Eeg

1
INTRODUCTION TO BIOMEDICAL
ENGINEERING (MEL426)
Study of Electroencephalogram (EEG)
waveform
Course Instructor:
Dr. Navin Kumar
Submitted by:
K.Raviteja
2012MEB1103

Study of Electroencephalogram waveform
Experiment No-3
Aim: To stimulate EEG waveforms
Objective:
 To stimulate EEG waveform/pattern,
 To understand various abnormalities associated with EEG, and
 To assist studying in-sleep patterns.
Theory: The electroencephalogram (EEG) is a dynamic non-invasive and relatively
inexpensive technique used to monitor the state of the brain. EEG has a number of clinical uses
that range from monitoring normal wakefulness or arousal states to complex clinical situations
involving seizure or coma. The brain contains unique information in many regions at any given
time. An EEG signal recorded with electrodes placed on scalp consists of many waves with
different characteristics. Arrays of electrodes are distributed over the entire scalp. The large
amount of data recorded from even a single EEG electrode pair presents a difficult
interpretation challenge. Signal processing methods are needed to automate signal analysis and
interpret the signal phenomena.
Basic Construction and Functions:
A neuron also known as a neurone or nerve cell is an electrically excitable cell that processes
and transmits information through electrical and chemical signals. A chemical signal occurs
via a synapse, a specialized connection with other cells. Neurons connect to each other to
form neural networks. Neurons are the core components of the nervous system, which
includes the brain, spinal cord, and peripheral ganglia.
Fig 1: Structure of a neuron cell

Fig 2: Composition of neuron cells in brain
Activities:
The activities in the CNS are mainly related to the synaptic currents transferred between the
junctions (called synapses) of axons and dendrites, or dendrites and dendrites of cells. A
potential of 60–70 mV with negative polarity may be recorded under the membrane of the
cell body. This potential changes with variations in synaptic activities. If an action potential
travels along the ﬁbre, which ends in an excitatory synapse, an excitatory post-synaptic
potential (EPSP) occurs in the following neuron. If two action potentials travel along the
same ﬁbre over a short distance, there will be a summation of EPSPs producing an action
potential on postsynaptic neuron providing a certain threshold of membrane potential is
reached.
Fig 3: Neuron Membrane Potentials
Action Potentials:
The information transmitted by a nerve is called an action potential (AP). APs are caused by
an exchange of ions across the neuron membrane and an AP is a temporary change in the
membrane potential that is transmitted along the axon. It is usually initiated in the cell body
and normally travels in one direction. The membrane potential depolarizes (becomes more
positive), producing a spike. The potential becomes more negative than the resting potential
and then return to normal. The action potentials of most nerves last between 5 and 10
milliseconds.
The conduction velocity of action potentials lies between 1 and 100 m/s. APs are initiated by
many different types of stimuli; sensory nerves respond to many types of stimuli, such as

chemical, light, electricity, pressure, touch, and stretching. On the other hand, the nerves
within the CNS (brain and spinal cord) are mostly stimulated by chemical activity at
synapses.
Fig 4: Processing of charged ions/current flow
EEG Generation:
An EEG signal is a measurement of currents that flow during synaptic excitations of the
dendrites of many pyramidal neurons in the cerebral cortex. When brain cells (neurons) are
activated, the synaptic currents are produced within the dendrites. This current generates a
magnetic field measurable by electromyogram (EMG) machines and a secondary electrical
field over the scalp measurable by EEG systems.
Action potentials:
1. When the dendrites of a nerve cell receive the stimulus the Na+ channels will open. If the
opening is sufficient to drive the interior potential from −70 mV up to −55 mV, the process
continues.
2. As soon as the action threshold is reached, additional Na+ channels open. The Na+ influx
drives the interior of the cell membrane up to approximately +30 mV. The process to this
point is called depolarization.
3. Then Na+ channels close and the K+ channels open. Since the K+ channels are much
slower to open, the depolarization has time to be completed. Having both Na+ and K+
channels open at the same time would drive the system towards neutrality and prevent the
creation of the action potential.
4. Having the K+ channels open, the membrane begins to repolarize back towards its rest
potential.
5. The repolarization typically overshoots the rest potential to a level of approximately −90
mV. This is called hyper polarization and would seem to be counterproductive, but it is
actually important in the transmission of information and it ensures that the signal is
proceeding in one direction.
6. After hyper polarization, the Na+/K+ pumps eventually bring the membrane back to its
resting state of −70 mV.

EEG Measurement
EEG recording techniques: Encephalographic measurements employ recording system
consisting of:
 Electrodes with conductive media
 Amplifiers with filters
 A/D converter
 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.
Scalp recordings of neuronal activity in the brain, identified as the EEG, allow measurement
of potential changes over time in basic electric circuit conducting between signal (active)
electrode and reference electrode Extra third electrode, called ground electrode, is needed
for getting differential voltage by subtracting the same voltages showing at active and
reference points. Minimal configuration for mono-channel EEG measurement consists of
one active electrode, one (or two specially linked together) reference and one ground
electrode. The multi-channel configurations can comprise up to 128 or 256 active
electrodes.
Fig 5: EEG setup
Brain Rhythms: There are five major brain waves distinguished by their different frequency
ranges. These frequency bands from low to high frequencies respectively are called delta (δ),
theta (θ), alpha (α), beta (β) and gamma (γ). Many brain disorders are diagnosed by visual
inspection of EEG signals. The clinical experts in the field are familiar with manifestation of
brain rhythms in the EEG signals.

Delta: refers to frequencies below 4 Hz. Delta activity, which is the slowest waveform, is
normal when present in adult during sleep. In normal elderly subjects, delta activity is
sometimes seen in the temporal regions during wakefulness, and in a generalized distribution,
maximal anteriorly, during drowsiness. It is usually abnormal under other circumstances.
Theta: ranges from 4 Hz to less than 8 Hz. It is often present diffusely in children and young
adults during wakefulness, whereas in adults it occurs predominantly during drowsiness. Like
delta activity, theta activity may occur in the temporal regions in normal elderly adults during
wakefulness.
Alpha: ranges from 8 to 13 Hz. Alpha waves appear in the posterior half of the head and are
usually found over the occipital region of the brain. They can be detected in all parts of
posterior lobes of the brain. Alpha waves have been thought to indicate both a relaxed
awareness without any attention or concentration.
Beta: above 13 Hz and below 30 Hz. This activity is usually most prominent anteriorly and is
often increased during drowsiness and in patients receiving sedating medication, particularly
barbiturates or benzodiazepines. Rhythmical beta activity is encountered mainly over the
frontal and central regions.
Gamma: correspond to the frequencies above 30 Hz (mainly up to 45 Hz, sometimes called
the fast beta wave). Although the amplitudes of these rhythms are very low and their
occurrence is rare, detection of these rhythms can be used for confirmation of certain brain
diseases. The regions of high EEG frequencies and highest levels of cerebral blood flow (as
well as oxygen and glucose uptake) are located in the front-o-central area. The gamma wave
band has also been proved to be a good indication of event-related synchronization (ERS) of
the brain and can be used to demonstrate the locus for right and left index finger movement,
right toes, and the rather broad and bilateral area for tongue movement.
There are two more wave-frequencies generated by human brain which are rarely used that is
Mu wave – (7.5 – 12.5 Hz) and SMR wave – (12.5 – 15.5 Hz).
Fig 6: Description of various brainwaves and their characteristics

Applications of EEG: The study of EEG is very much in need for diagnosis of many
neurological disorders and other abnormalities in the human body and acquired EEG signals
are used in diagnosis of various diseases, some of them are as follows: Monitoring alertness,
coma, and brain death.
 Locating areas of damage following head injury, stroke, and tumor
 Testing afferent pathways (by evoked potentials)
 Monitoring cognitive engagement (alpha rhythm)
 Producing biofeedback situations
 Controlling anesthesia depth (servo anesthesia)
 Investigating epilepsy and locating seizure origin
 Testing epilepsy drug effects etc.
Procedure:
 A person is selected to perform the experiment over him and he sits on a chair
comfortably. In our case, the person was Devdath.
 Run the EEG application.
 Select the type of patient from the waveform selector menu.
 Set the desired sampling rate and the number of samples to be displayed on screen.
 Place the EEG electrodes on the subject’s head in the placements given below. To
reduce the characteristic impedance/resistance between skin and electrode, a water
soluble lubricant/gel is used.
Fig 7: Positioning of electrodes on skull
 Run the Simulation.
 Record the waveforms of the different electrodes under different conditions.

Observations and Discussion: The following was obtained from the EEG experiment
performed on Devdath.
Opened Eyes: The brain waveform and brain map for opened eyes are shown below:
Fig 8: Waveform and brain map during opened eye
Closed Eyes: The brain waveform and brain map for closed eyes are shown below:
Fig 9: Waveform and brain map during closed eyes

Blinking eyes: The brain waveform and brain map for blinking eyes are shown below:
Fig 10: Waveform and brain map during closed eyes
Movement: The brain waveform and brain map during movement (mainly hand) are shown
below:
Fig 11: Waveform and brain map during movement

Talking: The brain waveform and brain map during talking/speech are shown below:
Fig 12: Waveform and brain map during talking
Asleep: The brain waveform and brain map during asleep are shown below:
Fig 13: Waveform and brain map during asleep

Result:
 The EEG of deva dath was simulated and recorded. The actions performed by the
patient was also accurately interpreted at real-time using EEG
 The magnitude and frequency of brainwave depend upon the activity we carry out.
 Portable EEG machines are relatively low sensitivity and specificity, which results in
errors.
References:
[1] Lab Manual
[2] https://en.wikipedia.org/wiki/Neural_oscillation
[3] http://www.spectrumhealth.biz/services/brain-mapping-know-more.php
[4] http://www.brainwavecollege.com/what-are-brainwaves.html
[5] www.hopkinsmedicine.org › Health Library
[6] https://www.epilepsysociety.org.uk/eeg-electroencephalogram

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