Electroencephalography or EEG is a neurodiagnostic medical test that monitors and records the electrical activity in the brain. By placing electrodes on the scalp, EEG detects post-synaptic potentials of pyramidal neural cell assemblies within the cerebral cortex. These results are recorded and can be interpreted to evaluate the state of the patient's brain and diagnose problems related to neurology such as seizures and epilepsy.
History and Development
The foundations of EEG were laid in the late 19th century by scientists like Richard Caton and Polish physiologist Edward Flatau. However, it was German psychiatrist and physician Hans Berger who is credited with developing the first human electroencephalogram in 1924. He discovered that it is possible to trace brain electrical signals by placing electrodes on the scalp and amplifying the tiny signals recorded. The invention of new technologies like amplifiers, filters, software that analyzes brainwaves gave a boost to EEG research and applications. Over the decades, advancements in instrumentation have led to digital EEG systems that provide more accurate and reliable recordings. It has now become an important neurodiagnostic procedure widely used across hospitals and research centers.
Working Principle
In simple terms, Electroencephalograph works on the principle that brain cells communicate through electrical and chemical signals. When large groups of neurons synchronize and fire together in the brain's cortex, tiny electrical charges are generated. The neuronal currents can be picked up through electrodes placed on the scalp. EEG devices amplify these signals that range between 0-100 microvolts and translate them into brainwaves which are recorded as waveforms with different frequencies - delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-30 Hz), and gamma (25-100 Hz). Variations in frequency and amplitude patterns carry important information that neurologists use to analyze and diagnose brain-related conditions. Sophisticated computer algorithms also help classify and map brain activity.
Components and Procedure
A modern digital EEG system consists of a cap with 21-32 electrodes fitted in standardized locations, an amplifier unit, data recorder and specialized EEG analysis software. The hair under electrodes is gently cleaned to facilitate better skin contact. A conducting gel or paste improves conductivity. Electrodes are attached to the scalp using adhesive paste. Patients lie on a bed in a soundproof, electrically shielded room during the 30-60 minute recording session. They remain relaxed but awake with eyes open or closed as instructed. Any movements or blinks are noted. Recordings obtained while patients hyperventilate or are shown flashing lights provide additional activation data. The digital EEG data is transferred to computers for expert analysis and diagnostic interpretation. Repeat tests may be required at times.
Applications in Clinical Neurology
EEG proves highly useful for evaluating epilepsy, seizures, head injuries, tumors, encephalitis and other neurological conditions. Some key clinical applications are discussed below:
- Epilepsy diagnosis: EEG plays a vital role in epilepsy evaluation. Interictal and ictal recordings can reveal abnormalpatterns like spikes and sharp waves diagnostic of seizure foci in the brain.
- Seizure monitoring: Ambulatory or long term video-EEG helps capture and correlate clinical seizures with EEG changes for accurate localization and treatment planning.
- Brain death determination: Isoelectric or flat EEG confirms brain death and lack of brainstem reflexes following severe brain injuries.
- Tumors and masses: Slowing of rhythms, attenuation or blocking of waves can indicate intracranial space occupying lesions like tumors pressing on brain tissues.
- Stroke detection: Changes in waves and asymmetries between hemispheres help identify infarction areas from occluded vessels.
- Encephalopathies: Generalized slowing patterns suggest toxic-metabolic encephalopathies from infections, liver-kidney failures, poisoning etc.
- Dementia evaluation: Progressive slowing and attenuation correlate with severity of dementia and Alzheimer's disease.
- Sleep studies: Polysomnography with EEG monitors sleep stages and disorders like insomnia accurately.
Thus EEG is invaluable for both diagnosing and managing a wide variety of acute and chronic neurological conditions seen in clinical neurology practice on a daily basis.
Neurophysiology Research
EEG has also profoundly contributed to advancing our understanding of brain physiology and functions through numerous research applications mentioned below:
- Mapping sensory and motor systems: Stimulating specific areas evoke corresponding responses that map functional cortical representations.
- Cognitive studies: Event related potentials (ERPs) time-locked to stimuli reveal temporal dynamics of attention, memory, language processing within 100-500 ms windows.
- Consciousness research: Studies on patients in comatose, vegetative or minimally conscious states use connectivity and complexity metrics to assess residual cognition.
- Neuroplasticity: Training-induced EEG changes document cortical reorganization mechanisms involved in learning, recovery from strokes and rehabilitation.
- Brain-computer interfaces: Recorded brain activity is decoded by computers to control prosthetic devices, spell words or computer cursors through thought alone in locked-in patients.
- Brain computer interface gaming: EEG measures player' brain engagement and attention for shaping games and interactive media experiences.
- Neurofeedback therapy: Patients learn to control brainwaves like SMR, theta using real-time feedback to treat ADHD, epilepsy, anxiety by inducing plastic changes.
- Neuroergonomics: EEG correlates of mental workload, fatigue, distraction are utilized to design human-centered technology and workplaces.
Therefore, EEG continues serving as a powerful research tool for expanding our fundamental understanding of complex brain-behavior relationships. Its non-invasive nature has led to new advances across diverse areas in neuroscience.
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