Extended Data Fig. 1: Recording challenges and lessons learned.

Recording challenges and lessons learned. a. Diagram of the different combinations of ground and referencing and connections that improved the electrophysiological signal or degraded the signal with the indicated ground and reference contacts (including the external and internal reference). Each circled number indicates a combination tested. A major step in reducing noise levels was to separate the ground and reference, with a single separate wire going to the grounding pads on one side and another wire going to a grounding pad on the other side of the Neuropixels probe, pictured here. We used the external reference tied to a sterile MedTronic grounding wire with a needle which, when placed in the scalp or CSF, improved the signal and reduced 60 Hz noise. Common 60 Hz noise and other noise decreased considerably if the anesthesia IV pump was unplugged from the wall and was run on battery during the recordings. Otherwise, turning off lights or other sources of noise (BOVIE cautery machine, AlphaOmega recording system, etc.) had no noticeable effect on noise. b. Diagram of the connections relative to the participant including the BOVIE pad placed on the leg connected to the IMEC ground via a BNC cable to the NIDAQ PCI chassis BNC. c. Securing the wires and the probe to the cannulae attached to the AlphaOmega manipulator with Tegaderm and Steristrips improved the stability. d. Mechanical stabilization of the probe involved two options, one using the ROSA robot combined with an AlphaOmega manipulator with the Neuropixels probe secured to cannulae. e. The second option involved securing the Neuropixels probe to a sterile syringe using Steristrips. f. Then, the syringe is attached to a 3-axis manipulator mounted on a Greenberg retractor over the craniotomy. g. Every Neuropixels probe is documented and checked several times during and after the procedure both via software (in the OpenEphys or SpikeGLX software) and a quick saline test is done in the side sterile table. h. Each probe is photographed before, during, and afterward in the OR to determine if they are intact. Regular checks of the connections are done as the electrode is being moved into place in the sterile field. The electrode is also checked and photographed afterward for whether it is intact and connected. i. In two cases, 60 Hz noise was reduced by tying a ground to a spare BOVIE pad placed on the thigh of the patient under the sterile drape (as shown in b), with the traces shown with the lower left LFP trace showing the signal grounded to the BOVIE versus grounded to a scalp needle electrode. j. Examples of considerable noise in the LFP and high frequency range (AP range) when crossing the ground and reference together. k. We did test using the internal reference on the Neuropixels probe and found the noise increased considerably in the two cases we attempted the switch. l. The placement of the sterile ground and reference leads made a difference. Ground and reference in the scalp had an improved signal. Placing the ground (but not the reference) in the saline in the craniotomy caused the LFP signal to saturate and degraded the signal.