When you are weighing two possible actions, your brain needs to decide what to do and how to do it.
For example, if a book and a cup sit side-by-side on a table, your brain must decide whether you want to read the novel or drink coffee – selecting the action to take. Your brain also needs to figure out if you need to reach 20 degrees one way for the book or 20 degrees the other way for the cup – specifying the action.
For decades, most neuroscientists thought that one area of the brain, the basal ganglia, was responsible for action selection and another area, the motor cortex, for action specification.
Now, using a novel system developed at Janelia, researchers led by the Dudman Lab found that a part of the basal ganglia called the striatum is not involved in action selection as previously thought. Instead, the striatum and the motor cortex work together to specify the movement parameters to get the action done.
The new work sheds light on the role of striatum in motor control, information that could help researchers better understand movement disorders, like Parkinson's and Huntington's.
"Now we can start thinking: how is the striatum controlling the speed of movement, how do we bring that back online, how do we improve that," says Janelia Senior Group Leader Josh Dudman. "A more accurate, conceptual model for how the striatum is working is ultimately going to be helpful for us to think more clearly about how to restore function."
Design, build, test
The project started a decade ago when Dudman and his team began to see data from the striatum that countered existing models about the role of the brain region.
Their data aligned with other observations about the striatum that questioned its traditional role. For example, the region is connected to the motor system, hinting that it might be involved with the more mechanical aspects of behavior. Further, patients with movement disorders that affect the basal ganglia, like Parkinson's or Huntington's, don't have trouble deciding what they want to do, but they have trouble moving their limbs to do it.
The team set out to test their hypothesis that the striatum may be involved in the more mechanical aspects of flexible, goal-directed action.
To tease apart different possibilities, the researchers designed an experiment where the actions are almost the same – picking up a book involves movement only slightly different from picking up a cup. In this case, the neural activity patterns would be similar for specification but different for selection. This would enable the researchers to distinguish between the two actions.
The researchers worked with Janelia Experimental Technology to design a novel "reach-to-pull" system where a joystick can be in one of two slightly different positions that require nearly similar movements to access.
The researchers simultaneously recorded neural activity in both the striatum and the motor cortex of a mouse as it reached for and then pulled a joystick to get a water reward. They found that neural activity in both brain regions was the same when the animal reached and pulled the joystick at the different locations. These results suggest that both that the striatum and motor cortex are involved in specification, not selection, and that they work together to enable the animal to choose how to carry out the action.
The team's work raises questions about how these two brain areas coordinate to contribute to the same function. Rather than having multiple brain regions carrying out the exact same function – potentially having "too many cooks in the kitchen" – it appears that each region likely contributes subtle variations. The new findings suggest that behavior doesn't result from a single agent but is a consequence of many partial agents working together.
The research also details a novel system for testing hypotheses about the role of different brain regions in flexible, goal-direction action – a project that the researchers say was empowered by Janelia.
"It makes a huge difference that we are in an environment where we can say, I believe we can build, in a timely fashion, a new piece of experimental hardware that we need to execute an experiment we designed," Dudman says. "It's not just speeding it up – it is taking it from above the threshold opportunity cost to below the threshold opportunity cost, and we can really try it."
