MicroECoG electrode arrays
Cortex (or Cerebral cortex): The cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It constitutes the outermost layer of the cerebrum. At the surface of the cerebral cortex is distributed the grey matter. Grey matter is formed by neuronal cell bodies, neuropil (dendrites and both unmyelinated axons and myelinated axons), glial cells (astroglia and oligodendrocytes) and capillaries. The white matter below the grey matter of the cortex is formed predominantly by myelinated axons interconnecting different regions of the central nervous system. The human cerebral cortex is 2–4 mm (0.08–0.16 inches) thick.
Electrocorticography (ECoG) is the practice of using electrodes placed directly on the exposed surface of the brain to record electrical activity from the cerebral cortex. In order to access the cortex, a surgeon must first perform a craniotomy, removing a part of the skull to expose the brain surface. This procedure may be performed either under general anesthesia or under local anesthesia if patient interaction is required for functional cortical mapping. Electrodes are then surgically implanted on the surface of the cortex, with placement guided by the results of preoperative EEG (electroencephalography) and magnetic resonance imaging (MRI). Electrodes may either be placed outside the dura mater (epidural) or under the dura mater (subdural). ECoG electrode arrays typically consist of sixteen sterile, disposable stainless steel, carbon tip, platinum, or gold ball electrodes, each mounted on a ball and socket joint for ease in positioning. These electrodes are attached to an overlying frame in a “crown” or “halo” configuration. The electrodes sit lightly on the cortical surface, and are designed with enough flexibility to ensure that normal movements of the brain do not cause injury. Individual electrodes are typically 5 mm in diameter. These brain-penetrating electrode arrays are used to help paralyzed people move a computer cursor, operate a robotic arm and communicate.
New Developments
Now researchers at the University of Utah have developed new microelectrodes that unlike ECoG electrode arrays do not penetrate the brain but sit on the brain and are able to detect brain signals controlling arm movements. The new kind of array is called a microECoG – because it involves tiny or “micro” versions of the much larger electrodes used for electrocorticography, or ECoG.
One of the researchers, assistant professor of bioengineering, Bradley Greger said, “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. 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, if the new array of microelectrodes are placed over the brain’s speech center in patients who cannot communicate because they are paralyzed by spinal injury, stroke, Lou Gehrig’s disease or other disorders, the electrodes would send speech signals to a computer that would convert the thoughts to audible words. In case of people who have lost a limb or are paralyzed, this device should allow a high level of control over a prosthetic limb or computer interface. It will enable amputees or people with severe paralysis to interact with their environment using a prosthetic arm or a computer interface that decodes signals from the brain.
But the researchers feel that it would take at least a few more years before this device can be actually used because more work is needed to refine computer software that interprets brain signals so they can be converted into actions, like moving an arm.
For their study the researchers obtained permission from two epilepsy patients (who were already having conventional ECoG electrodes placed on their brains) to allow them to place the microECoG electrode arrays at the same time (The microelectrodes were formed in grid-like arrays embedded in rubbery clear silicone.). Once they obtained the permission the researchers placed two identical microECoG electrode arrays, each with 16 microelectrodes arranged in a four-by-four square, in patient 1. Individual electrodes were spaced 1 millimeter apart (about one-25th of an inch). Patient 1 had the ECoG and microECoG implants for a few weeks. Months later, the second patient received one array containing about 30 electrodes, each 2 millimeters apart. This patient wore the electrode for several days.
In order to test how well the microelectrodes could detect nerve signals from the brain that control arm movements the two epilepsy patients were made to sat up in their hospital beds and use 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.
The study also showed that the optimal spacing between electrodes is 2 to 3 millimeters.
According to the researchers once a more refined software was developed to decode brain signals detected by microECoG in real-time, it will be tested by asking severe epilepsy patients to control a “virtual reality arm” in a computer using their thoughts.
According to the leader of the research group, University of Utah neurosurgeon Paul A. House, the findings represent “a modest step” toward the use of the new microelectrodes in systems that convert the thoughts of amputees and paralyzed people into signals that control lifelike prosthetic limbs, computers or other devices to assist people with disabilities.
In addition to House and Greger the research team also included Spencer Kellis, a doctoral student in electrical and computer engineering; Kyle Thomson, a doctoral student in bioengineering; and Richard Brown, professor of electrical and computer engineering and dean of the university’s College of Engineering.
Source: http://www.unews.utah.edu/p/?r=062409-1
June 29, 2009