Referencing--Thin-film grids, EEG neural recording/stimulation

Technical Note

Document Title: Referencing – Thin-Film Grids for EEG Neural Recording/Stimulation

Document Number: NN-TN-2025-008

Version: 1.0

Date: March 16, 2025

Author: NeuroNexus Technical Support Team

Revision History: N/A (Initial Release)

Status: Approved for Publication

Intended Audience: Researchers conducting electroencephalography (EEG) neural recordings and stimulation using NeuroNexus thin-film grids

Application Area: Electrophysiology techniques, electrodes, micro-scale neural interfaces, brain-computer interfaces

Associated Resources:

• surgical guide as a pdf document
• NeuroNexus website

1. Introduction

This technical note provides an overview of best practices for configuring reference and ground connections when using NeuroNexus thin-film grids in EEG neural recording and stimulation experiments. Proper referencing is critical for obtaining high-quality neural signals and minimizing artifacts during stimulation.

Detailed configurations may vary by laboratory based on experimental design, equipment, and institutional protocols. This document outlines a general approach to illustrate proper referencing and grounding within the basic procedural framework. Researchers should adapt these recommendations to their specific methodologies while complying with all applicable ethical and regulatory guidelines.

2. Importance of Proper Referencing

In EEG neural recording and stimulation, the reference electrode provides a stable baseline for neural activity measurements, while the ground electrode serves as a common electrical return path. Improper referencing can lead to:

• Increased Noise: Environmental interference that reduces the signal-to-noise ratio (SNR).
• Stimulation Artifacts: Electrical artifacts that obscure neural signals during stimulation.

Proper referencing ensures accurate data acquisition and reliable interpretation of EEG signals.

3. Reference and Ground Configurations

a. Reference Electrode Placement

• Common EEG Reference Configurations:
  • Linked Mastoid Reference: Electrodes placed on the mastoid bones behind the ears.
  • Midline Reference: A single electrode placed at a midline location such as Cz (for human EEG) or an equivalent location in animal models.

  • Ear Reference: Reference electrodes placed on the earlobes.

    Thin-Film Grid-Specific Reference Configurations:

  • Separate Reference Electrode:
    • Placement: A stainless steel or Ag/AgCl wire can be attached to a bone screw or placed in a small craniotomy outside the recording area.
    • Connection: Attach the reference wire to the designated reference input on the recording system.
  • No Separate Reference (Common Reference):
    • Configuration: If a separate reference is not used, the reference input on the grid can be connected to a bone screw positioned near but outside the recording grid.
  • Internal Reference Site (for Grids with Built-in References):
    • Some thin-film grids include an internal reference electrode on the array itself. If used, ensure the reference site is positioned over an electrically neutral area or an inactive brain region.

b. Ground Electrode Placement

• Ground Electrode:
  • Placement: Secure the ground wire to a bone screw positioned away from the recording grid to minimize interference.
  • Connection: Attach the ground wire to the ground input on the recording system.

4. Mitigating Stimulation Artifacts

During simultaneous recording and stimulation, artifacts can occur due to electromagnetic interference. Strategies to minimize these artifacts include:

• Impedance Matching:

  • Utilize electrodes with similar impedance values to reduce differential signal drift during stimulation.

    Differential Referencing:

  • Use a differential recording configuration where possible to minimize common-mode noise and stimulation artifacts.

    Artifact Rejection Algorithms:

  • Implement real-time subtraction or post-processing techniques to remove predictable artifacts from the recorded signals.

    Faraday Shielding:

  • If external noise sources are problematic, use shielding such as a Faraday cage to isolate the setup.