12 Biomedical Engineering Fundamentals
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12.4.1 Electroencephalogram (EEG)
In the biological system, the monitoring and control over the different parts are
processed through the brain. The action potentials are used to generate neural
activity in the brain and this brain activity can be recorded with the help of
electrodes. The signals obtained with these electrodes are known as EEG signals.
The history behind the use of EEG signal is based on an experiment performed
in 1929 in which a German psychiatrist named Hans Berger was performing an
experiment on his daughter’s head to verify the hypothesis that the brain exhibits
electrical activity. He observed that the electrical activity increased when she was
trying to solve some difficult multiplications. Thus, he deduced from this experiment
that the wave patterns observed in the brain recordings reflected the depth of the
brain activity [30].
It has been observed that the approximate range of nerve cells in the brain is in
the order of 1011. The potential of a nerve cell in steady state is typically around
−70 mV, and it is generally negative. On the other hand, the action potential peak
is +30 mV and it approximately falls for 1 ms. Thus, the nerve impulse has a
peak-to-peak amplitude of approximately 100 mV. In the gray matter, each neuron
releases the action potentials during the process of sensing inputs transmitted from
other neurons or external stimuli. The spatially weighted sum of all these action
potentials at the surface of the skull can be measured by EEG signal. The instrument
which is used to record EEG signals is less expensive and accurately measures the
brain’s electrical activity from the skull. These EEG recordings can be possible in
unipolar or bipolar manner. The depolarization signals from the nerve cells may
attenuate while passing through the skull because it has complex impedances. Thus,
the collections of these signals are possible with quality contact of electrodes with
the skull in order to overcome the impedance mismatch created by the hair and
dead skin on the skull. The collected EEG signals from the surface of the skull are
amplified to represent these signals on electric potential versus time graph [30].
The electric activity in a brain is simultaneously present at many different locations of the head. The most common recording technique of EEG signals utilized 21
electrodes to record these simultaneously occurring electrical activities. The number
of these electrodes varies from 64 to 256 for other measuring techniques. The
frequency range of amplifiers used to record EEG signals should cover the range
from 0.1 to 100 Hz for the analysis of all activities [30].
The EEG signals are broadly used for the diagnosis of various diseases or
disorders such as epilepsy, sleep disorders, neurodegenerative diseases, and brain
death. The EEG signals of a normal person with eyes-open condition and an
epileptic patient during seizure are depicted in Figs. 12.25 and 12.26, respectively
[26]. These signals are also obtained from Bonn University database.
The EEG signals are also used in research of brain functional activity. The
analysis of evoked potentials (EPs) and event-related potentials (ERPs) of the brain
using EEG signal is most common of them. In such applications, the responses
of EEG signals are recorded providing specific stimuli such as auditory and visual
571
12.4.1 Electroencephalogram (EEG)
In the biological system, the monitoring and control over the different parts are
processed through the brain. The action potentials are used to generate neural
activity in the brain and this brain activity can be recorded with the help of
electrodes. The signals obtained with these electrodes are known as EEG signals.
The history behind the use of EEG signal is based on an experiment performed
in 1929 in which a German psychiatrist named Hans Berger was performing an
experiment on his daughter’s head to verify the hypothesis that the brain exhibits
electrical activity. He observed that the electrical activity increased when she was
trying to solve some difficult multiplications. Thus, he deduced from this experiment
that the wave patterns observed in the brain recordings reflected the depth of the
brain activity [30].
It has been observed that the approximate range of nerve cells in the brain is in
the order of 1011. The potential of a nerve cell in steady state is typically around
−70 mV, and it is generally negative. On the other hand, the action potential peak
is +30 mV and it approximately falls for 1 ms. Thus, the nerve impulse has a
peak-to-peak amplitude of approximately 100 mV. In the gray matter, each neuron
releases the action potentials during the process of sensing inputs transmitted from
other neurons or external stimuli. The spatially weighted sum of all these action
potentials at the surface of the skull can be measured by EEG signal. The instrument
which is used to record EEG signals is less expensive and accurately measures the
brain’s electrical activity from the skull. These EEG recordings can be possible in
unipolar or bipolar manner. The depolarization signals from the nerve cells may
attenuate while passing through the skull because it has complex impedances. Thus,
the collections of these signals are possible with quality contact of electrodes with
the skull in order to overcome the impedance mismatch created by the hair and
dead skin on the skull. The collected EEG signals from the surface of the skull are
amplified to represent these signals on electric potential versus time graph [30].
The electric activity in a brain is simultaneously present at many different locations of the head. The most common recording technique of EEG signals utilized 21
electrodes to record these simultaneously occurring electrical activities. The number
of these electrodes varies from 64 to 256 for other measuring techniques. The
frequency range of amplifiers used to record EEG signals should cover the range
from 0.1 to 100 Hz for the analysis of all activities [30].
The EEG signals are broadly used for the diagnosis of various diseases or
disorders such as epilepsy, sleep disorders, neurodegenerative diseases, and brain
death. The EEG signals of a normal person with eyes-open condition and an
epileptic patient during seizure are depicted in Figs. 12.25 and 12.26, respectively
[26]. These signals are also obtained from Bonn University database.
The EEG signals are also used in research of brain functional activity. The
analysis of evoked potentials (EPs) and event-related potentials (ERPs) of the brain
using EEG signal is most common of them. In such applications, the responses
of EEG signals are recorded providing specific stimuli such as auditory and visual