`Mental agility' research could help paraplegics

Life has gotten a bit easier lately for some Stanford lab monkeys. Instead of earning their reward of fruit juice by using their arms to chase blinking cursors on a computer screen, the primates in electrical engineer Krishna Shenoy's lab merely have to think about approaching their targets.

That's because Shenoy's team is using tiny implanted electrodes to decipher the brain signals that specify where the animals intend to move their arms.

The ability to extract this ``plan activity'' is driving the latest generation of ``neural prosthetics'' -- communication devices that could one day help paraplegics by converting their cognitive signals into electronic commands that move a computer cursor, for instance. A patient could place phone calls using a keypad, type an e-mail message or select menu options such as ``turn on the light'' or ``switch off the TV.''

During the past decade, scientists have shown that such devices work. But for the Stanford group, the bigger question is just how well they can work.

``We're really trying to push the limits of how quickly and accurately someone can communicate using a prosthetic device,'' said Gopal Santhanam, an electrical engineering graduate student in Shenoy's lab. With Stanford neurosurgeon Stephen Ryu, Santhanam presented the team's findings Sunday at the annual meeting of the Society for Neuroscience in San Diego.

The earliest neural prosthetics focused on restoring movement in paralyzed patients by harnessing signals from the brain's motor cortex to maneuver a mechanical arm or leg. But for communication prosthetic systems where the goal matters more than the process of getting there -- selecting the number ``4'' on a keypad, for instance -- scientists realized they didn't need to bother decoding the instructions for continuous movement.

Rather than translate the motor signals for ``move my hand in a particular direction,'' they began to home in on regions of the brain that begin firing before movement begins. Within these so-called planning areas, researchers could tap into the neurons that shout, ``I want to touch that 4 key.''

``It's not representing the motor parameters of the movement but rather the intention of the subject,'' said Richard Andersen, a neuroscientist at the California Institute of Technology, whose work in plan-based prosthetic systems was published in Science magazine in July. Shenoy was a postdoctoral fellow in the Andersen group before starting his own lab at Stanford in 2001.

To develop their prosthetic device, Shenoy's team devised a scheme to get the monkeys to behave like paraplegics -- using their thoughts alone to select a computer cursor occupying one of eight different positions. As a first step, the monkeys were trained to reach for new targets once they appeared on the screen. Every correct touch earned a drop of juice.

``Eavesdropping'' on the brain's planning regions, the researchers were able to identify signature patterns of nerve cell activity corresponding to each of the eight cursor positions. They created algorithms to extract and decode this plan activity during the split second before the monkey prepared to reach for its target.

If the scientists correctly interpreted these plan signals, the animal got a drop of juice and the next cursor would appear. ``Because the monkey is rewarded, he just realizes there's no point in making the reach,'' Santhanam said.

The system can make up to 3.6 selections per second on a computer screen when two possible targets are presented, and it did this with more than 94 percent accuracy, the researchers said.

Acknowledging that it can be difficult to quantify and compare how well these devices work, Shenoy estimates that his team's device can achieve nine to 10 words per minute -- about a threefold increase over previous systems.

``It's a field where an extra word per minute is substantial,'' Shenoy said. ``We're desensitized to it in Silicon Valley, where the doubling of a processor's speed is sort of expected. It's not a given in this field.''

But neural prosthetics have not even begun to approach the complexity of human cognition, others caution. ``This could be a lifesaver for a paraplegic. To be able to move something is wonderful,'' said Jeff Hawkins, author of ``On Intelligence,'' a new book about brain-based machines, and founder of Redwood Neuroscience Institute in Menlo Park. ``But even when you're making four decisions a second, that's tiny compared to the normal capacity of the brain.''

Still, Shenoy approaches the challenge with an engineer's intuition -- likening the brain-to-computer link in neural prostheses to data transmission in telecommunications. ``For a long time, we lived with our 56K modems,'' he said. ``If we're smarter about our signal processing, you can transmit a lot more.''