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This year is the centenary of the first human EEG, produced by German physiologist Hans Berger.
In 1912, Vladimir Pravdich-Neminsky produced the first mammalian EEG, of a dog’s brain.
Berger succeeded him in 1924 with the human counterpart.
He is also credited with inventing the EEG, naming it, and introducing its utility in clinical settings.
What is An electroencephalogram (EEG)
‘Electro’ pertains to electricity; ‘encephalo’ refers to the brain; and ‘graphy’ is a suffix meaning to show or to represent.
Neurons in the brain perform various functions by moving electrically charged particles such as ions.
The movement of these particles gives rise to electrical activity that a health worker can use an EEG test to visualise.
Researchers have also been able to relate data obtained from an EEG with different levels and modes of brain activity, and used it to distinguish reliably between normal and abnormal states
EEG is not an uncommon diagnostic test in clinical settings.
Among other applications, it is the reference standard — that is, the best test available — to diagnose epilepsy.
An EEG test can also reveal the effects of anaesthesia, sleeping patterns, neurological activity during a coma, and availability of oxygen
EEG can also confirm brain death, one of the two legally recognised forms of death in India.
In research, scientists use EEG for neuroscience, cognitive psychology, neurolinguistics, neuromarketing studies and to develop brain-computer interfaces
What is volume conduction?
During an EEG test, a health worker will place electrodes on ones scalp.
There are many layers of skin, fluid, and bone between the electrodes and the neurons.
When a neuron produces electrical activity, charged particles will move through all these media before reaching the electrodes, and will be reflected, refracted, scattered, etc. en route.
Volume conduction refers to the movement of electrical activity through this three-dimensional volume.
It also stands for the fact that the electrical activity is produced in one place whereas the detectors that detect it are located at some distance.
The raw data collected by the electrodes will first need to be corrected for the effects of volume conduction, and then for noise in the data arising from faulty electrodes and incidental physiological activity (such as blinking or muscle activity).
A clinician finally interprets the processed data.
How does an EEG test work?
The neurons that make up the human brain are constantly exchanging atoms, molecules, proteins, etc. with their surroundings.
Sometimes neurons will push ions out into the space between neurons.
Since ions of the same charge repel each other, this ‘motion’ can push away other ions, which push away even other ions, and so on.
When a large number of neurons start this cascade at the same time, a (relatively) big wave of electrical activity flows through the brain.
The electrodes on the scalp are made of metal and track the changes in voltage as the waves move past them, creating an electroencephalogram.
What does and doesn’t EEG show?
The changes in voltage recorded at the electrodes are transmitted to a computer, which plots the readings on a graph with voltage on one axis and time passed on another.
Health workers are typically interested in two types of data in the graph: the voltage (measured in millionths of a volt) and the frequency of the variations (measured in hertz)
They will also factor in, among other things, the location of the neurons responsible for the electrical activity recorded by the test (for example, in the neocortex or the allocortex, the two types of cortices of the cerebral cortex)
On the downside, it is biased towards electrical signals generated closer to the surface of the cortex, and significantly so towards currents generated by neurons’ dendrites and against those generated by the axons.
The process to pinpoint where some electrical activity originated within the brain, to result in some electrical data, is also less than straightforward.
Are EEGs affordable?
EEG setup is also relatively simple and cost-effective.
The equipment involved doesn’t take up much space, doesn’t emit high-energy radiation or sounds, doesn’t confine patients to small spaces (like MRI), is non-invasive, and is portable
Similarly, aside from its diagnostic downsides, setting up an EEG test requires time — including applying a gel on the person’s head and placing the electrodes in precise locations according to the 10-20 System — and its readings can be affected if the person has so much as thicker hair.
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