The EEG is a recording of the
electrical activity in the brain. It results from the activity of large
populations of neurons. There are three ways in which a signal can be measured:
‘Action potentials along axons connecting neurones, currents through synaptic
clefts connecting axons with neurone, currents along dendrites from the synapse
to the soma of neurones’ (Hauk 2013).

The electrical activity occurs from the cortical neurons in the brain which generates
electrical currents. These currents spread to the surface of the scalp where
they are detected by the electrodes in the form of voltage changes. An EEG
signal can be explained by considering the electrical activity of a solitary
pyramidal cell activated by an afferent pathway. The received signal of the
synapse allows a change in membrane potential of the postsynaptic membrane as a
consequence of the cations rushing into the cell. As this current transmits
down the conductive dendron of the neurone, the size of the excitatory
postsynaptic potential (EPSP) decreases. This results in a negative charge in
the extracellular space immediately surrounding the synapse. Thus, this creates
a dipole with partial charges usually orientated in the cortex. Every neuron
that receive somatic inputs can therefore be thought as a dipole with specific
orientation and polarity. Signals cannot be detected from single neurons as the
potentials are very small magnitude and on a cellular level are considered quite
a distance from cell to scalp surface. The measurable signal detected is the
summation of thousands of neurones. (Hauk
2013) These episodes might occur as hyperventilation triggers “typical”
seizures and is performed by the child during EEGs. EEGs are measured using the
International Federation 10-20 system. In preparation, 21 silver-silver
electrodes are placed at specific anatomical points on the head of the patient.

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A differential signal is amplified and sprayed as a channel of EEG activity.

Montage is the representation of the EEG channels and there are many types:
Bipolar (the difference between two adjacent electrodes), common reference
(difference between one electrode and a common electrode) and finally average
reference (activity from all electrodes summed and averaged and then used as a
reference electrode. (ERS 2016)


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