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BrainBit has four dry EEG electrodes, a reference and a common electrode, and a miniature electronic module. An EEG signal using a BrainBit device is recorded at a sampling frequency of 250 Hz. It is possible to obtain a signal both without the use of filters (the so-called "raw signal") and using a notch (48-52 Hz) and digital (1-40 Hz) filters.
The device amplifies and digitizes the received signal and transmits it via the Bluetooth LE protocol to a computer, smartphone, or tablet.
The BrainBit device has the following electrodes:
The T3 and T4 electrodes provide direct contact with the scalp in the temporal lobe areas, and the O1 and O2 electrodes in the occipital region.
BrainBit can record EEG signal in monopolar and bipolar mode:
By default, signal registration is carried out in monopolar mode. Bipolar signal registration mode is achieved as a result of mathematical processing of the EEG signal and allows to reduce common-mode interference.
For each mode, it is possible to select the required number of channels:
To obtain the usual type of EEG, it is required to use filters that provide the necessary frequency band of the signal. During registering a "raw" signal, BrainBit transmits an EEG in the frequency band from 0 to 70 Hz. Usually it is a quite wide band. In addition, recording a signal from 0 Hz leads to a shift in the signal from its zero line on the visual display. It is not always clear and comfortable. The regular frequency band of the EEG for daily use is determined by the range of 1-25 Hz. This range allows you to register without distortion four basic frequency rhythms - delta, theta, alpha and beta. Delta activity (pathological activity) is not used in most applications, so while working with theta, alpha and beta rhythms it is recommended to use a band of 4-25 Hz, and with alpha and beta rhythms - 7-25 Hz.
It is worth noting that when recording EEG using dry electrodes, electrochemical processes, and electrode movement lead to a significant deviation (increase) of the delta rhythm from the usual values described in the medical literature.
– the location of the reference and general electrodes on the forehead;
– the direction of the arrows in the area of the common electrode (should be directed upwards);
– the location of the T3 and T4 electrodes in the temporal region, and the O1 and O2 electrodes in the occipital region.
Recommendations for channels using depend on the goals of software development and the planned functionality.
The presence in the device of the FpZ electrode and electrodes pairs with its presence (monopolar method of signal registration) make it possible to identify blinking, eye movements, forehead tension, swallowing, which is useful in applications for sleep analysis, concentration training, etc.
The presence in the device of electrodes O1 and O2, placed in the occipital region, makes it possible to record with high accuracy the alpha rhythm, which is an indicator of the level of human relaxation. In any other areas of the brain, alpha activity is significantly reduced and, as a rule, cannot be used as an objective measure of the degree of relaxation.
The presence of T3 and T4 electrodes placed in the temporal region in the device makes it possible to record with high accuracy the beta rhythm, which is an indicator of the level of concentration of attention. In any other areas of the brain, beta activity is represented to a lesser degree and, as a rule, cannot be used as an objective criterion for assessing the level of concentration of attention.
If the functionality of the application being developed doesn't imply tasks related to fixing blinks, eye movements, forehead tension (in this case, these physiological manifestations are artifacts for the electroencephalogram), we recommend using pairs of channels O1 - T3, O2 - T4, which are obtained by subtracting the T3 – FpZ signal pair from the O1 – FpZ pair and the T4 – FpZ pair from the O2 – FpZ pair, respectively.
This procedure makes it possible to exclude from the signals used all biological artifacts generated by the reference frontal electrode of the device. This is achieved due to the symmetrical arrangement of the electrodes relative to the reference and, accordingly, in-phase signal subtraction.
Thus, pairs of signals O1 - T3, O2 - T4 allow to provide registration of alpha and beta rhythms for further processing and analysis, while minimizing biological artifacts.
The presence of symmetrical (in the left and right hemispheres of the brain) electrodes T3 / T4 and O1 / O2 is due to the need to solve the problems of duplication of areas and to ensure the possibility of assessing the asymmetry of the brain.
The problem of duplication may be relevant in the event of a possible "poor" quality of one of the electrodes. Its solution involves the use of a symmetrical electrode on the other side. This can be performed even in real time, when suddenly a low-quality EEG signal is recorded in one of the leads, for example, as a result of manipulations by the user with the device on the head.
Summing up, it is worth noting that to solve most problems, it is sufficient to use the optimal pair (with a higher quality EEG signal, without interference) among the pairs O1 - T3 and O2 - T4, working on one bipolar channel.
The use of dry spring electrode technology can lead to high impedance values. However, the BrainBit device makes it possible to record a high-quality EEG signal even under the condition of high impedance values. It is important to focus on the threshold allowable value for conducting sessions, which is 1 MΩ.
The BrainBit device is characterized by long transient processes. Within a few minutes after placing the device on the head of the subject, the resistance level is normalized to optimal values without additional measures.
Nevertheless, high resistance remains one of the most common causes of poor EEG signal quality. The higher the resistance value, the worse the signal-to-noise ratio, the worse the quality of the recorded signal. However, resistance is not the only criterion for the quality of an EEG signal, but only an additional (auxiliary) tool for its evaluation. When assessing the quality of the signal, it is recommended to focus primarily on its objective parameters (amplitude) and shape.
If the resistance value is above 1 MΩ, it is recommended to perform the following actions to reduce it:
EEG quality can be affected by power source noise (50 Hz). The level of power source noise does not depend on the resistance. To get rid of it, it is necessary to filter the signal with a notch filter (48-52 Hz).
In general, in order to obtain the highest quality EEG signal, it is recommended to observe the following rules during registration: