You can read this article, and more generally, function in the world, thanks to precise communications between circuits of millions of nerve cells in your brain and spinal cord. Once a mystery, new studies now decipher some of this complex chatter.
“The clinical implications of this type of basic science research are enormous,” says György Buzsáki, MD, PhD, of Rutgers University , an expert on neural circuits. “By understanding the communication process of normal neural circuits, we can determine ways to help those who experience an injury or have a disorder that has damaged their circuits and impaired their function.”
For example, the research will likely aid the development of “thought-controlled prosthetic devices,” which could help amputees or those with paralysis who can't use their limbs. Normally circuits of nerve cells in the brain constantly compute and transmit complex signals through the spinal cord and out peripheral nerves in order to move an arm, hand, leg, or foot. An injury that causes the loss of a limb or damage to the spinal cord can sever this communication line and prevent desired movement. In theory, a thought-controlled prosthetic device, also known as a brain-machine interface, could bypass the loss or damage by directly interpreting nerve cell signals and then launching movement in a prosthetic limb.
The development of these devices requires a two-pronged, parallel approach. “The first is the understanding of the self-organized nature of healthy neural circuits,” says Buzsáki. “Second is the use of this information to control prosthetic devices and feed back information from them to the brain.” Researchers will report the latest advances in device development in the press conference on prosthetic devices, at the Society For Neuroscience annual meeting. This session will focus on the first issue and detail the progress in the basic science fields that help decipher neural circuit communication.
One new analysis that decoded some circuit activity may help simplify the development process of a thought-controlled prosthetic device. “Our findings represent a significant step toward the development of thought-controlled prosthetic devices, such as robotic arms for patients with severe neurological injuries, including those who suffer from complete paralysis,” says study author Miguel Nicolelis, MD, PhD, of Duke University.
In the study, the researchers examined 11 awake patients who were undergoing surgery to treat the brain disorder Parkinson's disease. Before their surgical treatment was performed the researchers used electrodes to simultaneously record signals from groups of nerve cells in two regions of the brain that aid movement control. Nerve cells talk to each other by sending electrical signals. One common way to study this activity is to place tiny wires, or electrodes, in the brain. This allows researchers to “see” the electrical signals from individual neurons.
During the recordings, the patients performed a visual-feedback hand-gripping task. “Remarkably, small groups of 3 to 35 simultaneously recorded nerve cells were sufficiently information-rich to predict gripping force during a 2.5-minute test period with considerable accuracy,” says Nicolelis. “This finding could simplify the development process of thought-controlled prosthetic devices.”
A thought-controlled prosthetic device requires a computer program that can read and decode the complex language of nerve cells in order to set off an intended movement in a robotic limb. “Our study suggests that the computer program may only need to understand the language of just a few hundred randomly selected small groups of cells to drive the robotic limbs,” says Nicolelis.
Another new study from the Nicolelis laboratory at Duke also may help simplify the development of computer programs for thought-controlled prosthetic devices. “We found that certain common components seen in the signals from groups of nerve cells better predict a variety of movement directions than the information from an individual nerve cell,” says Nicolelis.
In the study, researchers implanted signal-recording electrodes into brain areas known to aid reaching movements in two monkeys. Then Duke researchers trained the implanted monkeys to control a cursor on a computer screen using a joystick. Targets appeared on the screen, and the monkeys reached for them with the cursor. In the next series of experiments the monkeys controlled movements of a robot arm using the same joystick. The robot was invisible to the monkeys, but the cursor provided visual feedback of the robot's position.
An analysis determined that common components from the signaling activity of a large population of nerve cells better predicted movement directions than activity from individual nerve cells. “We believe that we can use the information from the large cell populations to make the robot arm perform a variety of movement tasks,” says Nicolelis. “Together the research may help simplify the development of the computer program portion of thought-controlled prosthetic devices.”
Other researchers at the Massachusetts Institute of Technology developed two new techniques that will help scientists better analyze the activity of neural circuits.
“The combination of new electrical and optical techniques will give us unprecedented opportunities to study how circuits in the brain work,” says study author Michale Fee, PhD.
On the electrical side, Fee and his colleagues developed a method that improves the standard use of electrodes to measure the electrical signals between nerve cells. “We have made a tiny device, called a microdrive, which holds up to three electrodes at a precise position in select brain areas,” says Fee. “In addition, tiny motors in the device allow us to move the electrodes around by remote control, find electrical signals from individual nerve cells, and then record their patterns of activity.”
This setup is ideal for examining animals, like songbirds, during their normal behaviors. “We were able to position the electrodes and find nerve cells in the songbird's brain without bothering the bird at all,” says Fee. “Because of this ability, we discovered that a particular group of nerve cells control the precise timing of the bird's songs.”
Another way to study nerve cell activity in the brain is to look at cells under a microscope. When nerve cells signal each other electrically, chemical changes happen inside the cell. For example, electrical activity in a nerve cell causes tiny amounts of calcium ions to flow into the cell, and optical techniques can highlight this change. “There are particular fluorescent dyes that change the amount of light they give off depending on how much calcium is around,” says Fee. “So if we put some of this dye inside a nerve cell, we can see the cell flash as it does its work and signals other nerve cells.” But in the past, this technique couldn't be used for moving animals.
Fee and his colleagues have changed this by developing a special miniature microscope that can be placed on the head of a small animal like a rat. “This allows us to watch changes in fluorescence deep in the brain in an animal that is free to move around,” says Fee. “In the future, we should be able to watch whole groups of nerve cells in the brain flashing their signals to each other as an animal goes about its business.”
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