Surgical Implantation of Chronic Neural Electrodes for Recording Single Unit Activity and Electrocorticographic Signals

The overall goal of this procedure is to implant an electrode for chronic electrophysiological recordings. This is accomplished by first anesthetizing and preparing the animal for surgery. Once ready, an incision is made down the midline and the top of the skull is exposed.

Next holes are drilled to hold screws and a craniotomy is performed. Finally, the electrode is implanted and a protective cap is built from dental acrylic. When finished, the electrodes are connected to a signal acquisition system, and recordings can be made of electrophysiological signals including action potentials and local field potentials.

The main advantage of chronic electrophysiology recordings is it allows us to study awake, behaving animals free of anesthesia. Now, this message could help us answer key questions, and neuroscience are difficult to ascertain from acute surgeries such as learning in memory, attention, decision making, and much more. Today’s surgeries are going to be performed by Sarah Nik and Colin Stetner, Having induced a surgical plane of anesthesia in the animal, which was confirmed by the absence of a pain reflex via a toe pinch, prepare the animal for surgery.

Shave the animal from between the eyes to behind the ears, then apply a lubricant to the eyes. Then scrub the shaved region three times, alternating between Isop profile, alcohol and Betadine. Attach measurement devices and mount the animal in Stereotaxic ear bars.

Now, carefully position each ear bar tip in the auditory ATU by first shifting the animal to position the first air bar, and second, using slight inward pressure to direct the second ear bar. Grasp the animal behind the ears and wiggle back and forth firmly. If the animal is mounted properly, the head will feel rigid and secured.

Re sterilize the head using a scalpel, make an incision down the midline. Use clamps to hold back the skin using sterile swabs. Remove underlying connective tissue that adheres to the bone.

The swab tip can absorb and minimize bleeding. At this point, clamps can be repositioned to tissue beneath the skin, disinfect and clean the bone’s surface using hydrogen peroxide. Then dry the bone using a gauze pad, using the BMA and lambda points, a stereotaxic tower is used to identify the drilling position of the craniotomy.

A pen or shallow drilling will mark the surface. Now drill screw holes into the bone for anchoring screws and ground place anchor screws around the perimeter and place the ground screw posterior of Lambda. Leave the area next to the craniotomy, free from screws.

After implanting screws, drill the craniotomy begin by drilling four shallow pilot marks for the corners of the craniotomy, followed by drilling out the perimeter of the craniotomy. If possible, avoid the sagittal sinus because it can cause extensive bleeding. Now, remove the center mass bone using a pair of sturdy forceps and hydrate the exposed dura with artificial CSF or saline fashion.

A small 90 degree hook from a 28 gauge or smaller hypodermic needle, avoiding blood vessels. Hook the surface of the dura and lift it off the brain. Use micro scissors to trim any connective tissue.

CSF will leak out when the dura is resected. Secure the linear motor actuator on a stereotaxic apparatus. Carefully attach the ground wire of the silicon electrode around the base of the ground screw.

The electrode is placed so that the cables hover over the craniotomy. Next, attach the recording electrode to the insertion device with heated PEG. Then manually lower the tip of the electrode to the desired stereotaxic location on the surface of the brain.

Using the software interface to the linear actuator, move the electrode into the desired depth within the brain tissue. Secure the recording electrode cable to the brain by attaching it to an adjacent screw using quick seal. Once secured, remove the electrode from the insertion device by dissolving that PEG connection with saline.

Raise the insertion device from the brain. When finished, cover the exposed craniotomy with saline soak, gel foam, or an alginate gel, and apply a silicon polymer to cover and protect each electrode cable. Apply acrylic to secure the implant site and allow it to harden.

Next, position the connector to its final location and apply dental acrylic to make a robust head cap. Allow the cement to harden. Then proceed with the section titled Postoperative Recovery.

For the implantation of a five by five millimeter device, prepare a six by six millimeter craniotomy between Lambda and Bgma to reduce the time the craniotomy is open and keep it clean. Apply a UV curable dental acrylic to the dry periphery of the cap and craniotomy site before drilling. Now, use a 1 0 7 bur to drill off the surface of bone in the general shape of the craniotomy down to a thin transparent layer followed by a number 1 0 6 bur.

To finish thinning the edges, lift off the remaining piece of bone using wrongs. Keep the dura well hydrated with artificial CSF or saline soak gel foam. Otherwise, the electrode will not slide into the tissue.

Using sterile tape, secure the micro ECOG electrode to the stereotaxic. Calm over the open skull. Lower the electrode, making sure the electrode sites are facing downward to contact the dura or peel surfaces.

Now place small pieces of saline soak gel foam around the electrode where there is dura or peer exposed and on top of the thin film electrode. Then stabilize the head cap by applying UV curable dental acrylic over the gel foam and up to the connector. Take care not to cover the connector beyond the bottom of the cap.

Leave the screws uncovered and accessible to wires. Once the cap is stable, cut the tape and back away the tower. Connect the ground wire to any screw touching the dura by wrapping it around that screw at least three times over and under itself.

Then connect the reference wire to another screw in the same manner. Finally, the cap is finished with some additional acrylic. Once the cement has hardened, the surgical site will be ready for closure.

To begin the postoperative recovery, remove the animal from the frame. Keep the animal warm and provide supplementary oxygen until the animal can independently maintain normal oxygen levels. House each chronically implanted animal in a separate cage.

It usually takes four to seven days for the animal to recover completely from the surgery. A successful implant will have predictable impedances measured at one kilohertz immediately post surgery. The neural signal shows spikes on the implanted electrodes and slow wave oscillations on the thin film surface electrode.

After watching this video, you should have a good understanding of how to do chronic surgeries with practice. You should be able to complete it within a couple hours.