What
is EEG?
Electroencephalography
is a method that measures and records the fluctuations of electrical activity
resulting from the ions flowing across a membrane in neurons. It measures this
activity over a short period of time such as 20 – 40 minutes and records in a
computer or any other technological device. EEG machines can be used to detect
diseases such as epilepsy where there is an abnormal amount of electrical
activity in the brain. It can also diagnose other conditions such as coma,
sleep disorders and brain death. In other diseases there is an abnormally low
amount of EEG activity. Because they only tell part of the picture EEG are
often combined with other neuroimaging techniques such as MRI, fMRI, PET to get
a better understanding of what is going on in the patients brain.
How
does EEG work?
The way EEG works is
by placing small electrical conductors or “electrodes” on the scalp of the
patient. A conductive gel or paste is also applied on the electrodes to get a
better reading of brain activity. The locations where the electrodes are
applied on the scalp are exact and relevant. Typically 19 electrodes are used
in a normal EEG scan however this number may increase or decrease depending on
the patient’s individual case. After the electrode detects the electrical
signals of the brain they must be amplified 1000 – 10,000 times before they are
recoded. This is because of the miniature nature of electrical brain activity
across the synapses of neurons. While the EEG is recording data, “activation procedures”
are used to recreate the effects of the patients disorder to better record
their abnormal brain activity. For example a patient with epilepsy or any other
disorder maybe told to withdraw on their medication for the disorder. Other
types of “activation procedures” include photic simulation (basically flashing
a strobe light into the patients eyes while they are closed), sleep or sleep
deprivation (patient maybe told not to sleep before the day of the EEG) or mental
activity (the patient is asked to recall events or do simple arithmetic
problems).
EEG
Limitations
Despite being a
useful tool for doctors and researchers the EEG has several limitations. One of
these is that the EEG only records the activity of certain neurons in the
brain. The neurons that are deeper in the brain have a weaker effect on the EEG
signals. Because the EEG averages the activity of several thousand neurons at
once a very large electrical change is required to be shown by the EEG. The EEG
does not reflect normal action potentials that do not influence many cells but
are important. Mostly dendritic currents are
captured by the EEG not axonal ones. Also the cerebrospinal fluid which is
meaningless to brain activity can blur the EEG signal. The EEG compared to
other neuroimaging techniques cannot give specific locations in the brain where
neurotransmitter drugs are found. Often background noises created by the brain
the skew the EEG signals.
Different EEG Band Waves
EEG
can pick up different types of brain waves after each wave is used a new type
of deal activity with a distinct
frequency (measured in hertz). These waves are shown as rhythmic activity
meaning in an up down motion with time in x-axis and hertz in y-axis. Alpha
waves for example have a distinct frequency of 8 – 15 Hertz and are found at
the posterior regions of the head. Relaxation and closing of the eyes is
related to these waves. When people are in a coma these are the main waves that
reflect the status of the individual.  |
| Alpha Waves |
Beta
waves have a frequency of 16 – 31 Hertz and are found on both sides of the
head. They are affected by active thinking or when an individual goes from a
relaxed state into an anxious or alert one. Beta waves are affected by the drug
benzodiazepine that treats/acts as a multitude of things. These include being a
muscle relaxant; hypnotic, anxiolytic and can treat alcohol dependence,
seizures, anxiety attacks, agitation and insomnia. The drug works by enhancing
the effect of the GABA neurotransmitter. This means there is a relationship between
beta waves and the neurotransmitter.
 |
| Beta Waves |
Gamma
waves have the frequency of 32 Hertz and above (up to 100 Hz) and are located on the
somatosensory cortex. They have been shown to be related to short-term memory
and matching of familiar sounds, objects or tactile sensations. Their rhythmic
band pattern has been associated with neurons carrying out major motor
functions. A decline in the gamma wave maybe related to a decline in general
cognitive abilities.
 |
| Gamma Waves |
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