An 'internal guidance system'
"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 Terrence Sejnowski of the Salk Institute, a computational neuroscientist who is familiar with the findings but who was not involved with the study. "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."
One question that Graybiel hopes to examine in future research is how the signal arises within the brain. Rats and other animals form cognitive maps of their spatial environment, with so-called "place cells" that are active when the animal is in a specific location. "As our rats run the maze repeatedly," she says, "we suspect they learn to associate each point in the maze with its distance from the reward that they experienced on previous runs."
As for the relevance of this research to humans, Graybiel says, "I'd be shocked if something similar were not happening in our own brains." It's known that Parkinson's patients, in whom dopamine signaling is impaired, often appear to be 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," Graybiel says.
|Contact: Kimberly Allen|
Massachusetts Institute of Technology