Introduction to Intracranial EEG (iEEG)

1. Overview

Intracranial EEG (iEEG) is a powerful neurophysiological method that involves recording electrical activity directly from the brain. It is primarily used in clinical settings to monitor and diagnose epilepsy and other neurological disorders. By placing electrodes directly on the surface of the brain or within its structure, iEEG provides high-resolution data about neuronal activity[1]. This technique offers unparalleled insight into the brain's electrical functions, making it a critical tool in both clinical and research settings.

For an in-depth EEG preprocesing and data visualization tutorial check out: EEG tutorial on Google Colab.

2. Key iEEG Terms

Term Definition
Waveform A graphical representation of the shape and form of a signal, such as electrical activity in the brain, over time.
Local Field Potential (LFP) Electrical recordings from a small area of the brain, reflecting the summed electrical activity of a population of neurons.
Event-Related Potential (ERP) Measured brain response resulting from a specific sensory, cognitive, or motor event, often used to study perception and attention.
Electrocorticography (ECoG) Recording of electrical activity directly from the exposed surface of the brain using electrodes placed on the cortex.
Depth Electrodes Electrodes implanted into the brain tissue to record activity from deep brain structures.
Subdural Electrodes Electrodes placed on the surface of the brain beneath the dura mater to record cortical activity.
Neural Oscillations Rhythmic or repetitive patterns of neural activity in the central nervous system.
Seizure Focus The specific area of the brain where seizures originate, crucial for planning surgical interventions for epilepsy.
Gamma Band Activity High-frequency brain waves associated with cognitive functions such as attention, memory, and consciousness.
Spike-and-Wave Discharge A pattern of brain activity often seen in epilepsy, characterized by a spike followed by a slow wave.

3. How iEEG Works

The iEEG process begins with the surgical implantation of electrodes. These electrodes can be placed subdurally (on the brain's surface) or depth electrodes can be inserted into brain tissue. The placement is guided by neuroimaging techniques like MRI and CT scans, ensuring precision in targeting the regions of interest[2]. The electrodes capture electrical signals generated by neuronal activity, which are then recorded and analyzed to understand brain function and identify abnormalities.

Types of Electrodes

There are two main types of electrodes used in iEEG:

  • Subdural Electrodes: These are placed on the surface of the brain to record activity from the cortical areas. They are often used in grid or strip configurations to cover larger surface areas.
  • Depth Electrodes: These are inserted into the brain tissue to record from deeper structures. They are used to target specific brain regions, such as the hippocampus, which is often involved in epilepsy[3].

Recording and Analysis

Once the electrodes are in place, the electrical activity of the brain is recorded over a period of time, which can range from a few days to several weeks. The data is then analyzed to identify patterns of abnormal activity, such as epileptic spikes and seizures[4]. Advanced software and algorithms are used to process the vast amounts of data collected, providing detailed maps of brain activity.

4. Applications of iEEG

iEEG is most commonly used in epilepsy surgery evaluations. By identifying the precise regions where seizures originate, surgeons can plan more effective interventions[5]. Additionally, iEEG is used in research to study brain function, cognition, and to map out critical brain areas before neurosurgery[6]. These applications extend our understanding of the brain and improve outcomes for patients undergoing surgical treatments.

Epilepsy Surgery

In cases where medication is not effective in controlling seizures, iEEG is used to locate the seizure focus. This allows for precise surgical removal of the epileptogenic zone, which can significantly reduce or eliminate seizures[7]. The detailed information provided by iEEG helps neurosurgeons avoid critical brain regions, minimizing the risk of post-surgical deficits.

Research Applications

iEEG is also a valuable tool in neuroscience research. It is used to study the neural mechanisms underlying various cognitive processes, such as memory, language, and sensory perception. iEEG provides high temporal and spatial resolution, making it ideal for investigating the dynamics of brain activity[8]. Researchers use iEEG to explore brain connectivity, neuroplasticity, and the effects of different stimuli on brain function.

Advantages of iEEG

iEEG offers several advantages over non-invasive EEG, including:

  • Higher spatial resolution: iEEG provides detailed localization of brain activity[9].
  • Less interference: iEEG recordings are less affected by artifacts from muscle activity or external electrical sources[10].
  • Direct measurement: iEEG measures the electrical activity directly from the brain, providing a more accurate reflection of neuronal activity[11].

5. Challenges and Risks

While iEEG is a valuable tool, it comes with inherent risks and challenges. The surgical procedure carries risks of infection, bleeding, and damage to brain tissue. Additionally, the high cost and invasiveness limit its use to specific clinical and research scenarios[12].

Surgical Risks

The implantation of electrodes involves a craniotomy, which is a major surgical procedure. Risks include infection, bleeding, and potential damage to brain tissue. Careful surgical planning and post-operative care are essential to minimize these risks[13]. Despite these challenges, the benefits of iEEG often outweigh the risks, particularly for patients with severe, drug-resistant epilepsy.

Cost and Accessibility

iEEG is an expensive procedure that requires specialized equipment and expertise. This limits its availability to major medical centers and research institutions. Efforts are ongoing to develop less invasive and more cost-effective alternatives[14]. Advances in technology and surgical techniques continue to improve the safety and feasibility of iEEG, potentially expanding its use in the future.

6. Future Directions

Advancements in iEEG technology and techniques continue to improve its safety and efficacy. Innovations such as high-density electrode arrays and wireless recording systems are expanding the possibilities for iEEG in both clinical and research contexts. As our understanding of the brain deepens, iEEG will remain a crucial method for unlocking the mysteries of neural function[15].

Technological Innovations

Recent developments include high-density electrode arrays that provide even greater spatial resolution. Wireless recording systems are also being developed to allow for more naturalistic recordings, free from the constraints of wired connections. These advancements enhance the ability of researchers and clinicians to monitor brain activity in real-time and in more natural settings.

Clinical Applications

Beyond epilepsy, iEEG is being explored for use in other neurological and psychiatric conditions, such as depression and obsessive-compulsive disorder. These applications involve targeting specific brain regions for therapeutic interventions. As research progresses, iEEG may provide new insights and treatment options for a wide range of brain disorders.

References

  1. Engel, J. (1993). Surgical Treatment of the Epilepsies. Raven Press.
  2. Spencer, S. S. (1994). "Neural networks in human epilepsy: evidence of and implications for treatment." Epilepsia, 35(S4), S48-S58.
  3. Fried, I., Wilson, C. L., MacDonald, K. A., & Behnke, E. J. (1997). "Single neuron activity in human hippocampus and amygdala during recognition of faces and objects." Neuron, 18(5), 753-765.
  4. Lachaux, J. P., Rudrauf, D., & Kahane, P. (2003). "Intracranial EEG and human brain mapping." Journal of Physiology-Paris, 97(4-6), 613-628.
  5. Lüders, H. O., Najm, I., Nair, D., Widdess-Walsh, P., & Bingman, W. (2006). "The epileptogenic zone: general principles." Epileptic Disorders, 8(S2), S1-S9.
  6. Engel, J., McDermott, M. P., Wiebe, S., Langfitt, J. T., Stern, J. M., Dewar, S., ... & Jacobson, M. (2012). "Early surgical therapy for drug-resistant temporal lobe epilepsy: a randomized trial." JAMA, 307(9), 922-930.
  7. Jerbi, K., Ossandón, T., Hamamé, C. M., Senova, S., Dalal, S. S., Jung, J., ... & Lachaux, J. P. (2009). "Task-related gamma-band dynamics from an intracerebral perspective: review and implications for surface EEG and MEG." Human Brain Mapping, 30(6), 1758-1771.
  8. Creutzfeldt, O., & Houchin, J. (1974). "Neuronal basis of EEG waves." Mechanisms of Action of Neuroleptic Agents, 1-26.
  9. Niedermeyer, E., & da Silva, F. L. (2005). Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. Lippincott Williams & Wilkins.
  10. Fisher, R. S., Webber, W. R. S., Lesser, R. P., Arroyo, S., & Uematsu, S. (1992). "High-frequency EEG activity at the start of seizures." Journal of Clinical Neurophysiology, 9(3), 441-448.
  11. Blume, W. T., Lüders, H. O., Mizrahi, E., Tassinari, C., van Emde Boas, W., & Engel, J. (2001). "Glossary of descriptive terminology for ictal semiology: report of the ILAE task force on classification and terminology." Epilepsia, 42(9), 1212-1218.
  12. Asano, E., Juhász, C., Shah, A., Muzik, O., Chugani, D. C., Shah, J., & Sood, S. (2005). "Surgical treatment of West syndrome." Brain and Development, 27(5), 317-322.
  13. Cardinale, F., Casaceli, G., Raneri, F., Miller, J., & Cossu, M. (2019). "Brain stereo-electroencephalography." World Neurosurgery, 123, 156-165.
  14. Chang, E. F., & Rao, V. R. (2017). "Electrical stimulation of the brain: clinical applications and safety." Brain Stimulation: Handbook of Clinical Neurology, 140, 107-120.
  15. Parvizi, J., & Kastner, S. (2018). "Human intracranial EEG: promise and limitations." Nature Neuroscience, 21(4), 474-483.