Electroencephalogram
199
to accomplish the desired result. The spinal cord also conveys the sensed signals
from the peripheral nervous system (PNS) to the brain.
Due to the importance of the brain and its delicate nature, the brain is protected
by three membranes called the meninges. The space between the meninges is filled
with a liquid called the cerebrospinal fluid. Despite these layers of protection, different types of damages to the brain can cause physiological and mental problems
for the patients. Specifically, damage to a specific region of the cerebrum results in
impaired functions associated with that part. For example, a stroke in the motor area
of the right hemisphere will cause paralysis of all or part of the body’s left side. Such
damages may also affect speech. It was observed that with training and determination, surviving neurons in the neighboring regions can be taught to take over at least
a portion of the original functionalities.
Now that we have familiarized ourselves with the physiology and the functions of
the brain, next we explore how EEG is created and measured.
10.3 ELECTROENCEPHALOGRAM: SIGNAL OF THE BRAIN
It has been estimated that the number of nerve cells in the brain is in the order of 10 11
nerve cells. Especially, the neurons in the cortex are strongly interconnected. A cortical axon may be covered with between 1,000 and 100,000 synapses. The steady-state
nerve potential is negative and is typically around −70 mV. The peak of the action
potential is positive 30 mV and lasts approximately 1 ms. The peak-to-peak amplitude
of the nerve impulse is thus approximately 100 mV. Every neuron in the gray matter
displays a release of action potentials throughout the course of receiving and processing sensory inputs coming from other neurons or external stimuli. ECG is the spatially
weighted summation of all these action potentials measured at the surface of the skull.
Since the initial discovery of the electric activity of the brain, the ability to
measure this activity using EEG has been perfected. The EEG technology is very
inexpensive and accurately measures brainwave activity in the outer layer of the
brain. Sensitive electrodes are attached to the skull, and signals are recorded in
either unipolar or bipolar fashion. The depolarization signals from the brain cells
are attenuated while passing through the connective tissue, the brain fluid, and the
skull and skin, which have complex impedances. In order to collect the relatively
small signals from the brain activity, the skull needs to be prepared for quality
contact to at least overcome the impedance mismatch created by the hair and dead
skin on the skull, which prove to be poor conductors.
The collected signals on the surface of the skull are amplified to give a graph of
electric potential versus time. Usually, the electric activity of the brain needs to be
compared at different spots on the head simultaneously. The most common recording technique applies 21 electrodes and an equal number of channels for EEG measurement. Other measuring techniques are in use that may record from 64 electrodes
to as many as 256 electrodes. The frequency range of the amplifiers used to record
the brain waves needs to be from 0.1 to 100 Hz to ensure proper registration of all
periodic details.
The most common EEG measurements are made with the electrodes placed in
an internationally recognized configuration illustrated in Figure 10.2. This standard
199
to accomplish the desired result. The spinal cord also conveys the sensed signals
from the peripheral nervous system (PNS) to the brain.
Due to the importance of the brain and its delicate nature, the brain is protected
by three membranes called the meninges. The space between the meninges is filled
with a liquid called the cerebrospinal fluid. Despite these layers of protection, different types of damages to the brain can cause physiological and mental problems
for the patients. Specifically, damage to a specific region of the cerebrum results in
impaired functions associated with that part. For example, a stroke in the motor area
of the right hemisphere will cause paralysis of all or part of the body’s left side. Such
damages may also affect speech. It was observed that with training and determination, surviving neurons in the neighboring regions can be taught to take over at least
a portion of the original functionalities.
Now that we have familiarized ourselves with the physiology and the functions of
the brain, next we explore how EEG is created and measured.
10.3 ELECTROENCEPHALOGRAM: SIGNAL OF THE BRAIN
It has been estimated that the number of nerve cells in the brain is in the order of 10 11
nerve cells. Especially, the neurons in the cortex are strongly interconnected. A cortical axon may be covered with between 1,000 and 100,000 synapses. The steady-state
nerve potential is negative and is typically around −70 mV. The peak of the action
potential is positive 30 mV and lasts approximately 1 ms. The peak-to-peak amplitude
of the nerve impulse is thus approximately 100 mV. Every neuron in the gray matter
displays a release of action potentials throughout the course of receiving and processing sensory inputs coming from other neurons or external stimuli. ECG is the spatially
weighted summation of all these action potentials measured at the surface of the skull.
Since the initial discovery of the electric activity of the brain, the ability to
measure this activity using EEG has been perfected. The EEG technology is very
inexpensive and accurately measures brainwave activity in the outer layer of the
brain. Sensitive electrodes are attached to the skull, and signals are recorded in
either unipolar or bipolar fashion. The depolarization signals from the brain cells
are attenuated while passing through the connective tissue, the brain fluid, and the
skull and skin, which have complex impedances. In order to collect the relatively
small signals from the brain activity, the skull needs to be prepared for quality
contact to at least overcome the impedance mismatch created by the hair and dead
skin on the skull, which prove to be poor conductors.
The collected signals on the surface of the skull are amplified to give a graph of
electric potential versus time. Usually, the electric activity of the brain needs to be
compared at different spots on the head simultaneously. The most common recording technique applies 21 electrodes and an equal number of channels for EEG measurement. Other measuring techniques are in use that may record from 64 electrodes
to as many as 256 electrodes. The frequency range of the amplifiers used to record
the brain waves needs to be from 0.1 to 100 Hz to ensure proper registration of all
periodic details.
The most common EEG measurements are made with the electrodes placed in
an internationally recognized configuration illustrated in Figure 10.2. This standard