Neuralink is developing a fully-implanted, wireless, high-channel count brain-machine interface (BMI) with the goal of enabling people with paralysis to directly use their neural activity to operate computers and mobile devices with speed and ease.
In a 2019 white paper, Neuralink outlined the design of their novel electrodes and unique surgical approach, along with preliminary electrophysiology obtained in a rodent model. That generation of the Link had wired leads and a connector that protruded through the skin, and was an important platform for developing and validating Neuralink's robotic surgical approach and their ultra low-power custom application-specific integrated circuits (ASICs) for amplifying and processing neural signals. In 2020, Neuralink publicly shared a wireless version of the Link that was able to stream 1,024 channels of action potentials (also called "spikes") wirelessly and in real time. Its functionality was demonstrated by recording somatosensory (touch) signals in pigs exploring their environment. The electrodes were placed in a part of the brain involved in processing signals from the pig's exquisitely sensitive snout. As it snuffled about, the responses of the neurons to sensory cues could be readily observed. For further information see the IDTechEx report on Invasive and Non-Invasive Neural Interfaces: Forecasts and Applications 2018-2028.
This demonstration was a step towards the team's vision of providing direct neural control of a computer cursor to people with paralysis. Swine will continue to be an important animal to validate the safety of the Link. However, to develop and advance the functionality of the Link, it is necessary to employ an animal model whose brain similarity (homology) and behavioral abilities enable the development of a hand and arm-based motor cortical BMI. The rhesus macaque model allows the team design, validate, and advance the performance and robustness of a complete "closed-loop" motor BMI system intended to improve the quality of life of people with neurological disorders.
Wireless, Fully Implanted BMI
Neuralink have now revealed the Link's capability to enable a macaque monkey, named Pager, to move a cursor on a computer screen with neural activity using a 1,024 electrode fully-implanted neural recording and data transmission device, termed the N1 Link. The Link has been implanted in the hand and arm areas of the motor cortex, a part of the brain that is involved in planning and executing movements. Links were placed bilaterally: one in the left motor cortex (which controls movements of the right side of the body) and another in the right motor cortex (which controls the left side of the body).
Neurons in somatosensory cortex respond to touch, and neurons in the visual cortex respond to visual cues. Analogously, neurons in motor cortex modulate their activity prior to and during movement, and are thought to be involved in planning, initiating and controlling voluntary movements. Many neurons in motor cortex are directionally tuned, that is, more active for particular movement directions than others. Different neurons are tuned to different movement directions.
By modeling the relationship between different patterns of neural activity and intended movement directions, a model can be built (i.e., "calibrate a decoder") that can predict the direction and speed of an upcoming or intended movement. Researchers can go further than simply predicting the most likely intended movement given the current pattern of brain activity: they can use these predictions to control, in real time, the movements of a computer cursor, or a MindPong paddle. The neurons with upward preferred directions clearly increase their firing rates as the monkey moves his MindPong paddle upward, and the ones with downward preferred directions increase their firing rates as Pager moves his paddle downward.
Decoding Neural Signals
The Link amplifies and digitizes the voltage recorded from each of its 1024 electrodes. These tiny voltage traces contain signatures of the activity of nearby neurons (called action potentials or "spikes"). Custom algorithms running aboard the Link automatically detect spikes on each electrode, which are then aggregated into vectors of spike counts (1 count every 25 ms x 1024 channels). Every 25 milliseconds, the Link transmits these spike counts over bluetooth to a computer running custom decoding software. First, this software re-aggregates the spike counts at several timescales, from the most recent 25 ms to the past 250 ms, to account for differing temporal properties in the activity of the motor neurons. Next, the weighted sum of these current and recent spike counts are computed for each dimension of control by passing their firing rates through a decoding model. The output of the decoder is a set of velocity signals for each 25 ms bin, which are integrated over time to direct the movement of a cursor (or MindPong paddle) on a computer screen.
On the Horizon
With monkeys, the team calibrates the decoder by mapping neural activity patterns to actual (joystick) movements. However, they won't be able to use such a strategy for people with paralysis. Prior research by the BrainGate consortium have shown that neurons in the motor cortex remain directionally tuned to movement intention even in people with paralysis, and that it is possible to calibrate a decoder as the person simply imagines moving a mouse on a mousepad or a finger on a trackpad to guide a cursor that automatically moves to presented targets. After the decoder is calibrated, the person is able to type emails and text messages, browse the web, or anything else that can be done with a computer, just by thinking about how they want the cursor to move.
Neuralink's technology builds on decades of research. The BMI systems used in previous studies have no more than a few hundred electrodes, with connectors that pass through the skin, requiring a technician or caregiver to "connect" the BMI. Neuralink's mission is to build a safe and effective clinical BMI system that is wireless and fully implantable that users can operate by themselves and take anywhere they go; to scale up the number of electrodes for better robustness and higher information throughput; and to automate the implant surgery to make it as rapid and safe as possible. Recent engineering advances in the field and new technologies developed at Neuralink are paving the way for progress on each of these key technical hurdles.
Neuralink's first goal is to give people with paralysis their digital freedom back: to communicate more easily via text, to follow their curiosity on the web, to express their creativity through photography and art, and, yes, to play video games. After that, it is intended to use the Link to help improve the lives of those with neurological disorders and disabilities in other ways. For example, for people with paralysis the Link could also potentially be used to restore physical mobility. To achieve this, Neuralink would use the Link to read signals in the brain and use them to stimulate nerves and muscles in the body, thereby allowing the person to once again control their own limbs.
MindPong is an initial demonstration of the potential capabilities of the N1 Link. However, it's important to remember that it is a small slice of what the device is intended to achieve.