WASHINGTON U.-ST. LOUIS (US) — Although not yet a reality, researchers are developing brain implants designed to communicate with prosthetics—with the goal of turning thoughts into movement.
A team of biomedical engineering is testing a multi-channel grid of disk-like electrodes implanted just under a macaque’s skull. They hope the implant will allow them to train the monkey to control—strictly by thinking about it—a computational model of a macaque arm.
A typical primate arm uses 38 independent muscles to control the positions of the shoulder and elbow joints, the forearm and the wrist. To fully control the arm, a brain-computer interface (BCI) system would need 38 independent control channels.
Daniel Moran, associate professor of biomedical engineering and neurobiology at Washington University in St. Louis, and colleagues recently completed experiments to define the minimum spacing between the EECoG electrodes that preserves the independence of control channels.
EECoG, or epidural electrocorticography, is a type of BCI that uses grids of disk-like electrodes placed inside the skull but outside the dura mater, a membrane that covers and protects the brain.
Together with Justin Williams at the University of Wisconsin, Moran has built a 32-channel EECoG grid small enough to fit within the boundaries of the sensorimotor cortex of the brain.
Model monkey arm
Moran’s biomechanical model of a monkey arm, described in the Journal of Neural Engineering, includes 38 musculotendon units. Given an arm position, it will calculate the joint angles, joint torques, musculotendon lengths, and muscle forces needed to place the arm in that position.
The arm has seven degrees of freedom, including rotation about the shoulder joint, flexion and extension of the elbow, pronation and supination of the lower forelimb, and flexion, extension, abduction, and adduction of the wrist.
The monkey, unharmed, will be persuaded by a virtual reality simulator to treat the virtual arm as though it were its own.
The monkey will be asked to trace with its virtual hand three circles that intersect in space at 45 degrees to one another, like interlocked embroidery hoops. Because this task separates degrees of freedom, it will allow scientists to map cortical activity to details of movement, such as joint angular velocity or hand velocity.
Should this experiment be successful, Moran would like eventually to connect his EECoG BCI to a new peripheral nerve-stimulating electrode he is developing with doctoral student Matthew MacEwan. By connecting these two devices they will create a neuroprosthetic arm—a paralyzed arm that can move again because the mind is sending signals to peripheral nerves that stimulate muscles to expand or contract.
Neuroprosthestics like the one Moran and colleagues are designing may one day help people suffering from spinal cord injury, brainstem stroke, or amyotrophic lateral sclerosis, which paralyzes the body while leaving the mind intact.
Unlike similar BCIs known as ECoGs, which lie directly on the brain, Moran’s grid rests on the dura mater, the outermost of the three membranes surrounding and protecting the brain and spinal cord.
In 2009, Moran published the first studies of epidural electrocorticography. Recording sites over the motor cortex of monkeys were arbitrarily assigned to control a cursor’s motion in the horizontal and vertical directions as the monkey traced circles on a computer screen.
In the latest set of experiments, Moran sought to define the minimal separation between electrodes that preserved independence of control. Once a monkey gained control of the cursor, the initial electrodes were abandoned and control was given to two electrodes that were closer together. The next week, the control electrodes were closer still.
Moran found that the electrodes, which were initially a centimeter apart, maintained their independence until they were only a few millimeters apart.
“So now that we know how many electrodes we can pack into an area, we have some idea how many degrees of freedom we’ll be able to control,” he says.
Together with Williams, he designed the 32-channel EECoG supported on a sheet of plastic thinner than Saran Wrap that sucks down to the dura and sticks like glue.
“Eventually,” he says, “we’ll have a little piece of Saran Wrap with telemetry. We’ll drill a small hole in the skull, pop the bone out, drop the device in, replace the bone, sew up the scalp, and you’ll have what amounts to Bluetooth in your head that translates your thoughts into actions.”
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