Brains plan for action, not limbs

The brain isn’t going out on a limb when choosing to grasp things.

Instead, new research shows, there is an overarching system where the brain uses action-specific regions to respond to a choice of action, such as reaching or grasping, regardless of the limb used to perform that action.

In a study described in the journal PNAS, a team led by Yuqi Liu of Georgetown University Medical Centre, US, found that reaching for an object with your feet engages the same part of your brain as reaching with your hand.

As well as advancing the basic science of brain organisation and function, the finding, they suggest, offers potential future clinical advances, including improving limb prostheses.

The choice of reaching or grabbing is controlled by different neural pathways, but, surprisingly, the researchers found that the brain areas controlling these pathways showed no preference for limbs.

Instead, the reaching part of the brain plans the action independently of which limb the central nervous system engages.

This means the brain organises itself to first make choices based on the task, not necessarily the limbs available. The action part of the brain follows the logic “I want to grab something, but I don’t care what limb does the grabbing”.  

This is considered a higher level of function than when the central nervous system moves an available limb. As such, this could be a new place to link prosthetics to, so action commands can come directly from the brain.

“In the long run, if we better understand action representations at a higher command level, maybe we can guide prostheses not only at motion of the hand, but also at a higher action aim level, which could lead to lower rejection of prostheses,” says corresponding author Ella Striem-Amita.

The team mapped the brain activity of people with upper limb dysplasia – born without hands or arms or with shortened limbs – to get a picture of which parts of the brain were used when they reached for or grabbed an object with their foot.

They compared this activity to a control group of people who reached with their hands and found that the same part of the brain lit up on an MRI when reaching with either an arm or a leg. This also happened with grabbing an object with a hand or foot. However, the action of reaching or grabbing lit up separate parts of the brain.

“This is more abstract than the way the brain controls fine-tuned flexion of a specific body part, such as a limb,” says Liu.

“Fine control of an arm, for example, corresponds to one area in the central nervous system and control of a leg corresponds to a different area. This is point-to-point organisation.”