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Brain Implant Teaches Neurons New Tricks

For release: Monday, February 26, 2007

Using an electronic implant, researchers have altered the connections between cells in the motor control region of the monkey brain, subtly altering the cells’ influence over movement.  Similar devices might one day be used to “rewire” damaged parts of the human brain and restore movement to people paralyzed by traumatic injury or neurological disease.

The implant, called the Neurochip, consists of a battery-powered microprocessor and stimulator that sit on top of the head, connected to electrodes that reach into the brain.  It works by recording electrical signals from neurons at one site in the brain and stimulating another site in lockstep.

Eberhard Fetz, Ph.D., a professor of physiology and biophysics at the University of Washington in Seattle, worked with Andrew Jackson, Ph.D., and graduate student Jaideep Mavoori to develop the Neurochip, which is modeled after a device used to monitor the brain activity of moths during flight.  Dr. Fetz is supported by a Javits Award from the National Institute of Neurological Disorders and Stroke (NINDS), given to just a handful of distinguished researchers each year.  The group’s experiments with the chip were published in Nature.* 

The Neurochip is the latest addition to a family of futuristic devices called brain-computer interfaces (BCIs).  Another type of BCI, developed in part by researchers at NINDS, is designed to bypass damaged areas of the brain and spinal cord by connecting the brain’s high-level motor control center – the motor cortex – to a personal computer.  In some cases, those BCIs have enabled people who are completely paralyzed, and unable to speak, to use their thoughts to spell out words on a computer screen – albeit at slow speeds.

The Neurochip is unique from other BCIs because it has the potential to be “therapeutic, rather than assistive, technology,” said Joseph Pancrazio, Ph.D., a program director in the NINDS Division of Extramural Research.

“[Dr. Fetz’s] work shows that the chip can actually reorganize the motor cortex,” he said.  “It might provide a way to recruit intact neurons to replace the function of damaged ones.”

Dr. Fetz’s experiments involved implanting the Neurochip into the motor cortex of freely behaving monkeys.  During a conditioning period, the chip recorded activity in a part of the cortex that controls the wrist on the opposite side of the body.  For each neural impulse recorded at that site (site A), the chip sent an electrical pulse to another site nearby (site B). 

Before conditioning, the firing of site A neurons caused the wrist to flex, while the firing of site B neurons caused it to extend.  After 2 days of conditioning, both sites were artificially stimulated to see if their effects on wrist movement had changed.  The authors found that movements evoked by stimulating site A neurons had shifted to resemble those evoked from site B.  The change persisted for 1 week.

The change points to the existence of weak connections between neurons at sites A and B that were reinforced by synchronizing their activity through the Neurochip, Dr. Fetz said.  At the same time, he said, the shift in neural connections was subtle enough that there was no obvious change in the monkeys’ voluntary behavior.

The monkeys in Dr. Fetz’s study were healthy, but he’s hopeful that the Neurochip could be used to treat neurological diseases and injuries by fortifying connections between undamaged areas of the brain – in effect, creating a bridge across damaged areas.

This work provides strong support for a theory known as Hebbian reinforcement, which holds that a synapse – a site of contact between two neurons – becomes stronger when the neurons are simultaneously active.  In other words, “neurons that fire together wire together.”  Scores of studies have demonstrated Hebbian reinforcement between neurons given a series of electrical jolts from an artificial source.  This work is the first to show that Hebbian reinforcement can be driven by normal activity in an intact brain.

“Tuning the chip to normal behavior may have been part of the reason it was so efficacious,” Dr. Fetz said, noting that enhancement of synaptic strength in other studies has lasted on the order of hours, rather than days.

“Clinical use [of the Neurochip] at this point is very much conjecture, but these are the kinds of persistent synaptic changes that could support repair of the nervous system in people with traumatic brain injuries, stroke, and other conditions,” Dr. Pancrazio said.

*Jackson A, Mavoori J, Fetz EE.  “Long-Term Motor Cortex Plasticity Induced by an Electronic Neural Implant.”  Nature, November 2, 2006, Vol. 444, pp. 56-60.

-By Daniel Stimson, Ph.D.

Last Modified February 26, 2007