For 56-year old Cathy Hutchinson who had been paralyzed by a stroke for fifteen years, taking a sip of coffee unassisted was the highlight of her life. Cathy cannot move her limbs. But with the help of electrodes implanted in her brain, she was able to control an external robotic arm. Cathy's enormous grin was well worth it for the scientists working on ways to help amputees and paralyzed people become independent.
The sensor, about the size of a small pill, eavesdropped on the electrical activity of a few dozen brain cells. When Cathy imagined moving their arms, the chip then sent signals to a computer, which translated them into commands to the robotic arms. The computer was taught how to interpret the brain's signals through practice.
Lets start with how a normal limb movement happens. When you want to lift a book, your brain sends an electrical command down your spinal cord. From here, the message travels through peripheral nerves to muscles that control your hand movement. The muscles contract or relax to perform the function -- in this case grasping a book and lifting it up.
Now imagine what would happen to this information pathway if the limb was amputated. The peripheral nerves would still carry electrical command signals from the brain, but the signals would meet a dead end and never reach the amputated muscles. The same is true for paralyzed patients whose brains are capable of generating electric signals, but the peripheral nerves are damaged.
Triggering the cortex
For the two patients under study, 96 electrodes were implanted in the motor cortex -- an area of the brain that controls voluntary movement. One of the first detailed maps of the human motor cortex was described by Leyton and Sherrington who showed that each point in motor cortex influences a range of muscles and joints. By studying the brains of amputees, it was noticed that a person who had lost an arm would over time apparently lose some of the neuronal mass in the part of the motor cortex that normally controls the arm. Likewise, a person who had lost a leg would show degeneration in the leg part of motor cortex. In this way the motor map could be established.
By reversing the activity in the brain and having the intent to do an action capture into an electric signal, scientists hope to create more of these neuro-assisted devices. The success rate of having the brain interpret the signals in the two patients was around 60%. Currently the sensor attached to the brain communicates with the computer through wires. Researchers are looking into replacing this with a wireless connection. Clearly a lot more work needs to go in to make the technology robust and deliver it at an affordable price, but this is a great first step!