Neurotrophic electrode
[2] Victims of locked-in syndrome are cognitively intact and aware of their surroundings, but cannot move or communicate due to near complete paralysis of voluntary muscles.
In early attempts to return some degree of control to these patients, researchers used cortical signals obtained with electroencephalography (EEG) to drive a mouse cursor.
[3] Patients with other motor diseases, such as amyotrophic lateral sclerosis and cerebral palsy, as well as those who have suffered a severe stroke or spinal cord injury, also can benefit from implanted electrodes.
Cortical signals can be used to control robotic limbs, so as the technology improves and the risks of the procedure are reduced, direct interfacing may even provide assistance for amputees.
Initial studies with Rhesus monkeys and rats demonstrated that the neurotrophic electrode was capable of chronic implantation for as long as 14 months (human trials would later establish even greater robustness).
[1] The glass cone is only 1–2 mm long, and is filled with trophic factors in order to encourage axons and dendrites to grow through its tip and hollow body.
[6] The cone sits with its tip near layer five of the cortex, among corticospinal tract cell bodies, and is inserted at an angle of 45° from the surface, about 5 or 6 mm deep.
The antenna, amplifiers, analog switches, and FM transmitters are all contained in a standard surface mount printed circuit board that sits just under the scalp.
The gold wires are cut to the correct length, coiled, and then bent to an angle of 45° just above the point of contact with the cone in order to limit the implantation depth.
The wires are then placed on the shelf and a methyl methacrylate gel glue is applied in several coats, with care taken to avoid covering the conductive tips.
Researchers implanted the electrode in the area of the motor cortex associated with the movement of speech articulators because a pre-surgery fMRI scan indicated high activity there during a picture naming task.
Recent data from a locked-in person implanted for 13 years clearly show no scarring and many myelinated neurafilaments (axons)[12] [9] So the longevity question has been answered for the Neurotrophic Electrode.
The Neurotrophic Electrode was limited in the amount of information it could provide, however, because the electronics it used to transmit its signal required so much space on the scalp only four could fit on a human skull.
Electroencephalography (EEG) involves the placement of many surface electrodes on the patient's scalp, in an attempt to record the summed activity of tens of thousands to millions of neurons.
The temporal and spatial resolutions and signal to noise ratios of EEG have always lagged behind those of comparable intracortical devices, but it has the advantage of not requiring surgery.
[13] Electrocorticography (ECoG) records the cumulative activity of hundreds to thousands of neurons with a sheet of electrodes placed directly on the surface of the brain.
In addition to requiring surgery and having low resolution, the ECoG device is wired, meaning the scalp cannot be completely closed, increasing the risk of infection.
He experienced a short lived episode of focal motor seizures and brain swelling leading to temporary weakness on the contralateral side of the body.
[16] When Johnny Ray was implanted in 1998, one of the neurotrophic electrodes started providing an intermittent signal after it had become anchored in the neuropil, and as a result, Dr. Kennedy was forced to rely on the remaining devices.
