“If you’re paralyzed and can’t speak but your cortex is still okay, the ability to transmit a few words like ‘yes’ or ‘no,’ ‘food’ and ‘water,’ could be very useful,” says Schieber. “But the question is, will we be able to decode all the phonemes of human language from ECoG signals? Can you get enough specific information to distinguish different kinds of grasps, like a pinch versus how you hold a hammer?”
In order to use the information to control a prosthesis or computer, scientists will also need to be able to extract the relevant information in real time. (In the current project, the analysis was done after the neural recording.)
Schalk and others are studying ECoG as a possible alternative to electrodes implanted into brain tissue. Scientists have made rapid progress using the latter as an interface for prosthetic devices, recently showing that monkeys can feed themselves with a robotic arm controlled by a brain-computer interface, and paralyzed patients can move a cursor on a computer screen using similar equipment. 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.