The study involved several sets of research sessions to probe how the motor cortex responds to observed movements. In one series of sessions, the participants watched animations of hand-like effectors as they performed simple tasks like a pinch grip or a fist-like power grip. The effectors varied in how human-like they looked. There was a fully human hand, an anthropomorphic robot hand, a three-pronged robot claw and a cube, which was included as a clearly non-humanlike gripping object. The BCI recorded the participants’ neural activity as they watched each of the animations.

The experiment found that observational neural activity was strongest for the most human-like effectors and decreased gradually as the effectors became less human-like. The pattern held both at the network level — more neurons fired when the activity was more human-like — and at the level of individual neurons, where the firing rate was faster with more human-like stimuli.

“What this suggests is that observational activity isn’t driven by specialized ‘mirror neuron’ cells,” Gusman said. “It’s really a network effect that’s sensitive to how anthropomorphic the observed stimuli are.”

The findings became more nuanced, however, when participants were asked to attempt the grip themselves while watching the effectors. Under those conditions, all of the effectors elicited similar levels of neural activity, regardless of how human-like they looked. That finding, the researchers say, suggests that when a person is actively trying to move, that movement-related signaling in the motor cortex is substantially greater than the observational signals, hinting that an effector's human-likeness may matter less once someone is actually engaged in controlling it.

An “aha” moment

The findings from the effector sessions were broadly echoed in a second set of research sessions involving dot-pattern animations of human hands. High-fidelity dot patterns that were easily recognizable as hands elicited stronger observational responses than sparser, more amorphous renderings. But the dot-pattern sessions also revealed a potential cognitive aspect to observational neural activity.

During an exploratory session using dot-pattern animations, one of the participants recognized a more abstract animation as resembling a human hand before being told that’s what it was, saying, “Oh, I figured it out.” That “aha” moment enabled the researchers to compare neural activity before and after the participant became consciously aware that the more abstract animations were supposed to be human hands. They found that, after that moment, neural activity increased across degrees of human-like appearance, suggesting that cognitive context can influence observational neural activity. 

“At least for this one participant, his recognition that this dot-pattern stimulus resembled a human hand appeared to trigger this hand area of his motor cortex to begin responding while he watched the stimulus,” Gusman said. “At a minimum, this finding suggests that in addition to bottom-up visual input, top-down contextual cues may also be involved in producing this mirror-like response.”

In addition to providing guidance in developing brain-controlled assistive devices, the researchers say the work provides unprecedented insights into how the brain processes observed actions.

“The ultimate goal of the BrainGate clinical trial is to help people who have lost the ability to move or communicate after an injury or illness,” said study co-author Leigh Hochberg, a professor of engineering and neuroscience at Brown who leads the BrainGate clinical trial. “Along the way, we’re making important discoveries about how the brain plans and executes body movements, and this work is an example of the fundamental neuroscience that we learn only because of the engagement of our extraordinary clinical trial participants. These findings will help to inform the next generation of implantable BCIs for people with neurologic injury.”

The research was supported by the American Heart Association (19CSLOI34780000), the Department of Veterans Affairs (N2864C, A2295R, A4820R), the National Institutes of Health (NINDS-UH2NS095548, NIDCD U01DC017844), the Movement Disorder Foundation, the ALS Association and the Cerebral Palsy Alliance Research Foundation. CAUTION: Investigational Device. Limited by Federal Law to Investigational Use.