Sleep deprivation is also used as a provocative technique. The recordings of drowsiness and sleep are important components of any EEG procedure. These include eye-opening and closure, hyperventilation, and photic stimulation. During the recording, various activation procedures are performed in order to trigger epileptiform abnormalities and other EEG changes. These include recording a square wave signal and biological calibration. Calibration should be performed prior to beginning the study. Once the electrodes are placed, the impedance of all electrodes should be measured and ensured that it is less than 5 kohms. Typically at least 21 electrodes are placed on an adult scalp, including a reference and ground electrodes. The 10 to 20 international system is most widely used for scalp electrodes placement. The test should be performed by an EEG technician with appropriate and relevant training. Ī routine electroencephalogram is performed in a quiet room with controllable lighting levels. There are also several benign EEG variants and artifacts that one should learn in order to avoid reporting a false-positive test. Other notable components/waveforms that appear during the first year of life and are useful to differentiate sleep stages are sleep spindles and K-complexes. There is also an anterior-to-posterior distribution of waveforms with faster frequencies present in the anterior and slower frequencies in the posterior head regions. The slower waveforms are less during the wakeful state and dominate during later stages of sleep. The normal adult resting PDR of 8.5 Hz in the posterior head regions is noted after eight years of age. The EEG waveforms start with discontinuous backgrounds during the prenatal phase and mature to be continuous at a later age. The predominance of waveforms in an EEG varies based on the age and state of wakefulness of the individual. The commonly encountered waveform frequencies in EEGs are alpha (8 to 12 Hz), beta (13 to 30 Hz), theta (4 to 7 Hz), and delta (less than 4 Hz). EEG evaluation provides important information about the localization and the spread of such discharges.
In the event of a seizure, a large super-synchronous neuronal discharge is created from an abnormal brain network. During activation, the cortical activity desynchronizes, and the oscillatory activity is replaced with lower amplitude and faster frequency activity.
During the resting or relaxed state, the EEG records a sinusoidal rhythmic activity called the posterior dominant rhythm (PDR) that is believed to be due to oscillatory interaction between the cortex (visual cortex in this instance) and subcortical structures (thalamus). The cortical neurons and the subcortical structures are systematically connected through well-developed feedback linkages. The summation of EPSPs and IPSPs over a selected cortical region with synchronous discharge creates an electrical field with positive and negative ends (dipole). The dipole is typically parallel to the pyramidal cell orientation. Following the release of neurotransmitters at the endplate, excitatory or inhibitory postsynaptic potentials (EPSP/IPSP) are generated secondary to neuronal depolarization (in the case of EPSP with intracellular sodium influx resulting in extracellular negativity) or hyperpolarization (in the case of IPSP intracellular negativity). The pyramidal cell bodies are mostly present in layers 3 and 5 of the cerebral cortex. An estimated cortical area of 10 cm2 discharging synchronously is required to generate a deflection on scalp EEG. They measure the absolute electrical potentials generated by the neurons of the underlying cerebral cortex. The electroencephalogram recording electrodes are placed over the scalp.