EEG is a marvel of physics and neurobiology that opens a simple window into the human brain. This window is often small but has a lot to offer. But the most amazing thing is that they open anything without having to open the skull first. This year marks the centenary of the first human EEG, produced by the German physiologist Hans Berger.
How was the EEG invented?
Berger’s achievement was preceded by gradual but significant progress throughout Europe from the end of the 19th century. In 1875, the English physician Richard Caton reported evidence of electrical activity in the brains of monkeys and rabbits. Fifteen years ago, his Polish colleague Adolf Beck found evidence of fluctuating activity in the brains of dogs and rabbits when he stimulated the senses.
In 1912, Vladimir Pravdich-Neminsky produced the first mammalian EEG, from a dog’s brain. Berger replaced him in 1924 with a human counterpart. He is also credited with inventing the EEG, naming it, and introducing its utility in the clinical setting.
What is an EEG?
EEG stands for electroencephalography. ‘Electro-‘ relates to electricity; ‘-encephalo-‘ refers to the brain; and ‘-graphy’ is a suffix to show or represent.
Neurons in the brain perform various functions by moving electrically charged particles such as ions. The movement of these particles causes electrical activity that can be seen by healthcare workers using an EEG test. Researchers can also relate data obtained from EEG to different levels and modes of brain activity, and can be used to reliably distinguish between normal and abnormal states.
EEG is not an uncommon diagnostic test in the clinical setting. Among other applications, it is the reference standard – i.e. the best test available – for the diagnosis of epilepsy. EEG tests can also show the effects of anesthesia, sleep patterns, neurological activity during coma, and oxygen availability. An EEG can also confirm brain death, one of the two legally recognized forms of death in India.
In research, scientists use EEG for neuroscience, cognitive psychology, neurolinguistics, and neuromarketing studies and develop brain-computer interfaces.
What is volume conduction?
EEG measures electrical activity in the brain generated by neurons. During an EEG test, a healthcare worker will place electrodes on the scalp. There are many layers of skin, fluid, and bone between the electrodes and the neurons. When the neuron produces electrical activity, the charged particles will move through all these media before reaching the electrodes, and will be reflected, refracted, scattered, etc.
Volume conduction refers to the movement of electrical activity through this three-dimensional volume. It also stands for the fact that electrical activity is produced in one place, while the detector that detects it is located at some distance.
The raw data collected by the electrodes must be corrected first for volume conduction effects, then for noise in the data arising from incorrect electrodes, 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 environment. Sometimes neurons will push ions out into the spaces between neurons. Because like-charged ions repel each other, this ‘motion’ can repel other ions, which can repel other ions, and so on.
When many neurons initiate this cascade at the same time, a (relatively) large wave of electrical activity flows through the brain. Electrodes on the scalp are made of metal and track voltage changes as the waves pass through them, creating an electroencephalogram.
Where are the electrodes placed?
You can put it anywhere, but if you are comparing notes with scientists or want to follow clinical standards, you should follow the International 10-20 System. In this system, the distance between two adjacent electrodes is 10% or 20% of the total distance between the two points on the head where the electrodes are attached.
The four common reference points are the nasion (the depression between the eyes, above the bridge of the nose) and the inion (the peak at the back of the skull) going forward to back, and from tragus to tragus side to side. . (The tragus is a small flap-like projection on the outer ear; you push against the ear to close the ear when there is a loud sound.)
What does and does not EEG show?
The change in voltage recorded on the electrode is transmitted to the computer, which plots the reading on the graph with the voltage on one axis and the elapsed time on the other. Health workers are usually interested in two types of data on the chart: voltage (measured in millions of volts) and frequency variation (measured in hertz). They will also factor, among other things, the location of the neurons responsible for the electrical activity recorded by the test (for example in the neocortex or allocortex, two types of cortex of the cerebral cortex).
EEGs are better than other diagnostic tools at tracking relatively fast electrical activity in the brain, on the order of milliseconds. At the bottom, the bias is towards electrical signals generated closer to the surface of the cortex, and significantly towards currents generated by neuron dendrites and towards those generated by axons. The process of determining where some electrical activity originates in the brain, to generate some electrical data, is also less straightforward.
To overcome these and other challenges, researchers use EEG in conjunction with other tests, such as magnetic resonance imaging (MRI), and develop sophisticated data acquisition, processing, and reconstruction methods.
Is EEG affordable?
In addition to its metrological and diagnostic capabilities, the EEG setup is also relatively simple and inexpensive. The equipment involved does not take up much space, does not emit radiation or high-energy noise, does not confine the patient to a small space (such as MRI), is non-invasive, and is portable. (The invasive version of EEG is called electrocorticography, or ECoG).
In addition, aside from the diagnostic downsides, setting up an EEG test takes time – including putting the gel on a person’s head and placing the electrodes in the right location according to the 10-20 System – and the readings can be affected if the person has them. like thicker hair.