Caltech's Revolutionary Ultrasound Brain-Machine Interface: Advancing Mind Control Techniques

Recent developments in Brain-Machine Interfaces feature functional ultrasound (fUS), an unobtrusive method for studying brain activity. This new technique reveals potential in directing electronic devices with negligible delay and does not necessitate frequent recalibration. Credit is due to SciTechDaily.com

A considerable breakthrough in Brain-Machine Interface technology, functional ultrasound (fUS) provides a less intrusive tactic for the accurate control of digital devices by deciphering brain activity.

Brain–machine interfaces (BMIs) are apparatuses capable of assimilating brain activity and converting this activity to govern an electronic device like a computer cursor or an artificial arm. BMIs offer a means for individuals with paralysis to command prosthetic instruments using their thoughts.

Majority of BMIs call for invasive surgical procedures to embed electrodes into the brain to analyse neural activity. However, in the year 2021, research team from Caltech invented an approach to examine brain activity utilizing functional ultrasound (fUS), a significantly less intrusive method.

Recently, an evidence-of-concept study illustrated that fUS technology can serve as the basis for an "online" BMI—one that discerns brain activity, deciphers its implication with programmed decoders using machine learning and as a result, manipulates a computer that is capable of predicting action with very minor time delay.

Ultrasound is exploited to take two-dimensional pictures of the brain, which can then be compiled together to produce a 3-D image. Credit: Courtesy of W. Griggs

This investigative study was performed in the Caltech laboratories of two senior researchers; Richard Andersen, James G. Boswell Professor of Neuroscience and leadership chair of the T&C Chen Brain-Machine Interface Center, and Mikhail Shapiro, Max Delbrück Professor of Chemical Engineering and Medical Engineering. It also involved a joint contribution with the lab of Mickael Tanter, director of physics for medicine at INSERM in Paris, France.

Andersen expresses, “Functional ultrasound introduces an entirely new modality to enhance the toolkit of brain–machine interfaces that can provide assistance to people with paralysis. It presents appealing alternatives of being less intrusive compared to brain implants and does not demand continuous recalibration. The development of this technology was achieved through a truly collaborative endeavor that could not be realized by one lab on its own.”

Sumner Norman, former senior postdoctoral scholar research associate at Caltech and a co-author of the study, discusses that, “Overall, all tools for gauging brain activity have merits and demerits. Whereas electrodes can measure the activity of separate neurons with absolute precision, these must be implanted within the brain itself and scaling to cover more than a few minor brain regions is challenging. Non-intrusive tactics also bear tradeoffs. Functional magnetic resonance imaging [fMRI] offers access to the whole brain but is constrained by decreased sensitivity and resolution. Portable techniques, such as electroencephalography [EEG] are limited by inadequate signal quality and incapability to localize deep brain function.”

The vascular framework of the posterior parietal cortex as determined by functional ultrasound neuroimaging. Credit: Courtesy of W. Griggs

Ultrasound imaging functions by launching pulses of high-frequency sound and calculating how these sound vibrations reverberate all over a substance, for instance, varied tissues of the human body. These sound waves sail at dissimilar speeds across different tissue types and rebound at the borders linking them. This technique is regularly employed for diagnostic imaging and for taking images of a fetus in the womb.

Given that the skull itself does not allow sound waves to permeate, employing ultrasound for brain imaging necessitates the insertion of a transparent “window” into the skull. Whitney Griggs (PhD ’23), a co-author of the study, notes, “Crucially, ultrasound technology does not need to be implanted into the brain itself. This significantly lessens the probability of infection and keeps the brain tissue and its safeguarding dura perfectly unaffected.”

“As neurons’ activity changes, so does their use of metabolic resources like oxygen,” says Norman. “Those resources are resupplied through the blood stream, which is the key to functional ultrasound.” In this study, the researchers used ultrasound to measure changes in blood flow to specific brain regions. In the same way that the sound of an ambulance siren changes in pitch as it moves closer and then farther away from you, red blood cells will increase the pitch of the reflected ultrasound waves as they approach the source and decrease the pitch as they flow away. Measuring this Doppler-effect phenomenon allowed the researchers to record tiny changes in the brain’s blood flow down to spatial regions just 100 micrometers wide, about the width of a human hair. This enabled them to simultaneously measure the activity of tiny neural populations, some as small as just 60 neurons, widely throughout the brain.

Unlocking Movement: Helping Paralyzed People Use Thought to Control Computers and Robotic Limbs

The researchers used functional ultrasound to measure brain activity from the posterior parietal cortex (PPC) of non-human primates, a region that governs the planning of movements and contributes to their execution. The region has been studied by the Andersen lab for decades using other techniques.

The animals were taught two tasks, requiring them to either plan to move their hand to direct a cursor on a screen, or plan to move their eyes to look at a specific part of the screen. They only needed to think about performing the task, not actually move their eyes or hands, as the BMI read the planning activity in their PPC.

“I remember how impressive it was when this kind of predictive decoding worked with electrodes two decades ago, and it’s amazing now to see it work with a much less invasive method like ultrasound,” says Shapiro.

The ultrasound data was sent in real-time to a decoder (previously trained to decode the meaning of that data using machine learning), and subsequently generated control signals to move a cursor to where the animal intended it to go. The BMI was able to successfully do this to eight radial targets with mean errors of less than 40 degrees.

“It’s significant that the technique does not require the BMI to be recalibrated each day, unlike other BMIs,” says Griggs. “As an analogy, imagine needing to recalibrate your computer mouse for up to 15 minutes each day before use.”

Next, the team plans to study how BMIs based on ultrasound technology perform in humans, and to further develop the fUS technology to enable three-dimensional imaging for improved accuracy.

The paper is titled “Decoding motor plans using a closed-loop ultrasonic brain–machine interface” and was published in the journal Nature Neuroscience on November 30.