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100 Years of Electroencephalography (EEG) in Human Medicine
Electroencephalography (EEG)
Invented by Hans Berger a century ago, is a remarkable tool that non-invasively reveals the brain’s electrical activity. This breakthrough in neurobiology and physics provides crucial insights into brain function without the need for surgery.
The Invention of EEG:
- The development of the EEG was preceded by several significant advances in neurophysiology across Europe.
- In 1875, early research by Caton (monkeys and rabbits), Beck (sensory stimulation in animals), and Pravdich-Neminsky (first mammalian EEG) laid the groundwork.
- Hans Berge invented EEG in 1924, recording the first human EEG. He named the technique and introduced its clinical applications.
What is EEG?
EEG stands for electroencephalography. ‘Electro-’ pertains to electricity; ‘-encephalo-’ refers to the brain; and ‘-graphy’ is a suffix meaning to show or to represent.
- Neurons in the brain create electrical activity by moving charged particles like ions. This activity is detected and visualised using an EEG test, providing insights into brain function without invasive procedures.
- Researchers can correlate EEG data with various brain activity levels and patterns, effectively distinguishing between normal and abnormal states.
- It is the reference standard — i.e. the best test available — to diagnose epilepsy.
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Other Measuring Instruments
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Applications:
The test can detect the effects of anaesthesia, monitor sleeping patterns, assess neurological activity during coma, and indicate oxygen availability.
- It is also used to confirm brain death, which is legally recognised as a form of death in India.
- Scientists use EEG for neuroscience, cognitive psychology, neurolinguistics, and neuromarketing studies and to develop brain-computer interfaces.
Volume conduction
- It is the distortion of electrical signals as they travel through different tissues (like skin, skull, and brain fluid) before reaching the EEG electrodes. The EEG signal doesn’t directly reflect the exact electrical activity of the brain source.
- The raw EEG data must be corrected for volume conduction and noise from faulty electrodes and incidental physiological activity like blinking or muscle movement.
How does an EEG test work?
- Neurons in the brain exchange ions with their surroundings. This pushes ions out, creating a cascading effect of electrical activity.
- When many neurons are active simultaneously, they generate detectable electrical waves.
- Metal electrodes on the scalp track voltage changes caused by these waves, resulting in an EEG.
Electrode Placement:
- Standard system for placing electrodes: International 10-20 System.
- In this system, the distance between two adjacent electrodes is either 10% or 20% of the total distance between two points on the head along which electrodes are being placed.
- Reference Points: Nasion (Depression between the eyes above the nose bridge), Inion(Crest at the back of the skull), Tragus (Flap-like projection on the outer ear).
Affordability:
- Cost-effective setup: The equipment is compact, emits no high-energy radiation or loud sounds, and does not require patients to be confined in small spaces like MRI machines do.
- Non-invasive and portable: In contrast, the invasive version, electrocorticography (ECoG), involves placing electrodes directly on the brain’s surface.
- Limitations: The test involves time-consuming steps such as applying gel to the person’s head and placing electrodes accurately according to the 10-20 System. Thicker hair can affect the accuracy of EEG readings.
Challenges and Opportunities:
- It is biased towards detecting electrical signals near the brain’s surface, mainly from neurons’ dendrites rather than axons.
- Pinpointing the exact origin of electrical activity within the brain using EEG is a complex and challenging process.
- Combined use with MRI and advanced data processing techniques help overcome these challenges.