EEG

BACKGROUND:

Electroencephalography (EEG) is an electrophysiological investigation technique
used to record bioelectric activity of the brain at the scalp. It is a non-invasive
method that acquires measures of instantaneous activities within the cerebral
hemispheres (in particular in the cortex).

Brainwaves (EEGs) reflect the brain’s electrical activity. A neuron at rest is like a
little battery. Whenever a neuron is active, its voltage briefly changes. If millions of
neurons all fire at the same time, this produces electrical activity detectable to an
electrode placed on the head.

For example, if you hear a tone, many different groups of neurons activate to process
that tone. EEGs can tell us when and where these groups of neurons fire. Doctors
often use this technique to diagnose hearing disabilities, since EEGs can reveal which
groups of neurons are damaged.

ELECTRODES FOR EEG:

Macroelectrodes only measure the coordinated activity of many millions of neurons.

Microelectrodes only measure the activity of one or very few neurons.

The most common recording setup is a scalp macroelectrode. While it is possible to
get data from as few as two electrodes, most labs use an electrode cap. These caps are
specially designed so that each electrode is over a general region of the brain. This
makes it easier to estimate the source of any EEG activity detected at each electrode.

These are standardized electrode locations, called the International 10-20
system.

The International 10–20 System of Electrode Placement is the most widely used
method to describe the location of scalp electrodes. It is based on the relationship
between the location of an electrode and the underlying area of cerebral cortex. Each
site has a letter (to identify the lobe) and a number or another letter to identify the
hemisphere location.

THE BEHAVIOR OF THE EEG SIGNAL:

1. Event related potentials (ERPs): Brain’s response to a specific event, such
as a tone or flash.

2. Spontaneous or free-running EEG: Naturally produced, rhythmic
brainwaves; do not require outside activity.

Well known free running EEGs include:

1. The Alpha waves have the frequency spectrum of 8-13 Hz and can be
measured from the occipital region in an awake person when the eyes are
closed. Amplitude: 30 – 50 μV.

2. The frequency band of the Beta waves is 13-30 Hz; these are detectable over
the parietal and frontal lobes; indicate alertness. Amplitude: Less than 20 μV.

3. The Delta waves have the frequency range of 0.5-4 Hz and are detectable in
infants and sleeping adults (deep sleep). Amplitude: Up to 100 – 200 μV

4. The Theta waves have the frequency range of 4-8 Hz and are obtained from
children and sleeping adults, during hypnosis and meditation. Amplitude: Less
than 30 μV.

5. The Mu waves have the frequency range of 8 – 13 Hz and are largest when
individual is not moving

NORMAL EEG

1. In adults who are awake, the EEG shows mostly alpha waves and beta waves.
2. The two sides of the brain show similar patterns of electrical activity.
3. There are no abnormal bursts of electrical activity and no consistently slow
brain waves detected on the EEG tracing.
4. If flashing lights (photic stimulation) are used during the test, one area of the
brain (the occipital region) may have a brief response after each flash of light,
but the brain waves remain normal.