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A serious drawback of devices that require implantation of electrodes on or in the brain is that they carry risks. Too much electricity sent into the brain can cause seizures, and removal of skull tissue to provide access is invasive and painful and brings danger of infection.

Not surprisingly, the images provided through all of these approaches so far are crude, allowing the wearer to distinguish little more than shapes or contrasts between two objects. For example, in the case of the artificial retina, it is difficult to generate high-resolution images using 16 electrodes to substitute for electrical pulses coming from more than one million rods and cones. This disparity should improve somewhat; Drs. De Juan and Humayun think they can soon implant a device with 60-100 electrodes, and their ultimate goal is a grid with 1,024 electrodes.

For the systems that attach directly to the visual cortex, the number of electrodes is also low compared with the number of brain cells involved in processing visual information. Even when the system is working, the patient can see only points of light, not colors or shades. They also seem to be unpredictable -- for reasons not clear, the impulses sent by the camera often fail to trigger a phosphene.

The relatively low number of electrodes is not the only hurdle. No one fully understands how to faithfully reproduce the digital signal from the retina to the optic nerve. The information is encoded in a bewildering pattern of delicately timed impulses. In addition, the brain many times has to be "retrained" to interpret visual information, especially if someone has been sightless for long periods.

Thought Into Action
To many researchers, the most exciting application of a mind-machine interface would involve tapping the brain for signals that trigger responses outside the body. The field of brain-machine interface (BMI) systems, as they are sometimes called, proposes to make it possible for thought to equal action.

In humans, rudimentary BMI systems have relied on monitoring a person's brain activity via electroencephalograph (EEG). Electrodes placed on the scalp read and amplify the activity before transmitting it to a computer. By thinking different types of thoughts, patients learn to exert control over a cursor on a video screen. In most cases, this simply moves the cursor in one of two directions. However, this binary code can communicate important thoughts. After the most obvious answers of "yes" or "no," a system can be set up to allow a person to slowly choose letters and thus spell words and make sentences. This research is still in its infancy, but its successful application could allow even those people who are completely paralyzed (a condition known as "locked in") to communicate with the outside world.

Aspirations for BMI, however, go far beyond communicating thoughts and wishes. The Holy Grail is finding ways for the brain to command the movement of other objects such as artificial limbs. In the mid-1990s, researchers at Duke University clarified how the brain sends out signals directing muscles to move. Miguel Nicolelis, one of the Duke researchers who made the discovery, has already proven that harnessing these signals can link thought and action.

Nicolelis and his colleagues monitored the brain activity of a monkey as it manipulated a joystick. The brain signals were picked up by hundreds of electrodes buried in the animal's scalp and connected to a cap on its head. Many repetitions of movement generated enough data for the researchers to recreate a "language" and thus enable the monkey to communicate with and control a robotic arm through its brain's neural signals. The animal was even able to control an artificial arm over the Internet 600 miles away. Further research at Duke and other institutions is exploring ways to allow the brain to achieve more delicate control of artificial limbs and also receive sensory feedback simultaneously about what the limbs are touching. Ultimately, when these motor and sensory systems are combined, someone commanding an artificial arm to pick up a glass of water would be able to "feel" where the glass was and control how hard to squeeze it.

In 2003, the BMI field received a tremendous boost from the U.S. military -- more specifically the Defense Advanced Research Projects Agency (DARPA), which manages research for the Department of Defense and specializes in funding explorations of high-risk, high-payoff technologies. In 2003, DARPA invested $24 million in BMI programs, split among six different laboratories, including the one at Duke.

A major challenge for BMI research is improving upon the weak, blurry signals provided by EEGs. One nascent technology that might solve this problem is a new kind of brain imaging known as magnetoencephalography (MEG), which measures the magnetic fields created by nerve cells as they produce the small electrical currents used for neurotransmission. MEG provides much better scanning speed and resolution and does not require physical contact to record signals. Current MEG scanners are massive and so sensitive they must be surrounded by shielded walls to prevent readings from being compromised by laboratory machinery or even nearby traffic. But DARPA is funding research into shrinking the scanners' size, with the ultimate goal being a device small enough to fit inside a helmet.

Ethical Questions
At some point, the public needs to carefully consider when it is acceptable to implant invasive devices in humans. Scientists are not even close to understanding all processes of the brain, still the most complex computer known. Such a delicate organ should not be tampered with lightly. In addition, conditions such as blindness, while limiting, are not life-threatening. Is it ethical to "cure" something that is not a disease? Cochlear implants have already been the subject of great controversy by advocates disputing the notion that deafness is something that needs to be "fixed." It will take decades for these issues to be settled, but the first, tentative steps toward fusing mind and machine have been taken.

Jim Stallard is a New York-based science and humor writer who has been published in SCIENCE Online, MCSWEENEY'S, MODERN HUMORIST, SWEET FANCY MOSES, and MIGHT magazine.

artificial leg
Erik Ax rides a stationary bike with his osseointegrated prosthesis.
The cochlear implant is already helping the hearing-impaired. Today, more than 20,000 people rely on cochlear implants to hear.

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