"Lay-On" Electrode Sheets: Neural Prosthetics

Brain electrodes that penetrate cortical tissue can quickly lose function due to scar tissue and "glommed on" bio-debris that collects over time and disrupts the electro-neural connection. If a longer-lasting direct-to-brain connection can be made using electrode sheets that merely "lay on" the surface of the cortex, then the less invasive approach would probably be the way to go.

Schalk and his colleagues studied epilepsy patients undergoing a procedure known as electrocorticography (ECoG), in which a flat array of electrodes is laid over an exposed section of cortex to record electrical activity. Normally, surgeons use this information to pinpoint the source of seizures and to map the location of specific brain functions, which must be avoided during surgery. The technique generates a better spatial resolution than electroencephalography (EEG), a noninvasive approach that records activity through the scalp. ECoG is now being explored for use in brain-computer interfaces. "There's a growing interest in use of ECoG signals because nothing penetrates into the brain, and that appeals to people more than penetrating electrodes," says Marc Schieber, a physician and scientist at the University of Rochester Medical School, who was not involved in the research.

... It's not yet clear that ECoG, which records extracellular electrical activity and thus averages information coming from different cells, will be able to provide the same accuracy as implanted electrodes, which record activity from single cells. "As far as limb control, I think it will be somewhat basic," says Andrew Schwartz, a neuroscientist at the University of Pittsburgh.

However, ECoG possesses some significant advantages. With implanted electrodes, the quality of the recorded signals degrades over time, and the stiff electrodes can sometimes move within the squishy brain, thus requiring recalibration of the system. ECoG devices are less sensitive to movement. And because they lie on the surface of the brain, they may be less susceptible to the immune reaction thought to impair implanted electrodes. "Surface electrodes are more likely to be fit for long-term use," says Schalk.

Miniaturized ECoG devices now under development may make this technology even more appealing. With the current procedure, a surgeon must remove a large piece of skull to insert the electrode array. But Justin Williams, a biological engineer at the University of Wisconsin-Madison, is developing a miniature ECoG device that could be fed through a small hole in the skull and then unfurl to cover a larger area of the cortical surface. Made of platinum wires embedded in a flexible polymer called polyimide, which is frequently used in electronics, the electrode array is flexible and sticks to the wet brain. That means it moves as the brain moves, capturing a better signal. "It acts like Saran wrap on a Jell-O mold," says Williams. _TechnologyReview

The problem is one of signal to noise, and resolution of complex signals. Penetrating electrodes are better than "lay-on" cortical electrode sheets (ECoG), which in turn are better than scalp electrodes (EEG). The ECoG approach when combined with advanced computational filters and "translators" may be the best approach for short-term to mid-term neural prosthesis research--until better penetrating electrodes are devised.

My preference for penetrating electrodes is using the individual's own neural stem cells to grow connections from an interfacing device fixed to the skull--actually functioning as a replacement for a small area of skull--which contains both living neural tissue and the electro-neural interfacing technology. Such a unit could be easily detached from the skull and serviced without involving major surgery.

For more on this general topic, see Brain Stimulant blog