Electroencephalography
Nico A.M. Schellart, Dept. of Med.
Physics, AMC
Basic Principles
Electroencephalography is the neurophysiologic
measurement of the electrical activity of the brain by recording from
electrodes placed to the scalp, or in special cases on the cortex
(sub-dural). The resulting traces are known as an electroencephalogram (EEG).
The recording is
obtained by placing electrodes on the scalp, usually after preparing the scalp
area by light abrasion and application of a conductive gel to reduce impedance.
Each electrode is connected to an input of a
amplifier, which amplifies the voltage (typically 1,000–100,000 times, or
60–100 dB of voltage gain), and then displays it on a screen or inputs it to a
computer. The amplitude of the spontaneous ongoing EEG is about 100 µV when
measured on the scalp, and about 1-2 mV when measured on the surface of the
brain. The amplitude of evoked potentials and pre-motor activity is up to some
15 µV. In general avering is necessary to elucidate these signals from the spontaneous EEG.
Methods
The
electrode-amplifier relationships are typically arranged in one of three ways:
Common reference
derivation
One terminal of
each amplifier is connected to the same electrode, and all other electrodes are
measured relative to this single point. It is typical to use a reference
electrode placed somewhere along the scalp midline, or a reference that links
one or both earlobe electrodes.
Average reference
derivation
The outputs of all
of the amplifiers are summed and averaged, and this averaged signal is used as
the common reference for each amplifier.
Bipolar
derivation
The electrodes are
connected in series to an equal number of amplifiers. For example, amplifier 1
measures the difference between electrodes A and B,
amplifier 2 measures the difference between B and C, and so on.
This distinction
has become void with the advent of computerized or paperless EEGs, which record all electrodes against an arbitrary
reference and will calculate the above montages post hoc.
The choice of the
reference is crucial for the resulting spontaneous or evoked EG and the
activity maps.
EEG has several
limitations. Scalp electrodes are not sensitive enough to pick out individual action
potentials, the electric unit of signaling in the brain, or whether
the resulting electrical activity is releasing inhibitory, excitatory or
modulatory neurotransmitters. Instead, the EEG picks up
synchronization of neurons,
which produces a greater voltage than the firing of an individual neuron.
Secondly, EEG has limited anatomical specificity when compared with other
functional brain imaging techniques such as functional magnetic resonance
imaging (fMRI). With the use of a large number of electrodes EEG brain activity
maps can be constructed (see Fig. 1) and the source of the activity can be
estimated (see MEG).
Fig. 1 Maps of the
response components a 68 year old subject to the presentation of a rare course
visual pattern within a continuous series of a fine pattern stimulus. The
component names and their supposed funtions are indicated at the top of the
map. The numbers below the maps are the times after the start of the stimulus
at which the maps are constructed from the responses of 64 channels.
EEG has several
strong sides as a tool of exploring the brain activity. As other methods for
researching brain activity have time resoloution between seconds and minutes,
the EEG has a resolution down to sub-millisecond. The brain is thought to work
through its electric activity. EEG is in addition to the highly specialized MEG
method the only method to measure it directly. Other methods for exploring
functions in the brain do rely on blood flow or metabolism which may be
decoupled from the brain electric activity. Newer research typically combines
EEG or MEG with MRI, SPECT or PET to get high temporal and spatial resolution.
Fig. 2 Three seconds of spontaneous
EEG recorded, shown for eight of the 64 channels. The sine-wave like signals represent the alpha activity. In the spectrum this activity
gives rise to the so-called alpha-peak.
Wave types
Historically four
major types of continuous (spontaneous) rhythmic sinus-like EEG waves are
recognized: delta (up to 4 Hz), theta (4-8 Hz), alpha (8-13 Hz; Fig. 2) and
beta (13-40 Hz). An alpha-like normal variant called mu is sometimes seen over
the motor cortex (central scalp) and attenuates with movement, or rather with
the intention to move. The sensorimotor rhythm (SMR) is a middle frequency (about 12-16Hz) associated with
physical stillness and body presence. Gamma
is the frequency range above 40 Hz (approximately 30-80 Hz to be precise).
Application
This device is
used to assess brain damage,
caused by tumors, CVAs and trauma's. It also applied
for supporting the diagnosis of neurological disorders like dementia, etc. and
psychiatric disorders.
Neuroscientists
and biological psychiatrists use EEGs to study the function of the brain by
recording the spontaneous and evoked EEG during controlled behavior of human
volunteers and animals in lab experiments. Theories to explain sleep often rely on EEG
patterns recorded during sleep sessions. In addition, the procedure is
routinely used clinically to assist in the diagnosis of epilepsy.
In some jurisdictions it is used to assess brain death.
The EEG has a history
of tens years and so there is a mass of journal publications and handbooks
about EEG theory and applications.
Reference
Regan D. Human brain electrophysiology: evoked potentials and evoked
magnetic fields in science and medicine.