3.3 Invasive recording methods (microelectrodes, ECoG)

Invasive neural recording methods offer unparalleled insight into brain activity. These techniques, like microelectrodes and electrocorticography, provide high-resolution data crucial for neuroprosthetics. They enable precise control of devices and targeted stimulation for various applications.

However, invasive methods come with risks and ethical concerns. While they offer superior spatial and temporal resolution compared to non-invasive techniques, they require surgical intervention. The trade-offs between data quality and potential complications are key considerations in neuroprosthetic development.

Invasive Neural Recording Methods

Principles of invasive neural recording

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• Microelectrodes penetrate brain or spinal cord tissue to record electrical activity from individual neurons or small neuron groups
  • Typically made of metal (tungsten, platinum-iridium) or silicon
  • Can be single-channel or multi-channel (array) electrodes
  • Provide high spatial resolution (< 100 μm) and temporal resolution (< 1 ms) enabling precise recording of neuronal activity
• Electrocorticography (ECoG) involves placing electrodes directly on the brain surface, typically beneath the dura mater
  • Records local field potentials (LFPs) generated by synchronized activity of neuronal populations
  • Provides higher spatial resolution than non-invasive methods (EEG), but lower than microelectrodes
  • Offers better signal-to-noise ratio and higher frequency content compared to non-invasive methods, improving data quality for neuroprosthetic applications

Resolution of recording methods

• Spatial resolution comparison
  • Microelectrodes: Highest spatial resolution (< 100 μm), allowing for single-neuron or small neuronal population recordings
  • ECoG: Intermediate spatial resolution (1-5 mm), recording from local neuronal populations
  • Non-invasive methods (EEG, MEG): Lowest spatial resolution (> 1 cm), recording from large neuronal populations and unable to precisely localize sources
• Temporal resolution comparison
  • Microelectrodes and ECoG: High temporal resolution (< 1 ms), capturing fast neuronal dynamics and spiking activity
  • Non-invasive methods: Lower temporal resolution (> 10 ms), limited by the filtering properties of the skull and scalp
• Implications for neuroprosthetic applications
  • High spatial resolution of invasive methods allows for precise control of neuroprosthetic devices (robotic arms) and targeted stimulation (visual or auditory implants)
  • High temporal resolution enables real-time, closed-loop control of devices based on fast neuronal dynamics, improving responsiveness and accuracy
  • Invasive methods are preferred for applications requiring fine-grained control (motor prostheses, sensory restoration)
  • Non-invasive methods may be sufficient for applications with lower spatial and temporal resolution requirements (communication prostheses, simple environmental control systems)

Surgical procedures for invasive recording

1. Pre-operative planning using neuroimaging (MRI, CT) to identify target brain regions and plan electrode trajectories
2. Stereotactic frame or neuronavigation system used for precise electrode placement during surgery
3. Craniotomy or burr hole created to access the brain surface or deeper structures
4. Dura mater incised for ECoG electrode placement or penetrated for microelectrode insertion
5. Electrodes secured in place using sutures, dental cement, or skull-mounted hardware to ensure stable long-term recording

• Safety considerations include strict aseptic techniques, use of biocompatible materials, careful selection of electrode trajectories, intraoperative monitoring, post-operative management, and long-term follow-up to minimize risks (infection, hemorrhage, seizures) and ensure proper functioning of implanted electrodes

Advantages vs limitations of invasive methods

• Advantages
  • High spatial and temporal resolution, enabling precise and responsive control of neuroprosthetic devices
  • Direct access to relevant neuronal populations, reducing the need for complex signal processing and decoding algorithms
  • Improved signal-to-noise ratio compared to non-invasive methods, facilitating more accurate and reliable device control
  • Potential for long-term, stable recordings that can adapt to changes in neuronal activity over time
• Limitations
  • Invasive nature of the procedures, requiring surgical intervention and associated risks (tissue damage, inflammation, scarring)
  • Limited spatial coverage compared to non-invasive methods, as electrodes can only record from specific brain regions
  • Ethical considerations and regulatory hurdles associated with invasive brain interventions
  • High cost and specialized expertise required for electrode implantation and maintenance
• Specific applications
  • Motor prostheses (robotic arms): Invasive methods are well-suited due to the need for precise, real-time control based on motor cortex activity
  • Sensory restoration (visual or auditory implants): Invasive methods can provide high-resolution stimulation of sensory cortices to elicit percepts
  • Communication prostheses (brain-computer interfaces for typing): Invasive methods may offer faster and more accurate communication, but non-invasive methods are often preferred due to lower risks
  • Seizure prediction and intervention: Invasive ECoG recordings can help identify seizure onset zones and trigger targeted stimulation for seizure suppression