EEG is an instrument used for recording of electrical activity of brain. EEG electrodes are smaller than ECG. The activity measured by EEG are electrical potential created by the post-synaptic currents. Its an effective method for diagnosing many neurological disorder such as epilepsy,tumour,etc.
ELECTRODESIn general 25 electrodes are used in generalEEG test but no of electrode may vary as perthe EEG requirement and area of investigationIn 25 electrode EEGsystem23 electrode areactive electrodes1 is ground electrode1 is ref electrode
It’s a method to describe and apply the location of scalp electrodes. This system is based on the relationship between the electrode and underlying cerebral cortex. The 10 and 20 refer the actual distance between the adjacent electrodes
Its frequency range is 1-4hz. It can be recorded with an electroencephalogram and usually associated with slow wave sleep (SWS).Location: frontally in adults, posteriorly in children; high amplitude waves
Two types of theta wave described as:Hippocampal theta wave which is observed in hippocampus and cortical theta wave which low frequency component of scalp EEG frequency range from 4-7Hz
Alpha waves are detected by electroencephalography and magnetoencephalography. Its mainly originated from occipital lobe during relaxation with closed eyes. Its frequency range is 8-12 hz.Location: posterior regions of head, both sides, higher in amplitude on dominant side. Central sites (c3-c4) at rest .
Its frequency range is 12-30hz. Beta activity is increased when movement have to be resisted. beta waves have three sections: high beta waves(19hz), beta waves(15-18hz), low beta waves (12-15hz).Location: both sides, symmetrical distribution, most evident frontally; low amplitude waves
Its frequency range is 25- 100hz.Location: Somatosensory cortex
EEG voltage signals represent differences between two electrode that can be between two active electrode or between reference electrode and active electrode. In adult EEG amplitude lie between 10 microvolts to 100 microvolts from scalp. And 10-20mv when measured from subdural electrode
Evoked potentials are used to measure the electrical activity in certain areas of the brain and spinal cord. Electrical activity is produced by stimulation of specific sensory nerve pathways.
Types of evoked potentials Visual Evoked Potentials (VEP): the patient sits before a screen in which alternating patterns are displayed. Auditory Evoked Potentials (AEP): the patient listens to a series of clicks in each ear. Sensory Evoked Potentials (SEP): short electrical impulses are administered on the arm or leg. Motor Evoked Potentials: these can detect disruption on a motor pathway of the brain or spinal cord.
Few applications of evoked potential recordingBERA (brain evoked responseaudiometry)VEP test (visual evokedpotential test)Brain evoked test for multiplesclerosis
For rapid and accurate interpretation, it need constant analysis of the EEG. Therefore, computerized EEG signal processing to extract the frequency and amplitude in simple form.