Brain Mechanism That Pauses Before Stimulus Response Revealed

The brain’s motor cortex, which helps control movement, registers the sensation of jostling almost immediately, but it pauses before ordering the muscles to react. Understanding why and how this occurs is critical to designing a system to allow a person with paralysis to use his or her brain to control a prosthetic arm. Neuroengineers worried that when the brain is directly connected to a prosthesis, its earliest responses to perturbations would cause the arm to move erratically every time the user encountered something unexpected. Now a team led by electrical engineer and neuroscientist Krishna Shenoy, PhD, director of Stanford University’s Neural Prosthetic Systems Lab, said it has identified the mechanism behind this brief pause between stimulus and response. The study describing the work was published June 15 in the journal Neuron. 

“The brain has a mechanism to keep us from prematurely reacting when we are jostled. Now that we understand it, we can design an electronic interface between the motor cortex and a prosthetic arm that works as nature intended,” said Shenoy, who is also the Hong Seh and Vivian W. M. Lim professor of engineering in the Departments of Electrical Engineering and, by courtesy, Bioengineering and Neurobiology.

The current findings build on Shenoy’s long-term collaboration with Stanford neurosurgeon Jaimie Henderson, MD. They are conducting clinical trials of a technology that allows people with paralysis to use a brain-computer interface (BCI) to type commands onto a virtual keyboard displayed on a computer screen. Electrical leads on the BCI, a tiny, implanted silicon chip, pick up signals from neurons in the motor cortex that reveal the person’s intended movement.

Developing a brain-controlled prosthetic limb is more complicated, but some of the technology developed for the thought-controlled keypad aids in the prosthetic arm challenge. In the preclinical experiments described in Neuron, the Stanford researchers used the BCI to reverse-engineer the mechanism that enables the motor cortex to pause between stimulus—like when the arm is jostled—and response. The experiments confirmed that during this delay, the motor cortex, while recognizing the need to react, briefly suppresses that urge while deciding what orders to send to the muscles.

“It’s like having a scratch pad where you can first prepare a rough draft that no one else will see,” said Sergey Stavisky, PhD, a postdoctoral fellow in the Department of Neurosurgery and first author of the paper.

This understanding is crucial to developing brain-controlled prosthetic arms. Shenoy said other researchers have already developed a prosthetic arm controlled by the brain, but they have not yet seen what happens when it encounters an unexpected perturbation.

“We knew that if we couldn’t separate out the different neural patterns, we’d have trouble designing a brain-controlled prosthetic that works like a biological arm,” Shenoy said.

Shenoy said Stanford researchers want to design brain-controlled prosthetic limbs that don’t overcompensate or overreact. The knowledge gained from this work will feed into early-stage efforts to develop human clinical trials of BCI-controlled prosthetic arms.

Editor’s note: This story was adapted from materials written by Glen Martin and provided by Stanford.