefly. But the dopamine signal did not vary with running speed or trial duration, nor did it depend on the probability of getting a reward, something that had been suggested by previous studies.
"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," Graybiel said.
The researchers also found that the size of the signal was related to the size of the expected reward. In some trials the T-shaped maze was extended to a more complex shape, requiring animals to run further and to make extra turns before reaching a reward. During these trials, the dopamine signal ramped up more gradually, eventually reaching the same level as in the shorter maze.
"It's as if the animal were adjusting its expectations, knowing that it had further to go," Graybiel said.
"This means that dopamine levels could be used to help animals in making choices on the way to the goal and to estimate the distance to the goal," said Terrence Sejnowski of the Salk Institute.
"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," said Sejnowski.
The finding may also explain why patients with Parkinson’s disease often have difficulty in sustaining motivation to finish tasks. Previous studies have linked dopamine to rewards, and have shown that dopamine neurons show brief bursts of activity when animals receive an unexpected reward.