Principles of analyses of electroencephalogram
Construction of the conclusion on electroencephalogram submits to algorithm and according to it can be divided into three basic stages.
- Definition of a functional condition of a brain of the patient.
- The instruction on presence or absence of local pathological changes of biopotentials.
- Revealing of the subcortical phenomena regarded as EEG of equivalent epileptic seizure.
These positions reflect three problems which are solved for each patient.
There are clinical (visual) and statistical methods of studying of EEG. The clinical (visual) analysis is strictly individual, has qualitative character, can show a degree of conformity or deviation EEG of the patient from the standard norms. In clinic there are special receptions of the description and clinical interpretation EEG. It is natural, that on occassion value of one of problems can prevail, and place priorities the attending physician directing the patient on inspection should. Skill "to read" EEG to allocate in it hidden and it is frequent very much вариативные pathological attributes, knowledge of features of the neurologic status, a leading syndrome of disease will allow a doctor to give exhaustive answer.
Well-known, that EEG is a record of total activity neurons of brain cortex, but at such narrow approach to a technique its diagnostic importance sharply decreases.
In electroencephalograpy record of potentials нейронов barks of the brain, its nonspecific systems being under constant influence is made.
F
ig.
8. Normal EEG.
Neurodynamic of a brain it is defined by interaction synchronizing and making active systems.
The first is localized in first departments of hypothalamus, its activity defines a condition of the weakened wakefulness and a dream. The second - provides a condition of vigil and settles down in oral departments of a trunk. The mutual relation of the specified systems creates optimum conditions for functioning a brain and that picture EEG which it is accepted named electroencephalogram of the healthy awake person or is normal EEG. Normally organized activity of a brain at the patient in condition wakefulnesses is presented by three basic rhythms:
α –rhythm
Fig. 9. The Alpha-rhythm.
The alpha-rhythm gives reaction of depression (activation) at opening and is restored at cutoff an eye (Fig. 10.).
F
ig.
10. Depression and restoration an alpha-rhythm.
β-waves dominate in EEG the person over an active condition, intensive physical and brainwork, an emotional pressure, realization rough and conditioned reflexes.
Fig. 11. Beta-rhythm.
The θ -rhythm is shown and at pathological conditions of a brain, and also at an extreme emotional pressure.
B
y
the form of electroencephalogram, on occurrence or disappearance of
the certain rhythms it is possible to judge character and a degree of
shifts of a functional condition of nervous structures of a brain,
about dynamics of changes, to find out area of a bark of a brain
where these changes are most expressed. So, at transition from vigil
to dream α and β - rhythms are replaced essentially slower (θ and
δ- rhythms). Statistical methods of studying of EEG use
mathematical transformations Fourier, establish on everyone making
distribution, for example, capacities of signals of different
frequency and amplitudes, and then receive spectra, cards of capacity
for further interpretation of EEG.
Fig. 12. Conditional card of EEG.
Example: at a cartographical method of electroencephalography the next problems are solved:
1. Find full capacity of signal EEG in each tap (the area under a corresponding spectral curve) and build a card of distribution of full capacity on a surface of a head, appropriating each of intervals into which all interval of values of capacity is broken, the color or a shade.
On a scale located below, intervals into which values of capacity are broken all from are presented Pmin up to Pmax, to each of which this or that type of shading is appropriated. Similar shading covers corresponding areas of a surface of a head.
2. Find not full capacity of a signal in this or that tap, and only the capacity corresponding this or that spectral interval. These intervals have received the name of rhythms.
Similarly build a color card of distribution of capacities in the chosen spectral interval on a surface of a bark of a brain. Sometimes the doctor at once analyzes some cards corresponding different rhythms.
A
nalysis
of EEG represents a challenge. For compression of the information and
its representation in a kind convenient for understanding build
frequency spectra of signals of EEG on some informative interval.
After that frequency spectra can be developed in time and to receive
time "landscape". Now, using computer techniques, electric
activity of a brain analyze by means mapping of surfaces of a head.
Fig. 13. Results of inspection
Magnetoencephalography is registration of parameters of the magnetic field caused by bioelectric activity of a brain. Record of these parameters is carried out by means of quantum interference sensors and the special chamber isolating magnetic fields of a brain from stronger external fields.
Registration of biopotentials directly from the naked brain is called electrocorticography and is usually spent during neuro-surgical operations.
1. To familiarize with devices and accessories.
2. To consider special contact electrodes which are put to a surface of a skin on a head. Electrodes should possess small transitive (electrode-leather) resistance, the minimal polarization and anticorrosive properties. For reduction of resistance of the leather contacting to an electrode, it process dissolving substances (for example, ether). On a head electrodes fix by means of special elastic helmets. To electrodes attach the conductors possessing isolation and employees for connection of electrodes on an input of electroencephalograph.
3. To study international system of installation of electrodes «10-20 %».
4. Possible points of an arrangement of electrodes to divide the intervals making 10 % or 20 % of these distances on a skull.
5. For convenience of registration all skull to break into the areas designated in figure by letters: F-frontal, About-occipital area, Р-parietal, Т-temporal, С-area central line.
Not even numbers of places of tap concern to left, and even - to the right hemisphere.
Letter Z - designates tap from a top of a skull. Sets of cables of taps EEG are noted everyone by the color.
By red color it is marked referent tap. By black color it is marked ground tap (it is connected to ground electrode). The given wire is present at a set of each hemisphere.
Taps of channels EEG of each hemisphere have following colors (this information is used at formation of the scheme of taps):
White - the channel 1 left and 0 right hemispheres;
Yellow - the channel 2 left and 3 right hemispheres;
Green - the channel 7 left and 5 right hemispheres;
Dark blue - the channel 8 left and 6 right hemispheres.
Conformably to the second standard tap the set of cables of the channel of an electrocardiogram has following colors:
Yellow - it is connected to an electrode established on the left hand (L);
Red - it is connected to an electrode established on the right hand (R);
Black - it is connected to an electrode established on the right leg (N).
Let's lead the order of connection of cables of taps to electrodes in standard 8-channel to the scheme of taps of "10-20 % " (Fig. 14). Such taps and such connection is used during registration EEG on electroencephalograph "Нейрон-Спектр-1" at a choice of any 8-channel installation from the list of standard installations.
Connection of a set of cables of taps of the left hemisphere:
Black wire – grounding electrode;
Red wire - ear electrode А1;
White wire - an electrode of tap Ғр 1;
Yellow wire - an electrode of tap 01;
G
reen
wire - an electrode of tap СЗ;
Dark blue wire - an electrode of tap ТЗ.
Connection of a set of cables of taps of the right hemisphere:
Black wire - grounding electrode;
Red wire - ear electrode А2;
White wire - an electrode of tap Ғр2;
Yellow wire - an electrode of tap 02;
Green wire - an electrode of tap С4;
Dark blue wire - an electrode of tap Т4. Fig. 14.
PHOTO-ELECTRIC CONVERTERS
In 1887 German physicist Henry Gerts experimented an arrester for radiation of electromagnetic waves - pair metal spheres; at the appendix of a voltage difference between them the spark slipped. When it shined one of spheres with ultra-violet beams, the category amplified. Thus, the external photoeffect has been detected.
In 1888 Wilhelm Galvaks has established, that the metal plate irradiated by ultra-violet light is charged positively. So there was a second opening a photoeffect. The third, not knowing about experiences of Hertz and Galvaks, it was observed the same year by Italian Augusto of Riga. It has found out, that the photoeffect is possible both in metals, and in dielectric.
Alexander Grigorevich Stoletov was fourth scientist, irrespective of others opened a photo effect. It two years investigated the new phenomenon and has deduced its basic laws.
There was, that a force of a photocurrent, first, in direct proportion intensity of falling light, and secondly, at the fixed intensity of an irradiation all over again grows in process of increase of a voltage difference, but, having reached the certain value (a current of saturation), does not increase any more.
In 1899 German Phillip Lenard and an Englishman Joseph Thomson have proved, that light falling on a metal surface beats out from it electrons which movement brings to existence a photocurrent. However to understand the nature of a photo effect by means of classical electrodynamics and it was not possible. Inexplicable remains, why the photocurrent arose only when frequency of falling light exceeded size strictly certain for everyone metal.
Only per 1905 Einstein has transformed this riddle into absolutely transparent picture.
It has assumed, that electromagnetic radiation is not simply let out by portions - it and extends in space, and is absorbed by substance too in the form of portions - light quantums (photons). Therefore for occurrence of a photo effect intensity of a falling light bunch is important at all. The main thing, whether suffices separate light quantum of energy to beat out electron from substance. The minimal energy necessary for it, name work of an output A. As a result Einstein has deduced the equation.
Clearly, that the light wave of rather high frequency can cause a photo effect only, and force of a photocurrent is proportional to intensity of absorbed light, that is number of the photons, capable to beat out electrons from substance. In 1907 Einstein has made one more specification of a quantum hypothesis. Why the body radiates light only in the portions? That is why, Einstein answered, that atoms have only discrete set of values of energy. Thus, the theory of radiation and absorption has accepted the complete type.
In 1922 American Arthur Kompton has detected, that is long waves of x-ray radiation changes at dispersion on electrons of substance. But, on classical electrodynamics, the length of a light wave at dispersion cannot change. Then Kompton has executed calculation, having assumed, that on electrons waves, and particles (photons) dissipate not. The result has coincided with experimental. It became the direct proof of a reality of existence of photons.