Layer-Dependent Effects of Chronic Neural Electrode Implants on Neural Functional Connectivity in Mice

Introduction

This study explores the long-term effects of chronically implanted microelectrodes on neural functional connectivity within the brains of C57BL6 wild-type mice. Implanted intracerebral electrodes enable the recording and electrical stimulation of neural signals, and have widespread applications in brain-computer interface (BCI) systems, such as restoring motor control and sensory perception. However, over time, the signals recorded by implanted electrodes gradually deteriorate, a degradation thought to be caused by the “foreign body response” (FBR). The specific ways in which FBR affects the function and stability of neural circuits around the implantation area remain unclear. This study aims to reveal how long-term FBR alters local neural circuit function and to deepen our understanding of its impact on BCI decoding devices.

Research Background and Objectives

Although implanted neural electrodes have potential applications, they still face issues in terms of recording sensitivity and stability. Previous studies have shown that implanted electrodes can trigger acute local inflammation, disrupt the blood-brain barrier, and lead to the infiltration of blood cells and plasma proteins. These changes further activate glial cells and astrocytes, forming encapsulating glial scars, reducing the signal-to-noise ratio, and affecting signal detection. Additionally, increases in oxidative stress, pro-inflammatory cytokines, and glutamate can affect neuron survival and synaptic integrity, leading to the silencing or degeneration of nearby neurons, reducing neural signal detection capabilities. Damage to myelin and oligodendrocytes, which play roles in action potential propagation and support axon function, may further restrict the signal detection capability of the electrodes.

Research Methods and Experimental Procedures

The study used single-shank, 16-channel Michigan-type microelectrodes with 100 µm spacing for recordings through all cortical layers and the hippocampal CA1 region. The subjects were gender-balanced, 11-13 week-old C57BL6 wild-type mice. The main experimental methods employed in this study include the following:

1. Electrode Implantation Surgery

A total of 8 mice (4 males and 4 females) were used as study subjects. The responsible electrode implantation process included the use of ketamine and diazepam to anesthetize the mice and fix them in a stereotaxic apparatus, removing the skin and connective tissue at the implantation site. The microelectrodes were vertically inserted into the left primary visual cortex, followed by electrochemical impedance spectroscopy measurements to ensure the implanted electrodes were functioning properly.

2. Electrophysiological Recording

Electrophysiological recordings were conducted with the mice awake and head-fixed, capturing both spontaneous and visually-evoked activity. The visual stimulation paradigm included drifting gradients of black-and-white stripes to study neural activity responses to visual stimuli.

3. Data Analysis

Data analysis included electrode depth alignment, single-unit classification, and cross-frequency synchronization analysis. Current source density (CSD) maps were used to identify layer depths along the implanted electrode. Single-unit waveforms were classified by peak-to-trough delay into presumed excitatory and inhibitory neurons. Phase-amplitude coupling (PAC) was used to quantitatively describe the functional connectivity of neural networks.

4. Unit Classification and Connectivity Analysis

Unit classification in the recordings was used to assess activity changes of presumed excitatory and inhibitory neurons, and local network activity was evaluated through frequency synchronization analysis. The results showed significant changes in the modulation index (MI) of LFP phase-amplitude coupling during different periods.

Results and Discussion

1. Layer-Specific Characteristics of Neuronal Activity

Analysis of discharge activity at different depths revealed that during chronic electrode implantation, excitatory connectivity in L4 remains stable, whereas L2/3 rapidly loses function, leading to the loss of appropriate inter-layer connectivity in downstream output layers (L5/6). L2/3 and L5/6 exhibited notable layer-dependent changes in functional connectivity during chronic electrode implantation. Firing activity of both excitatory and inhibitory neurons in the L2/3 layer progressively declined, especially the unidirectional connection from L2/3 to L5/6, while connectivity in the L5/6 layer itself increased under visual stimulation.

2. Directional Dependence of Inter-Layer Connectivity Function

The study further revealed cross-layer functional connectivity impacted by chronic electrode implantation. L4, as the information-receiving layer, maintains stable excitatory networks during the process, but L2/3 experiences early functional connectivity deficits. PAC analysis showed consistent coupling between low-frequency phase in L4 and high-frequency amplitude in L2/3 under visual stimulation, but this connectivity was affected early post-implantation and gradually recovered over time.

3. Interruption of Network Activity in the Hippocampal CA1 Region

Compared to the cortex, the hippocampal CA1 region exhibited weaker responses to visual stimulation. Although LFP analysis showed similar rhythmic oscillations to the cortex, LFP synchrony with local neurons was impaired, indicating long-term functional connectivity disruptions within the hippocampus due to chronic implantation, affecting neuronal responsiveness.

4. Research Significance and Applications

This study, for the first time, reveals the changes in functional connectivity between different cortical layers and within the hippocampal CA1 region during chronic microelectrode implantation, providing critical information for understanding regional neural network loss and its mechanisms caused by implanted electrodes. The findings have significant implications for developing new regeneration strategies and more durable and stable BCI decoding devices.

Conclusion and Outlook

This study systematically describes the effects of chronic microelectrode implantation on neural functional connectivity in the brains of mice, particularly the layer-dependent functional connectivity disruptions in the visual cortex and hippocampal regions. This research provides new insights into the long-term neural network degeneration caused by implanted microelectrodes and suggests possible interventions for improving tissue health around cerebral implants, anticipating further exploration and technological development in this field.

Keywords

Brain-Computer Interface, Neural Circuit, LFP Synchronization, Visual Cortex, Neuroinflammation