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There has been a real rash of improvements to neuroprosthetic systems just recently, and the micro ECoG system is no exception. ECoG, or electrocorticography is a method of brain-computer interaction, in which a mesh of electrodes is placed like a veil, directly over the outside of the brain itself.

Traditionally that has always been the exact same type of interface as with EEG - a mesh grid of metal electrodes each two to four millimetres in diameter and encased in a protective shell, spaced 10mm apart, covering the surface of the brain. Its invasive, and occupies a lot of space, but it's the best we've had.

Now a new ECoG system has been developed at the University of Utah, which shrinks ECoG arrays by several orders of magnitude, actually increasing signal fidelity and making permanent emplacements much more likely.

Above, you can see two photoshop-enhanced photographs of a live human brain, a surgical epilepsy patient who agreed to have an ECoG array placed in their brain alongside corrective surgery. The skull has been cut and a section removed, allowing the ECoG electrode placement. The large metallic discs are 4m in diameter each, and are individual numbered electrodes for the ECoG array.

On the left image, two circles have been drawn showing 16 electrode points in each. The array is actually there, but consists of clear wires in a clear silicone disk, so the black is for emphasis. As you can see, each array is barely larger than a single ECoG electrode, and allows for far greater signal penetration than normal. The upper array feeds into the orange wire, whilst the lower feeds into the green wire. These wires then connect up with the standard ECoG data signalling system before exiting the brain.

On the right, is another microECoG array. This time, 32 individual microelectrodes occupy a space filled with four normal electrodes. Also covered in a silicone disk just half a milimetre thick, the microelectrodes press much closer to the brain, taking up far less space inside the skull cavity in addition to heightened sensitivity. Whilst the metal discs are not really suitable for long-term implantation, these silicone arrays are.

In addition, these new microelectrodes sit on the surface of the brain, under the silicone disc which is surgically glued to the brain. The electrodes are not penetrating the grey matter, yet the researchers found they were still capable of increased signal fidelity over those that do poke just under the surface. This is in part due to the utter lack of scar tissue formed without penetration.

"The unique thing about this technology is that it provides lots of information out of the brain without having to put the electrodes into the brain," said Bradley Greger, an assistant professor of bioengineering and co-author of the study. "That lets neurosurgeons put this device under the skull but over brain areas where it would be risky to place penetrating electrodes: areas that control speech, memory and other cognitive functions."

For example, the new array of microelectrodes someday might be placed over the brain's speech centre in patients who cannot communicate because they are paralysed by spinal injury, stroke, Lou Gehrig's disease or other disorders, he adds. The electrodes would send speech signals to a computer that would covert the thoughts to audible words.

"If you're going to have your skull opened up, would you like something put in that is going to last three years or 10 years?" Greger asked.

"No one has proven that this technology will last longer," House says. "But we are very optimistic that by being less invasive, it certainly should last longer and provide a more durable interface with the brain."

The regular-size ECoG electrodes are too large to detect many of the discrete nerve impulses controlling the arms or other body movements. So the researchers designed and tested microECoGs in two severe epilepsy patients who already were undergoing craniotomies.

The epilepsy patients were having conventional ECoG electrodes placed on their brains anyway, so they allowed House to place the microECoG electrode arrays at the same time because "they were brave enough and kind enough to help us develop the technology for people who are paralyzed or have amputations," Greger says.

The researchers tested how well the microelectrodes could detect nerve signals from the brain that control arm movements. The two epilepsy patients sat up in their hospital beds and used one arm to move a wireless computer "mouse" over a high-quality electronic draftsman's tablet in front of them. The patients were told to reach their arm to one of two targets: one was forward to the left and the other was forward to the right.

The patients' arm movements were recorded on the tablet and fed into a computer, which also analyzed the signals coming from the microelectrodes placed on the area of each patient's brain controlling arm and hand movement.

The study showed that the microECoG electrodes could be used to distinguish brain signals ordering the arm to reach to the right or left, based on differences such as the power or amplitude of the brain waves.

Once the researchers develop more refined software to decode brain signals detected by microECoG in real-time, it will be tested by asking severe epilepsy patients to control a VR arm using their thoughts.


Dictionary: ECoG

Reading the brain without poking it

Human neocortical electrical activity recorded on nonpenetrating microwire arrays: applicability for neuroprostheses

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