EEG is a technique that measures electrical activity in the brain using electrodes placed on the scalp. It records brain wave patterns which are categorized by frequency into different types like beta, alpha, theta, and delta waves. EEG is used to diagnose brain conditions, locate seizures or lesions, and study cognitive processes. It involves placing electrodes on the scalp, amplifying the tiny electrical signals, filtering out noise, and analyzing the brain wave patterns.

1. ELECTROENCEPHALOGRAPHY
(EEG)

2. Introduction
 A medical imaging technique
 A measurement of the electrical activity of the brain.
 The recording of the brain's spontaneous electrical activity
over a short period of time, usually 20–40 minutes, as
recorded from multiple electrodes placed on the scalp.
An early EEG recording, obtained by Hans Berger in 1924. The upper tracing is
EEG, and the lower is a 10 Hz timing signal.

3. Principle
 The brain's electrical charge is maintained by billions of neurons.
 Neurons pass signals via action potential created by exchange
between sodium and potassium ions in and out of the cell -
Volume conduction.
 When the wave of ions reaches the electrodes on the scalp, they
can push or pull electrons on the metal on the electrodes, the
difference in push, or voltage, between any two electrodes can
be measured by a voltmeter. Recording these voltages over time
gives us the EEG.
 Scalp EEG activity shows oscillations at a variety of frequencies.
Several of these oscillations have characteristic frequency
ranges, spatial distributions and are associated with different
states of brain functioning.

4. Brain Wave Classification
 Brain patterns form wave shapes that are commonly
sinusoidal.
 Measured from peak to peak and normally range from 0.5
to 100 μV in amplitude.
 Signal is derived by means of Fourier transform power
spectrum from the raw.
 Brain waves have been categorized into four basic groups;
- Beta (>13 hz)
- Alpha (8-13 hz)
- theta (4-8 hz)
- Delta (0.5-4 Hz)

5. Type Frequency (Hz) Location Normally Pathologically
Delta Upto 4 frontally in adults, adults slow wave sleep, subcortical lesions,
posteriorly in children; high in babies, diffuse lesions,
amplitude waves Has been found during metabolic encephalopathy
some continuous attention hydrocephalus,
tasks deep midline lesions
Theta 4-8 Found in locations not young children, focal subcortical lesions,
related to task at hand drowsiness or arousal in metabolic encephalopathy,
older children and adults, deep midline disorders,
idling, some instances of
Associated with inhibition of hydrocephalus
elicited responses
Alpha 8-13 posterior regions of head, relaxed/reflecting, coma
both sides, higher in closing the eyes,
amplitude on dominant Also associated with
inhibition control, seemingly
side. Central sites (c3-c4) at
with the purpose of timing
rest
inhibitory activity in different
locations across the brain
Beta 13-30 both sides, symmetrical alert/working, benzodiazepines
distribution, most evident active, busy or anxious,
frontally; low amplitude thinking, active
concentration
waves
Gamma 30-100+ Somatosensory cortex Displays during cross-modal A decrease in gamma band
sensory processing, activity may be associated
Also is shown during short with cognitive decline,
term memory matching of especially when related the
recognized objects, sounds, theta band; however, this
or tactile sensations has not been proven for use
as a clinical diagnostic
Mu 8-13 Sensorimotor cortex Shows rest state motor Mu suppression could be
neurons indicative for motor mirror
neurons working, and
deficits in Mu suppression,
and thus in mirror neurons,
might play a role in autism

6. Applications
 Monitor alertness, coma and brain death
 Locate areas of damage following head injury, stroke,
tumour, etc.
 Test afferent pathways (by evoked potentials)
 Monitor cognitive engagement (alpha rhythm)
 Produce biofeedback situations, alpha, etc.
 Control anaesthesia depth
 Investigate epilepsy and locate seizure origin
 Test epilepsy drug effects
 Assist in experimental cortical excision of epileptic focus
 Monitor human and animal brain development
 Test drugs for convulsive effects
 Investigate sleep disorder and physiology

7. Methodology
 Non-invasive and painless
 To study the brain organization of cognitive processes such as
perception, memory, attention, language and emotion in normal
adults and children.
 Major components;
1. Electrodes with conductive media
2. Amplifiers with filters
3. A/D converter
4. Recording device
 Electrodes read the signal from the head surface, amplifiers
bring the microvolt signals into the range where they can be
digitalized accurately, converter changes signals from analog to
digital form and personal computer stores and displays obtained
data.

8. Recording electrodes
• Types of electrodes:
1. Disposable (gel-less, and pre-gelled types)
2. Reusable disc electrodes (gold, silver, s.s. or tin)
3. Headbands and electrode caps
4. Saline-based electrodes
5. Needle electrodes
• Electrode caps are preferred, with certain number of electrodes
installed on its surface.
• Commonly used scalp electrodes consist of Ag-AgCl disks, 1 to 3
mm in diameter, with long flexible leads that can be plugged
into an amplifier.
• Needle electrodes are used for long recordings and are
invasively inserted under the scalp.

9.  Electrode locations and names are specified by
the International 10–20 system for most clinical and
research applications
 Label 10-20 designates proportional distance in percents
between ears and nose where points for electrodes are
chosen.
 Electrode placements are labeled according adjacent brain
areas: F (frontal), C (central), T (temporal), P (posterior),
and O (occipital).
 The letters are accompanied by odd numbers at the left
side of the head and with even numbers on the right side.

10. Electrode caps Labels for points according to 10-20
electrode placement system

11.  Display of the EEG may be set up in one of several ways.
The representation of the EEG channels is referred to as
a montage.
1. Bipolar montage Each channel (i.e., waveform) represents
the difference between two adjacent electrodes. The
entire montage consists of a series of these channels.
2. Referential montage Each channel represents the
difference between a certain electrode and a designated
reference electrode.
3. Average reference montage The outputs of all of the
amplifiers are summed and averaged, and this averaged
signal is used as the common reference for each channel.
4. Laplacian montage Each channel represents the difference
between an electrode and a weighted average of the
surrounding electrodes.

12. Amplifiers and filters
 The input signal to the amplifier consists of five components:
1.Desired biopotential
2.Undesired biopotential
3.A power line interference signal of 50/60 Hz and its harmonics
4.Interference signals generated by the tissue/electrode interface
5.Noise
 The A/D converter is interfaced to a computer system so that
channels of analog signal are converted into a digital
representation.
 Analog low-pass filters prevent distortion of the signal by
interference effects with sampling rate, called aliasing, which
would occur if frequencies greater than one half of the sampling
rate survive.

14. Artefacts
 Among basic evaluation of the EEG traces belongs scanning
for signal distortions called Artefacts.
 The Artefact in the recorded EEG may be either patient-
related or technical.
Patient related: Technical:
- Any minor body movements - 50/60 hz
- EMG - Impedance fluctuation
- ECG (pulse, pace-maker) - Cable movements
- Eye movements - Broken wire contacts
- Sweating - Too much electrode paste/jelly
- Low battery

15. Alternative Neuroimaging Techniques
 Positron emission technique (PET)
- Picture image of brain giving information about glucose and
oxygen structures in the brain, blood flow, and blood volume in
the brain
- Advantage: compare cross-sections of brain regions
simultaneously
- Disadvantage: findings may be caused by inhibitory neurons
 Functional magnetic resonance imaging (ғmri)
- Picture image of anatomical structures, derived from magnetic
imaging
- Allows for measurement of blood oxygen concentration, blood
flow, and blood volume
- Advantage: see ongoing changes as well as strong spatial
resolution, and quick/effective data collection

16.  Biomagnetism
- Measures magnetic activity given off by the brain
- Super conductive quantum interfering device (SQUID)
- Disadvantage: very difficult to pick up these small magnetic
measures due to environmental magnetic forces
 Magnetoencephalogram (MEG)
- Similar to EEG in that it combines the activities of millions of
neurons
- Advantages: no reference electrode, some currents can only be
found magnetically, scans field patterns of brain allowing for
simultaneous area activity
- Disadvantage: data not as clear and device is very susceptible
to noise

17. Limitations
 Poor spatial resolution
 Most sensitive to a particular set of post-synaptic
potentials, those generated in superficial layers of the
cortex, on the crests of gyri, in dendrites and deep
structures or producing currents that are tangential to the
skull.
 It is mathematically impossible to reconstruct a unique
intracranial current source for a given eeg signal, as some
currents produce potentials that cancel each other out.
This is referred to as the inverse problem.