Biological Substrate being Captured
Neurons convey and process information by means of axonal conductance and synaptic
transmission (Lukatch, Kiddoo, & Maciver, 2005).
Excitatory electrochemical events associated with post-synaptic potentials radiate
current from the neural tissue where they occur through brain tissue, the meninges and
Each electrode detects the electro-cellular activity of approximately 10 billion cortical
Biological Substrate being Captured
As the resultant diffuse electrical currents migrate from the brain and meninges through
the skull, they are further scattered as they pass through the skull where they activate
surface electrodes placed on the skull. EEG reflects activity of neurons close to the
electrodes and is unable to detect deep structures (Luck, 2005)
Locating the electrodes
In order to locate the exact electrode position, EEG uses four anatomical landmarks
from which measurements can be made.
The nasion is the indentation between the forehead and the nose
The inion is a ridge that can be felt at the midline of the back of the skull
Over the occipital area
The preauricular points are defined as the indentations just above the cartilage
that covers the external ear openings.
The electrode locations and distances between the electrodes are then defined as 10%
or 20% of these anatomical distances. The typical electrode placements for clinical work
follows the "10-20" system.
Electrode locations are denoted by brain region F = frontal lobe; T = temporal
lobe; C = central lobe; P = parietal lobe; O = occipital lobe; A = auricular (ear).
Basic Frequency Bands
EEQ wave forms are analyzed according to time periods called epochs,
Historically, the frequencies have been divided into 5 groups:
Delta (0-4 Hertz); Theta (4-8 Hertz);
Alpha (8-13 Hertz;
Beta (13-32 Hertz); (13-21 Beta 1; 22-32 Beta 2)
Gamma (32-64 Hertz).
Associated Amplitudes of EEG Frequency Bands
The EEG curves are classed according to waves. There are four recognized wave
1. Delta waves. Frequency 0.5 – 4 Hz (amplitude 20-200 µV).
2. Theta waves. Frequency 4-8 Hz (amplitude 20-100 µV).
3. Alpha waves: Frequency 8 - 13 Hz (amplitude 20-60µV).
4. Beta waves: Frequency is 13-32 Hz (amplitude 2-20 µV).
Functional States and EEG Frequency Bands
Slower waveform activity (fewer cycles per second), as in the delta, theta or alpha
traces , indicate lowered blood flow and fuel (glucose) use in that part of the brain.
Faster activity as in the beta trace, shows increased brain activity.
These types of brain electrical activity also reflect the level of arousal and associated
functions of the person:
Delta activity (2-4 cps) accompanies deep sleep,
Theta (4-7cps) states of drowsiness,
Alpha (8-11 cps) relaxed but alert states
Beta range activity reflects an engaged or active brain, and, with very fast beta
activity, excited or urgent/emergency
Typical Clinical Purposes:
Diagnose epilepsy and see what type of seizures are occurring (Dam et al., 2007).
Epilepsy - EEG is commonly used to confirm the suspicion of epilepsy, however
further tests such as CT, MRI and PET may be required to get more specific
information on what is causing the seizures and where in the brain they are
occurring (de Jongh, de Munck, Goncalves, & Ossenblcok, 2005).
Check for problems with loss of consciousness or dementia (Kurlychek, 2006).
Help find out a person's chance of recovery after a change in consciousness (Travis &
Orme-Johnson, 1989). Find out if a person who is in a coma is brain-dead (Kaplan, 2004).
Study sleep disorders, such as narcolepsy (Smit et al., 2005).
Watch brain activity while a person is receiving general anaesthesia during brain
surgery (Becker et al., 2010).
Help find out if a person has a physical problem (problems in the brain, spinal cord, or
nervous system) or a mental health problem (Arbour, Headley, Leeper, & Rueden,
The amplitude and patterns of the EEG waves can be considered the dependent
variable, which can be distinguished by specific task parameters, stimulus and assist in
diagnosis or linkages for particular symptoms.
Emerging research is finding that EEG signals may confirm or exclude ischemic strokes
for lacunar and posterior circulation conditions (Sheorajpanday et al., 2011).
EEG is also an effective inverse index for detecting cerebral activity during the period
between each response and the next stimulus in Stroop task (Compton et al., 2011).
Support for right and left hemispheres and their role in behavioural activation and
inhibition were supported by the differences in alpha power during the intertrial intervals.
EEG studies have also confirmed theories that the magnitude of right-left
desynchronization for action perception and production varies among the
developmental stages, with smaller magnitudes in infants compared to either adults or
older children (Marshall et al., 2011).
Strengths of EEG
Provides direct rather than indirect evidence of epileptic abnormality.
Portable / ambulatory.
Provides summary of ongoing states in "real time".
Safely used on wide range of patients including infants.
Detects changes within a millisecond; important considering an action potential
takes between 0.5-130 milliseconds to propagate across a single neuron,
depending on the type of neuron. Compared to PET or fMRI which as time
resolutions between seconds and minutes. EEG measures electrical activity, where other methods record blood flow
Weaknesses of EEG
Variability wave frequencies according to age. Waves are lower in children and
adults over age 60.
Detects cortical dysfunction but rarely discloses its etiology.
Relatively low sensitivity and specificity.
Subject to both electrical and physiologic artifacts.
Influenced by state of alertness and drugs.
Small or deep lesions might not produce an EEG abnormality.
Limited time sampling (for routine EEG) and spatial sampling.
Poor spatial resolution
Locating cortical generators which are associated with scalp topographical
distributions is controversial (Wan, et al., 2006)
Plots of Evoked Potential amplitude vs. flicker frequency are different for different
retinal quadrants (Regan, 1976)