“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, we outlined the design of our novel electrodes and our 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 our robotic surgical approach and our ultra low-power custom application-specific integrated circuits (ASICs) for amplifying and processing neural signals. In 2020, we 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 (Fig. 1). We demonstrated its functionality 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.
This demonstration was a small but important step towards our 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 for us 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 us to 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
Today we are pleased to reveal 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. We have implanted the Link in the hand and arm areas of the motor cortex, a part of the brain that is involved in planning and executing movements. We placed Links 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. An example of this directional modulation can be seen in the raster plot to the right (Fig. 2).
By modeling the relationship between different patterns of neural activity and intended movement directions, we can build a model (i.e., “calibrate a decoder”) that can predict the direction and speed of an upcoming or intended movement. We can go further than simply predicting the most likely intended movement given the current pattern of brain activity: we can use these predictions to control, in real time, the movements of a computer cursor, or in the video below, 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.
The video below shows the spatial pattern of directional tuning on each of the electrodes in Pager’s implant while playing a 2D target acquisition game.”