Although babies lack much in the way of motor skills, they can still accurately bring their hands toward their mouth in a motion to feed themselves. Now scientists have discovered this self-feeding movement is encoded in a part of the human brain where many critical movements are hardwired in monkey brains. The findings, detailed in the Proceedings of the National Academy of Sciences, suggest simple hand-to-mouth movements are of high evolutionary significance.
Past research found that in monkeys, reaching, grasping, defensive and hand-to-mouth movements were encoded in the precentral gyrus, the primary motor area of the cerebral cortex. However, there was a dearth of evidence that humans possessed such “motor primitives.”
To help solve this mystery, cognitive neuroscientist Angela Sirigu and her colleagues examined 26 patients ages 2 to 60 undergoing brain surgery for brain tumors and other medical conditions.
A common procedure to minimize the risk of side-effects after brain surgery is to map the brain — direct electrical stimulation of the cortex to identify what regions are responsible for specific motor skills and other brain functions.
“The mapping time during the surgery is limited — around 15 minutes to perform stimulation,” says Sirigu, research director at France’s National Center of Scientific Research’s Center of Cognitive Neuroscience in Lyon. “The primary goal is to map patients’ motor areas to minimize potential deficits after surgery.”
The researchers applied electrical stimulation to 144 sites over the surface of the precentral gyrus. They identified 10 sites triggering both hand and mouth activity. During stimulation of these sites, the mouth gradually started to open while the hand moved toward the face, as if wanting to bring something to the mouth.
The scientists also discovered 10 others sites that both generated motions with the hands and arms while receiving sensory signals from the mouth. These likely help accurately produce coordinated hand-mouth movements.
“In our brain we have prewired motor primitives that allow us to easily perform hand-mouth movements from birth,” Sirigu says.
Currently the researchers are searching for other types of movements encoded in the brain, “such as defensive or manipulative movements,” Sirigu says.