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Electroencephalography

Clinical Applications of EEG: – EEG is the gold standard for confirming epilepsy. – Routine EEG has a sensitivity of 29-55% in detecting interictal epileptiform […]

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Clinical Applications of EEG:
– EEG is the gold standard for confirming epilepsy.
– Routine EEG has a sensitivity of 29-55% in detecting interictal epileptiform discharges.
– Interictal epileptiform discharges in EEG confirm epilepsy with high specificity.
– EEG aids in diagnosing brain tumors, brain damage, encephalopathies, sleep disorders, depth of anesthesia, and coma.
– It identifies cerebral hypoxia post-cardiac arrest and brain death.

EEG Monitoring Techniques:
– Epilepsy Monitoring Unit (EMU) allows for ictal and interictal recordings.
– Ambulatory video EEGs last 1-3 days for outpatient monitoring.
– EMU admissions last several days to a week for comprehensive monitoring.
– Continuous EEG monitoring helps characterize seizures and localize seizure origin.
– Quantitative EEG analysis complements visual analysis for quicker identification of abnormalities.

EEG Derivatives and Analysis:
– Evoked potentials (EP) and event-related potentials (ERPs) provide insights into sensory and cognitive processing.
– EPs and ERPs offer valuable information for cognitive psychology studies.
– EEG electrodes are placed using the International 10-20 system.
– EEG detects abnormal electrical discharges like sharp waves and spikes.
– EEG offers millisecond-range temporal resolution, valuable for research.

EEG Technology and Techniques:
– High-density arrays can contain up to 256 evenly spaced electrodes for increased spatial resolution.
– EEG signals are filtered, digitized, and output digitally.
– Conventional scalp EEG involves electrode placement with conductive gel or paste.
– Differential amplifiers amplify voltage between active electrodes and a common reference.
– EEG combined with other neuroimaging techniques like fMRI, MEG, and PET for comprehensive insights.

Advantages and Limitations of EEG:
– Advantages include lower hardware costs, high temporal resolution, and non-invasiveness.
– Disadvantages include low spatial resolution, false localization, and poor signal-to-noise ratio.
– EEG reflects synchronous activity of neurons with similar spatial orientation.
– EEG power in gamma band and phase in delta band relate strongly to neuron spike activity.
– EEG has limitations such as poor spatial resolution and inability to capture axonal action potentials directly.

Electroencephalography (Wikipedia)

Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex and allocortex. It is typically non-invasive, with the EEG electrodes placed along the scalp (commonly called "scalp EEG") using the International 10–20 system, or variations of it. Electrocorticography, involving surgical placement of electrodes, is sometimes called "intracranial EEG". Clinical interpretation of EEG recordings is most often performed by visual inspection of the tracing or quantitative EEG analysis.

Electroencephalography
Epileptic spike and wave discharges monitored EEG

Voltage fluctuations measured by the EEG bioamplifier and electrodes allow the evaluation of normal brain activity. As the electrical activity monitored by EEG originates in neurons in the underlying brain tissue, the recordings made by the electrodes on the surface of the scalp vary in accordance with their orientation and distance to the source of the activity. Furthermore, the value recorded is distorted by intermediary tissues and bones, which act in a manner akin to resistors and capacitors in an electrical circuit. This means not all neurons will contribute equally to an EEG signal, with an EEG predominately reflecting the activity of cortical neurons near the electrodes on the scalp. Deep structures within the brain further away from the electrodes will not contribute directly to an EEG; these include the base of the cortical gyrus, mesial walls of the major lobes, hippocampus, thalamus, and brain stem.

A healthy human EEG will show certain patterns of activity that correlate with how awake a person is. The range of frequencies one observes are between 1 and 30 Hz, and amplitudes will vary between 20 and 100 μV. The observed frequencies are subdivided into various groups: alpha (8–13 Hz), beta (13–30 Hz), delta (0.5–4 Hz), and theta (4–7 Hz). Alpha waves are observed when a person is in a state of relaxed wakefulness and are mostly prominent over the parietal and occipital sites. During intense mental activity, beta waves are more prominent in frontal areas as well as other regions. If a relaxed person is told to open their eyes, one observes alpha activity decreasing and an increase in beta activity. Theta and delta waves are not seen in wakefulness, and if they are, it is a sign of brain dysfunction.

EEG can detect abnormal electrical discharges such as sharp waves, spikes, or spike-and-wave complexes that are seen in people with epilepsy; thus, it is often used to inform the medical diagnosis. EEG can detect the onset and spatio-temporal (location and time) evolution of seizures and the presence of status epilepticus. It is also used to help diagnose sleep disorders, depth of anesthesia, coma, encephalopathies, cerebral hypoxia after cardiac arrest, and brain death. EEG used to be a first-line method of diagnosis for tumors, stroke, and other focal brain disorders, but this use has decreased with the advent of high-resolution anatomical imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT). Despite its limited spatial resolution, EEG continues to be a valuable tool for research and diagnosis. It is one of the few mobile techniques available and offers millisecond-range temporal resolution, which is not possible with CT, PET, or MRI.

Derivatives of the EEG technique include evoked potentials (EP), which involves averaging the EEG activity time-locked to the presentation of a stimulus of some sort (visual, somatosensory, or auditory). Event-related potentials (ERPs) refer to averaged EEG responses that are time-locked to more complex processing of stimuli; this technique is used in cognitive science, cognitive psychology, and psychophysiological research.

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