Microelectrodes allow recording the activity of individual neurons in the living brain. Thanks to recent developments in engineering, microelectrode arrays are becoming available for use in humans. They can give access to over a hundred individual neurons in the human cerebral neocortex.
So far, microelectrode arrays have been used in humans in two circumstances. As chronic implants to control brain-computer interfaces, and as temporary implants to investigate the neural underpinnings of epilepsy in patients undergoing assessment with intracranial EEG. Microelectrode arrays are invasive devices, and their implantation requires opening the skull and meninges, and inserting the array into the cortex.
Therefore, the implantation of microelectrode arrays in humans must be performed by a neurosurgeon. In addition to expertise with standard neurosurgical tools and techniques, the surgeon should be trained in anesthetic procedure of microelectrode array implantation. We designed the training procedure using a formaldehyde-fixed human cadaver.
Our goals were to provide surgeons an operating room-like environment, realistic anatomy, and the opportunity to practice and rehearse the critical steps of the procedure. The protocol shown here specifically applies to the Utah array. This device comprises of 4x4-mm array of 100 silicon-based microelectrodes, a bundle of 100 individually isolated gold wires, and a transcutaneous titanium pedestal that connects to the computer recording the neural signals.
The present tutorial video reviews the steps of the operative procedure starting with set up, exposing the cortical surface, fixing the pedestal, implanting the microelectrode array itself, positioning an electrocorticography or ECOG grid, and closing the craniotomy. Select a specimen with no history of disease or injury to the head, skull, and brain. If available, a postmortem head CT scan will ensure that there is no significant intracranial lesion.
Fix the head in the skull clamp, and cover with surgical drapes. Incise and recline the scalp and the temporalis muscle. Perform a large square craniotomy, for instance, 5x5 centimeters.
Remove the bone flap and expose the dura mater. Reserve the bone flap. Open the dura mater on three sides of the craniotomy.
Recline it, and expose the arachnoid membrane and the surface of the cerebral neocortex. The first step in fixing the pedestal is to select a cortical gyrus where the microelectrode array will be implanted. The surface of the gyrus must be approximately flat so that the microelectrode array will lie flush with it when inserted.
And there must be no visible blood vessel coursing on the cortical surface where the microelectrode array is to be inserted. Then, select a site for the fixation of the pedestal on the superior edge of the craniotomy close to the skin incision. Allow enough slack for the wire bundle so that the microelectrode array can reach the target gyrus.
Use bone screws to screw the pedestal onto the external table of the skull bone next to the craniotomy. Throughout the procedure, it is critical to always ensure that the microelectrode array does not touch anything, as it may be damaged or could lacerate the neocortical surface. Hold the wire bundle close to the microelectrode array with tweezers with plastic or rubber coated tips.
Position the microelectrode array parallel to the surface of the target gyrus. Bend the wire bundle as needed for that purpose. This is a critical step.
The wire bundle is somewhat stiff and does not easily conform to the surgeons wishes. Care and patience are required to obtain good alignment of the microelectrode array and cortical surface. Dog bone titanium straps can be used to secure the wire bundle to the skull bone and control its course towards the target gyrus.
Now bring the holder of the pneumatic impactor into approximate alignment with the back of the microelectrode array. The millimetric screws of the pneumatic impactor holder will be used later for finer adjustments. Fix the pneumatic impactor inside its holder.
Check the connections with the control box, and then turn on the control box. Now, use the millimetric screws to bring the impactor in close alignment with the back of the microelectrode array. Then, use the impactor to apply a tack to the back of the microelectrode array and push it into the cortical surface.
Check that the microelectrode array is flush with the cortical surface. Optionally, position a subdural electrocortical refragrade over the exposed cortical surface. Secure it by suturing it to the dura mater.
The microelectrode array will be covered by the ECOG grade. Reposition the dura mater over the exposed cortical surface and suture it. Reposition the bone flap.
Secure it with dog bone titanium straps. Make sure that the wire bundle of the microelectrode array as well as the ECOG cables do not get crushed between bone edges. Reposition and suture the skin flap.
Close the skin incision around the neck of the pedestal. Alternatively, the pedestal can egress the scalp through a separate stab incision made into the skin flap. We used a formaldehyde-fixed human cadaver to allow surgeons to practice the surgical procedure of implanting a microelectrode array into the human cerebral neocortex.
Advantages include an operating room-like environment and realistic human anatomy, as well as the opportunity to practice key steps of the procedure including the manipulation of the microelectrode array itself, and the use of the impactor. The obvious draw backs of using a cadaver are the absence of blood and cerebral spinal fluid circulation and the absence of the cerebral pulses caused by breathing and heartbeats. To conclude, the practice of microelectrode implantation on a formaldehyde-fixed cadaver is adequate for the training of neurosurgeons.
Formaldehyde-fixed cadavers are readily available and relatively inexpensive. Such a procedure bypasses the ethical and cost-related issues of training surgeons with non-human primates.
