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. It is one of the few mobile techniques available and offers millisecond-range temporal resolution, which is not possible with CT, PET, or MRI.ĭerivatives 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). Despite its limited spatial resolution, EEG continues to be a valuable tool for research and diagnosis. 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). ![]() It is also used to help diagnose sleep disorders, depth of anesthesia, coma, encephalopathies, cerebral hypoxia after cardiac arrest, and brain death. EEG can detect the onset and spatio-temporal (location and time) evolution of seizures and the presence of status epilepticus. ĮEG 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. Theta and delta waves are not seen in wakefulness, and if they are, it is a sign of brain dysfunction. If a relaxed person is told to open their eyes, one observes alpha activity decreasing and an increase in beta activity. During intense mental activity, beta waves are more prominent in frontal areas as well as other regions. Alpha waves are observed when a person is in a state of relaxed wakefulness and are mostly prominent over the parietal and occipital sites. 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). The range of frequencies one observes are between 1 and 30 Hz, and amplitudes will vary between 20 and 100 μV. Ī healthy human EEG will show certain patterns of activity that correlate with how awake a person is. 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. 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. ![]() 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. 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. Voltage fluctuations measured by the EEG bioamplifier and electrodes allow the evaluation of normal brain activity. Clinical interpretation of EEG recordings is most often performed by visual inspection of the tracing or quantitative EEG analysis. ![]() Electrocorticography, involving surgical placement of electrodes, is sometimes called " intracranial EEG". 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. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex and allocortex. Electroencephalography ( EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain.
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