Biological signals, or biosignals, are records of a biological event such as a beating heart or a contracting muscle. The electrical, chemical and mechanical activity that occurs during this biological event often produces signals that can be measured and analyzed from the body’s surface. Biosignals, therefore, contain useful information that can be used to understand the underlying physiological mechanisms of a specific biological event or system and that may be useful for medical diagnosis, treatment, peak performance, Brain-Computer Interface, etc. Biological signals can be acquired in a variety of ways—for example, by a physician who uses a stethoscope to listen to a patient’s heart sounds or with the aid of technologically advanced biomedical instruments. Following data acquisition, biological signals are analyzed to retrieve useful information. Basic signal analysis methods, such as amplification, filtering, digitization, processing, and storage, can be applied to many biological signals. These techniques are generally accomplished with simple electronic circuits or with digital computers. In addition to these common procedures, sophisticated digital processing methods are quite common and can significantly improve the quality of the retrieved data. These include signal averaging, wavelet analysis, and artificial intelligence techniques.
Bioelectric Signals are very low amplitude and low-frequency electrical signals that can be measured from biological beings, for example, humans. These signals are produced by the electrical activity that arises from the biological activity within different tissues and organs of the human body. The most common bioelectrical signals are electrocardiogram (ECG), electrogastrogram (EGG), electroencephalogram (EEG), electromyogram (EMG) and electrooculogram (EOG).
** Bioelectric Signals are the most common biosignals in research and application**
- ECG: ECG (electrocardiography) is a method of collecting electrical biosignals generated by the heart. This allows us to understand the level of physiological arousal that someone is experiencing, but it can also be used to understand someone’s psychological state better. ECG electrodes are typically wet sensors, meaning that they require the use of a conductive gel to increase conductivity between skin and electrodes and are usually placed on the arm or chest.
- ECG: An electrogastrogram (EGG) is a graphic produced by an electrogastrograph, which records the electrical biosignals that travel through the stomach muscles and control the muscles’ contractions.
- EEG: Electroencephalography, or EEG, is the physiological method of choice to record the electrical activity generated by the brain via electrodes placed on the scalp surface. For faster application, electrodes are mounted in elastic caps similar to bathing caps, ensuring that the data can be collected from identical scalp positions across all respondents.
- EMG: these types of signals are produced by the electric currents that are generated by the muscle contraction. The electrical activity of the selected muscle is detected by surface electrodes.
- EOG The electrooculogram (EOG) is an electrophysiologic test that measures the existing resting electrical potential between the cornea and Bruch’s membrane. EOG is a very useful tool in diagnosing hereditary macular diseases. Moreover, combining EEG with EOG data can help us to interpret EEG data with more precision.
Different organs, including the heart, brain, and lungs, also generate weak magnetic fields that can be measured with magnetic sensors. Typically, the strength of the magnetic field is much weaker than the corresponding physiological bioelectric signals. Biomagnetism is the measurement of the magnetic signals that are associated with specific physiological activity and that are typically linked to an accompanying electric field from a specific tissue or organ. With the aid of very precise magnetic sensors or SQUID (superconducting quantum interference device) magnetometers, it is possible to directly magnetic activity from the brain (magnetoencephalography, MEG), peripheral nerves (magnetoneurography, MNG), gastrointestinal tract (magnetogastrography, MGG), and the heart (magnetocardiography, MCG).
Mechanical functions of biological systems, which include motion, displacement, tension, force, pressure, and flow, also produce measurable biological signals. Blood pressure, for example, is a measurement of the force that blood exerts against the walls of blood vessels. Another example is Accelerometer (ACC). The ACC sensors are commonly found in motion tracking applications where this sensor can be used for the development of to measure physical activity, range of motion, as well as to conduct vibration analysis, which, for example, can be used to prevent ergonomic injuries or evaluate tremors of Parkinson patients.
Bioacoustic signals are a special subset of biomechanical signals that involve vibrations (motion). Many biological events produce acoustic noise. For instance, the flow of blood through the valves in the heart has a distinctive sound. Measurements of the bioacoustic signal of a heart valve can be used to determine whether it is operating properly. The respiratory system, joints, and muscles also generate bioacoustic signals that propagate through the biological medium and can often be measured at the skin surface using acoustic transducers such as microphones.
Bio-optical signals are generated by the optical or light-induced attributes of biological systems. Bio-optical signals can occur naturally, or in some cases, the signals may be introduced to measure a biological parameter with an external light-medium. For example, information about the health of a fetus may be obtained by measuring the fluorescence characteristics of the amniotic fluid. Red and infrared lights are used in various applications, such as to obtain heart rate and precise measurements of blood oxygen levels by measuring the light absorption across the skin or a particular tissue.