Chronic Cranial Window Technique for Repeated Cortical Recordings During Anesthesia in Pigs

This research aims to develop a scalable and reliable method for chronic cortical recordings in pigs, addressing challenges of device failure and tissue responses, improving neuroscience, diagnostics and research. To extract information from the brain for prolonged periods, current technologies focus on creating a stable and reliable interface between the electrode and the brain, ensuring consistent signals from the same neural population. Electrocorticography and intracranial recordings are typically used for prolonged brain recordings.

These electrodes stay in the same location, but the electrode tissue interface changes due to tissue responses and device failure. To begin, identify the bregma point by drawing straight lines between each ear and the contralateral eye. The bregma point is at the intersection of those lines.

Make an incision slightly lateral to the bregma point with a scalpel. Using a cauterizer, continue the incision through the skin. After placing the sterile drape, make the initial incision at the marked side in the dermis with a scalpel.

Using a cauterizer, continue the incision through the skin. After placing the retractor, lift the skin using forceps and cut the avascular connective tissue between the periosteum and the subcutus to provide flexibility for suturing later. Place one more retractor to accommodate the additional skin flexibility.

Drill a 10 millimeter diameter hole frontal to the sagittal suture line and lateral to the midline suture using a rounded drill bit. Once the dura becomes visible, switch to a smaller drill bit to achieve the maximal width of the hole at the dura level. Clip the edges of the hole using rongeurs.

Confirm that the cranial window fits completely into the drilled hole, reaching the bottom without gaps and determine the appropriate depth. While holding the cranial window in place, mark the screw hole positions with a hand drill. Remove the cranial window and drill the marked holes to a depth of at least four millimeters.

Then place the cranial window back. After removing the needle from the butterfly infusion set, measure the screw hole depth through the cranial window using the blunt butterfly. Insert and fasten a screw with a matching length into the hole.

Place a U-connector under the screw that perforates the skull and touches the dura. Insert two needles perpendicular to the nerve course approximately two centimeters apart. Pierce the skin again, approximately three centimeters away from the insertion point.

Pass the Cooner wires through the needles and remove the needles, leaving the wires partially under the skin. After dipping the micro electrocorticography electrode in alcohol, place it in the ZIF-Clip Headstage. Using a micro manipulator and gently guiding it with a cotton bud, bring the micro electrocorticography electrode into place.

Connect the ground wires of the electrode to the ground screw using a crocodile connector. Place sterile gauze on the electrode to maintain firm tissue contact. Drip body temperature saline on the gauze to prevent the tissue from drying.

Then apply electrical stimulation to the ulnar nerve via the peripheral wire to evoke small movements in the forelimb. Preview spontaneous and evoked brain signals. Place the cranial window cap to close the cranial window, ensuring that it fits precisely in the cranial window and requires no additional fastening.

Place an antibiotic pouch on top of the cranial window cap. Close the skin subcutaneously using the buried vertical mattress technique with single resorbable sutures placing them five to 10 millimeters apart. Tie the last three sutures after all the sutures have been placed.

Next, place intradermal sutures in a continuous fashion, starting one to two centimeters lateral to the incision site and tunneling under the skin to the dermal layer, but below its surface. Tie a knot on the lateral end to prevent irritation or suture loosening. Then place the continuous suture between the subcutaneous sutures until the other side of the incision is reached.

Make a new incision in the dermis with a scalpel slightly lateral to the midline. Using a cauterizer, continue the incision through the skin. Then incise the periosteum so the implant becomes visible.

Finally, using gentle circular motions with a cotton bud, remove any connective tissue from the opening until the dura becomes visible. This figure shows the stability and modulation of event related potentials or ERPs recorded at different time points and under varying stimulation conditions. ERPs recorded at three time points showed nearly identical waveforms demonstrating the high reproducibility of the cranial window technique over time.

Peak amplitudes and interchannel variability remain stable across sessions indicating consistent signal quality, while a slight delay in the first session likely reflects developmental changes. High frequency stimulation significantly enhanced the N1 peak amplitude confirming ERP sensitivity to neuroplastic changes linked to increased pain sensitivity. This approach reduces risks associated with permanent implants by eliminating device failure and reactions at the electrode.

This in turn increases recording fidelity and results in highly reliable recordings. This method also reduces research costs and improves animal welfare. It can further be implemented in other species or to access other brain areas.