Imagine being trapped in your own body, aware of what's going on around you but unable to move or even speak.
Thanks to a modern technological innovation known as a neural interface — a direct link between the human brain and a computer — there may be hope for sufferers of what's commonly known as "locked-in syndrome."
As portrayed in the 2007 movie "The Diving Bell and the Butterfly," locked-in patients are conscious, but fully paralyzed except for their eyes.
Thanks to advances in life-support technology and rising survival rates following brain-stem strokes, there may now be as many as 50,000 locked-in patients in the United States, the National Institutes of Health estimates.
A major breakthrough for these patients occurred in December 2004, when a neural interface consisting of a few thin electrodes was implanted in the brain of 24-year-old Erik Ramsey, who was "locked in" at age 16 when a car crash triggered a brain-stem stroke.
Dr. Frank Guenther and his team put the device, developed by Neural Signals Inc. near Atlanta, into Ramsey's speech-motor cortex, allowing him a chance to communicate for the first time in nine years.
"By measuring the brain signals in this area while Erik thinks about speaking, we have been able to build a decoder that translates the brain signals into the speech sounds Erik was thinking about," says Guenther, an associate professor of cognitive and neural systems at Boston University.
"The output of the decoder is used to drive a speech synthesizer, which creates sound output."
Signals from the brain are received using three tiny wires. The measurement of these electrical signals is very complex but eventually may be able to decode Ramsey's thoughts and translate them into recognizable speech.
"There are no obvious patterns in the signals," Guenther says. "For example, we don't find neurons that fire only when Erik thinks about 'ee' but not any other sound.
"Instead, all neurons are firing almost all the time, but buried within this electrical activity is information that can be extracted using sophisticated signal-processing techniques."
Guenther's team is relying on its expertise in understanding how neural signals relate to thoughts and actions and to coax out the underlying neurological patterns related to speech.
"So far we have decoded Erik's brain signals 'offline,' meaning that we collect the data one day, then analyze it to create the corresponding speech sounds on another day," Guenther explains. "[Soon], we will connect the synthesizer 'online,' meaning that Erik will be able to hear the sounds immediately as he thinks them."
This will give Erik the chance to practice and to get used to using the synthesizer. Guenther says the process works similarly to how a toddler learns to speak by babbling and talking.
The initial synthesizer will be able to produce only vowels. Erik is expected to learn how to control it within a year.
"We are also developing a more sophisticated synthesizer that will allow him to produce both vowels and consonants. We are hopeful that this synthesizer will allow him to communicate with words within two to three years," Guenther says.
Implants such as this one could also be used to help patients who have lost partial mobility due to spinal-cord damage.
An electrode implanted in the area of the motor cortex that controls one arm could send signals to a receiver mounted on the arm, which would then transmit electrical pulses to the muscles, causing movement.
A slightly different neural interface is also bringing hope to patients with lesser, though still severe, degrees of paralysis.
The BrainGate System, developed by Cyberkinetics Neurology Systems Inc., of Foxboro, Mass., is designed for people with severe motor impairment caused by spinal-cord injury or motor-neuron diseases.
It aims to give them mobility and a direct means of communicating with a computer using only their thoughts.
Its first test subject was Matthew Nagle, a Boston-area man whose spinal cord had been severed at age 21 when he was stabbed during a fight, leaving him a quadriplegic — fully paralyzed from the neck down — though he could still speak.
Soon after receiving his BrainGate implant in the summer of 2004, he was able to move a cursor on a computer screen, enabling him to turn on the TV, check his e-mail and even play Pong just by thinking about it.
"I pretty much had that mastered in four days," he told the New York Times in 2006.
Nagle died at the age of 27 from a full-body infection in July 2007, but the success of his implant led to two currently ongoing clinical trials, the first standardized trials for BrainGate.
One clinical trial is aimed at people with spinal-cord injury, brain-stem stroke or muscular dystrophy. This trial currently has three participants, two with spinal-cord injury and one with brain-stem stroke.
The other trial, for people with Lou Gehrig's disease and other motor-neuron diseases, currently has one participant.
"The goal is to determine the safety and the possible feasibility of using this particular device to allow people who currently do not use their hands to control a cursor on a computer screen simply by thinking about the movement of their own hand," explains Dr. Leigh Hochberg, a neurologist at Massachusetts General Hospital and the principal investigator for the feasibility study of the BrainGate System.
The BrainGate interface is connected to the motor cortex, the part of the brain that controls movement. The system records neural signals via an array of 96 electrodes arranged on a grid 4 mm by 4 mm (about a sixth of an inch) square.
The array is connected by fine wires to a pedestal which protrudes about 1 centimeter (half an inch) above the scalp. During recording sessions, a cable is connected to the pedestal and linked to a computer that records the neural activity.
"What we're hoping through the course of the trial is that as someone thinks about the movement of their own hands, we'll be able to record the neural signals that are changing during that intention to move the hand, and be able to convert those signals into the control of a cursor on a computer screen or other devices," Hochberg says.
Some scientists speculate that implants similar to BrainGate could also restore cognitive functions to victims of Alzheimer's and strokes. Another use might be to enhance the abilities of people in physically demanding occupations, such as rescue workers, deep sea divers and the military, who might become the first true cyborgs.