To register the SCP of the brain, direct current amplifiers are required, which make it possible to measure the potential difference of the millivolt range and its changes in the time range of a second – hours. In order for the change in the resistance of the object not to affect the results of measurements of the potential difference, the input resistance of the device should be much larger than the resistance of the object. Since human skin has a fairly high DC resistance (tens of kilo-ohms), the input resistance of the amplifier should be at least 1MΩ. In our devices of the Neuroenergometer type, the input resistance is 10 MΩ.
The level of constant brain potential is measured by non-polarizable electrodes. Non-polarizable electrodes consist of a metal on the surface of which a layer of sparingly soluble salt of the same metal is applied. The interaction of the metal and its salt ensures the stability of the electrode potential. You can read more about this in the special manuals . Most electrodes used in electroencephalography are polarized under the influence of direct current. This means that during the passage of direct current on the polarizable electrodes, a potential difference arises, which is opposite in sign to the measured voltage, which has a significant effect on the recorded potential difference. The magnitude of the artifacts of electrode origin can exceed the brain SCP by an order of magnitude.
Most often, non-polarizable silver-silver electrodes with a small stable interelectrode potential difference are used to register SCP. In highly stable electrodes, the electrode potential changes less than by 1 mV for 1 hour of registration. When registering the soft starter, it is necessary to measure the interelectrode potential difference so that it can then be subtracted from the registered soft starter value.
Artifacts of biological origin, the main of which are skin potentials, can also introduce a significant distortion into the measured value of brain SCP. There are two characteristic errors when registering constant potentials from the surface of the head. Some authors ignore the potentials of the skin and related artifacts, while others do not notice the potentials of brain origin. The first approach is characteristic of the so-called omegametry, when the potential difference between the arm and the head is recorded in response to some kind of impact. According to the authors, the dynamics of constant potentials at the same time indicates the features of the regulation of various reactions by the brain (biochemical, immunological, etc. ) . Another approach is directly opposite in its conclusions with a fundamentally similar derivation of constant potentials. SCP with symmetrical abduction of the SCP from the temporal-frontal areas of the right and left half of the head are interpreted as skin potentials. However, both approaches are erroneous, since it is almost impossible to completely get rid of potentials of skin or brain origin with the registration technique used.
To reduce the effect of skin potentials on SCP, removed from the surface of the head, allows knowledge of the origin and patterns of dynamics of skin potentials. It is known that between the outer and inner surface of the skin there is a potential difference of the millivolt range, and the outer surface is negatively charged relative to the morning morning . Skin potentials are not the same in different areas, in the so-called biologically active points (acupuncture points), the value of the skin potential and skin resistance significantly differs from neighboring areas. Skin potentials change with galvanic skin reactions that occur in response to various stimuli. This determines a significant difficulty in isolating the contribution of skin potentials to the recorded SCP value .
However, monitoring and minimizing the influence of skin potentials when recording SCP is possible. The magnitude of the skin potential depends on the skin resistance. This is evidenced by the genesis of skin potentials, as well as the fundamental similarity of skin-galvanic reactions using skin resistance and skin potential as an indicator . With a skin- galvanic reaction, a decrease in skin potential occurs in parallel with a decrease in skin resistance . Therefore, ensuring a minimum value and a stable level of skin resistance makes it possible to minimize the contribution of skin potentials to the recorded value of SCP. Reducing skin resistance and potentials is achieved by degreasing the skin with alcohol and then applying it to the skin 3-5 minutes before recording a saturated NaCl solution. When recording SCP, it is necessary to simultaneously register skin resistance. The low level of skin resistance by direct current (within 5 kOhm), the equality of its values in the places of abduction, and the stability during the recording of the SCP indicate a small value and the same level of skin potentials in the corresponding areas. The identical and minimal contribution of skin potentials in each of the regions allows one to significantly reduce the effect of skin origin atefacts on the recorded SCP value.
To record the SCP, as a rule, a monopolar lead is used, in which the potential difference between the active electrodes located above different parts of the brain and the reference electrode is estimated. All parts of the body have one or another constant potential, therefore, for the location of the reference electrode, zones with minimal and stable potentials are chosen. Such characteristics have areas located above the bone structures, for example, the patella and wrist . Using the wrist area as a reference has certain advantages: the reference electrode is conveniently placed on the wrist, the potential of the wrist is more stable, since it is the result of averaging the potentials of a sufficiently large area .
On the contrary, it is not recommended to place the reference electrode on areas of the body with significant and changing potentials, which are recorded in the areas of muscle location, on the palms and soles, where the skin-galvanic reaction is most pronounced.
Some, especially foreign authors, have a reference electrode on the earlobe, just like when registering an EEG. This localization of the reference electrode has a number of technical conveniences, however, it deprives of information about the potential difference between the head and remote areas of the body. As will be shown below, this characteristic is useful in many cases, especially when studying the dynamics of functional states.