Electroencephalography is a medical imaging technique that reads scalp electrical activity (brainwaves) generated by brain structures. The electroencephalogram (EEG) is defined as electrical activity of an alternating type recorded from the scalp surface after being picked up by metal electrodes and conductive gel.
EEG are used extensively in neuroscience, cognitive science, cognitive psychology, and psychophysiological research. Many EEG techniques used in research are not standardized sufficiently for clinical use. EEG monitoring is silent, which allows for better study of the responses to auditory stimuli and EEG does not aggravate claustrophobia, like fMRI can ( inside a very noisy magnetic cylinder.)
Electrode locations and names are specified by the International 10–20 system for most clinical and research applications. This system ensures that the naming of electrodes is consistent across laboratories. In most clinical applications, 19 recording electrodes (plus ground and system reference) are used. Our brainmachines use 2 or 4 channels, and they monitor the frontal lobe at positions Fp1 and Fp2 ( left and right above each eyebrow)
A typical adult human EEG signal is about 10µV to 100 µV in amplitude when measured from the scalp, and the speed of the EEG is generally from 0.1 upto 45 Hertz, in human conscious and other states.
The frequencies are as follows :
Delta 1-3 Hz, Theta 3-7 Hz, Alpha 8-12 Hz, Beta Low 8-12 Hz, Mid Beta 13-25 and High Beta 25 to 35 Hz, anything faster are Gamma waves.
Electroencephalographic reading is a completely non-invasive procedure that can be applied repeatedly to patients, normal adults, and children with virtually no risk or limitation.
When brain cells (neurons) are activated, local current flows are produced. EEG measures mostly the currents that flow during synaptic excitations of the dendrites of many pyramidal neurons in the cerebral cortex.
Only large populations of active neurons can generate electrical activity recordable on the head surface. With any EEG recording there are many barriers between electrode and neuronal layers as current penetrates through skin, skull and several other layers. Weak electrical signals detected by the scalp electrodes are massively amplified, and then displayed
Due to capability to reflect both the normal and abnormal electrical activity of the brain, EEG has been found to be a very powerful tool in the field of neurology and clinical neurophysiology.