Download EFFECT OF ELECTRICAL STIMULATIONS ON EEG Harikanth1

EFFECT OF ELECTRICAL STIMULATIONS ON EEG
Harikanth1, Sanjeev Kumar2, Amod Kumar2, Sneh Anand3
1M.Tech.
Biomedical Engineering,
Vellore Institute of Technology, Vellore
E mail: [email protected]
’B’,
Biomedical Instrumentation Unit,
Central Scientific Instruments Organization
Sector 30 C, Chandigarh
2Scientist
2Scientist
’G’ & Head,
Biomedical Instrumentation Unit,
Central Scientific Instruments Organization,
Sector 30 C, Chandigarh
3Professor,
Centre for Biomedical Engineering
Indian Institute of Technology, Delhi
Abstract: Generally, Electro Enchephalo Graphic signals (electrical activity of the brain,
EEG) are complex in its structure and nature. The observation and analysis of EEG is also
difficult because it is highly subjective, random in nature and not able to predict the signal
pattern. But, proper EEG analysis in different conditions is very much helpful for further
research and in diagnosing the abnormalities and disorders related to the central nervous
system. In present study, an electrical stimulation isolated circuit was designed for giving
electrical stimulations (somato sensory evoked potentials). For attaining the maximum
stimulation, pulse width and burst were maintained at constant values. The stimulations were
given at various rates. The EEG signals were recorded from P3, C3, P4 and C4 locations in
both normal and under electrical stimulation conditions in real time. Beta, Alpha, Theta and
Delta waves were extracted from the raw EEG signals. The frequency and amplitude of beta
wave was found to be increased and the amplitude of theta wave was found to be reduced at
stimulation period in all the locations.
Key Words: Electro Enchephalo Graph, Somato Sensory, Evoked Potentials, Electrical
Stimulator, Beta wave, Theta wave
INTRODUCTION:
Electro Encephalo Graphy is the study of the electrical activity of the brain [1-2]. Monitoring of
the Electro Enchephalo Gram is proven to be an effective method of diagnosing different
neurological illnesses and diseases such as epilepsy, tumor, cerebrovascular lesions,
ischemia and problems associated with trauma [3]. It is also effectively used in the operating
room to facilitate anesthetics and to establish the integrity of the anesthised patient’s nervous
system. Consequently, EEG monitoring and its analysis has becoming very popular.
Different types of electrodes can be used for EEG such as peel and stick electrodes, silver
plated cup electrodes and needle electrodes depends on the requirement. The EEG can be
picked up with these electrodes either from the scalp or directly from the cerebral cortex.
Electrode gel is used to reduce the impedance between the scalp and electrodes. EEG
signals are stochastic in nature, highly subjective and vary randomly for the same person
from time to time. Because of the lack of repeatability in a given person from one occasion to
another and there is no standard pattern for EEG makes the analysis of the EEG a difficult
task.
The variations in EEG signals both in amplitude and frequency are of diagnostic value. The
frequency of the EEG is seems to be affected by the mental activity of the person. The
frequency range of EEG is about 0.5 to 20 Hz (Below and above of this range likely to be
artifactual, under standard clinical recording techniques) [4-5]. This range is classified into
different frequency bands such as Delta (δ) up to 4 Hz, Theta (θ) 4-7Hz, Alpha (α) 8-12 Hz
and Beta (β) 12-30 Hz depends on the activity of the brain. These frequencies represent
different activities of the brain in different conditions.
Generally, human brain is divided into four lobes according to the anatomical positions,
frontal, parietal, temporal and occipital lobe. Each lobe is responsible for different actions.
According to the diagnostic requirement, the EEG will be recorded either from the respective
locations, or from the whole brain. Acquisition of the EEG signals is also difficult because the
neural potentials that are acquired from the scalp are in micro volts range [6]. The acquired
EEG is needed to be amplified for further analysis. The placement of electrodes plays an
important role in acquiring EEG. There is a standard system for placing electrodes called 1020 lead system. The notation of electrode placement was done based on the region of the
brain such as frontal, central, parietal, temporal and occipital etc. 10% and 20% indicates the
percentage of distance between Nasion and Inion.
The comparative study of the EEG signals under different conditions helps the researchers in
analyzing the brain’s activity which will helps further to study the nature of the brain in deep. It
also helps in the field of depth of anesthesia monitoring for maintaining the accurate
balanced anesthesia state.
Another important measurement of the EEG is the recording of ‘evoked response’ [7-8]. An
evoked potential is an electrical potential recorded from the nervous system of a human or
other animal following presentation of a stimulus, as distinct from spontaneous potentials as
detected by electroencephalography (EEG) [9]. The stimuli could be a flash of light or a click
of sound. Since, these responses are repeatable; the evoked responses can be easily
distinguished from the rest of the EEG activity. This analysis helps in the evaluation of the
functioning of different parts of the brain.
The main objective of this study is to analyze the EEG under normal and under electrical
stimulation conditions. This analysis might be helpful for further analysis for the depth of
anesthesia monitoring. Different types of stimulations can be applied for the experimentation
such as electrical, mechanical, thermal, chemical and cold pressure. In this study, electrical
stimulation was used because of its efficient results and constant accurate administration.
The frequency and amplitude of EEG of all frequency bands at different locations were
observed.
METERIALS AND METHODS:
An electronic circuit was designed for giving the electrical stimulation [10] which is shown in
fig. 1. This circuit consists of the following components.
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M100 series 10 Watt DC-DC converter: having different features which include high
outputs (up to 1000Vdc), single and dual inputs, wide input ranges, output isolation,
continuous short circuit protection and economical. It is mainly used to convert the
normal range of DC to high output ranges based on the requirement.
IC 7805: it is a 5 V voltage regulator used to produce constant Vcc to the whole
circuit.
IC 7404: it consists of six not gates out of which two not gates were used for the
protection of the function generator.
1:1 transformer: This is 1:1 opto isolator used to protect the circuit components from
the back current and for making the isolation of the patient from the external circuitry.
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10K potentiometer: This potentiometer was used to vary the input voltage to get the
required output voltage.
Function Generator: function generator was used to produce the TTL pulses which in
turn give the pulsed DC voltage to the transformer as an input. For producing the
pulsed DC, three factors were taken into consideration, pulse width, burst and rate.
Pulse width is defined as the duration of one single pulse. Burst is defined as the
number of pulses that are applied in a single stretch. The rate at which these bursts
can be applied is called the rate. These three factors were varied individually and
tested for getting the maximum stimulation. In this observation, it was found that, at
35 µsec. of pulse width and a burst of 50, the subject was feeling maximum stimulus.
The rate was changed in different intervals and the EEG was observed at these rates.
Fig. 1 Circuit for electrical stimulator
Hardware used:
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Electro cap of 32 electrodes was used for recording EEG.
Bio-amplifier (g-tec, g.BS amp, biosignal amplifier) was used to amplify the signal.
This is a general purpose amplifier which is able to amplify any kind of biological
signals. In this study, this amplifier was used to amplify the raw EEG signals.
Data Acquisition Card (AD Instruments, Powerlab 16/32) was used for acquiring the
EEG signals from bio-amplifier to feeding to computer through USB port.
Labchart software was used for recording, filtering and analysis purpose. All kinds of
filters on the raw signals can be applied from this software itself. Different
components from the raw EEG signal were extracted by applying respective options.
METHODS:
Subject was instructed to lie down on the bed with eyes closed. According to the 10 - 20 lead
system, electro cap was connected. The signals were recorded from four channels P3, C3, P4
and C4 as an active points and CZ as a reference point. Nasion point has considered as a
ground (GND). All the electrodes were connected properly with electro gel for reducing the
electrode impedance and to make proper contact with the scalp. Recording of the normal
EEG was started after the subject was completely relaxed. All the signals were recorded with
a sampling rate of 1000 and the complete signals were saved in the computer.
The electrical stimulations were applied on the skin surface at wrist because the capability of
sensing the stimulations is more at this region. Disposable pad electrodes were used for
applying the stimulations. This electrodes itself consists of gel which reduces the impedance
between skin and electrode. The output terminals were directly connected to these
electrodes directly. The electrodes were placed side by side for inducing maximum
stimulation. As the distance increases, the capability to sense the electrical stimulation
reduces.
The recording of EEG has done mainly in two sessions.
Session1: normal EEG is recorded at relaxed state with eyes closed.
Session2: this session includes recording of EEG at different rates.

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3 sec.
2 sec.
10 m sec. (continuous)
Raw EEG was recorded at these rates and signals were observed. Alpha, Beta, Theta and
Delta waves were extracted from the raw EEG by applying band pass filters of their
respective frequencies in the Labchart software. The filtered and extracted EEG signals were
taken into consideration for the observation purpose.
RESULTS:
The EEG signals under normal condition and under electrical stimulations were compared.
All frequency components P3, C3, P4 and C4 of the raw EEG signal were observed at
different rates. Amplitude and frequency of the signals were taken into consideration for the
analysis. The normal EEG was recorded which was shown in fig. 2. Then EEG was recorded
by applying electrical stimulations at different rates. Continuous stimulation was given and
EEG was recorded which is shown in fig. 3. The electrical stimulations were also applied at
rates of 3sec., 2sec. and 1sec. During these electrical stimulations, it was found that, among
four frequency components, beta and theta waves have shown a significant variation in the
signal. It was also found that the same results were reflecting in all locations that are
recorded. These results were shown in fig. 5(a-d). The recording under electrical stimulation
was classified into two categories depends upon the rate of stimulation that is applied.
Evoked responses:
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The frequency of Beta wave was found to be increased during the stimulation.
The amplitude of the Theta wave was found to be reduced after applying the
stimulation.
Continuous stimulation:
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During the continuous electrical stimulation, the complexity of the signal was found to
be increased.
The frequency and amplitude were varying randomly during the stimulation.
Fig. 2 EEG under normal condition
Fig. 3 EEG under continuous stimulation
(a)
(b)
(c)
(d)
Fig 4 (a-d) α,β,θ and δ waves under electrical stimulations at p3, C3, P4 and C4
DISCUSSIONS:
There are different methods for giving the pain stimulus such as electrical, mechanical,
chemical, thermal, cold pressure, etc. In this study, electrical stimulation was selected [11].
Electrical circuit was designed which gives the pulsed DC output voltage which will not
harmful to the subject. Opto isolation transformer was used to protect the circuit from back
current and to isolate the subject from the external circuitry. Not gate was used for protecting
function generator. This observation was done only in awake state. Both amplitude and
frequency have shown significant variation for beta and theta waves. These evoked
responses observation can be done in sleep state also which will be useful for the analysis of
hypnosis in depth of anesthesia monitoring.
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http://www.beteredingen.nl/downloads/eeg_electrodes_10-20.pdf
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http://en.wikipedia.org/wiki/Electroencephalography
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[8].
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http://en.wikipedia.org/wiki/Evoked_potential
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