This unit covers Sources of artifact (III-B), Identification and elimination of electrical shock hazards (III-C) and Essential electronic terms and concepts for biofeedback applications (III-D).
Students completing this unit will be able to discuss:
The matter comprising our universe occupies space and possesses mass. Matter can assume solid, liquid, gaseous, and plasma states. Atoms are basic units of matter consisting of a central nucleus, that contains protons and neutrons, and orbiting electrons. The positively-charged nucleus contains the majority of an atom's mass in the form of positively-charged protons and uncharged neutrons. Each proton carries a positive charge that is equal and opposite to the negative charge of the electron. Negatively-charged electrons rotate around the nucleus at varying distances and participate in chemical reactions. The number of electrons equals the number of protons in the atom, balancing the electrical charge of the nucleus. In other words, the atom’s net charge is zero and the atom is said to be neutral.
Elements are substances that contain identical atoms and cannot be reduced by common chemical reactions. Of the 116 elements confirmed to date, calcium (Ca), carbon (C), hydrogen (H), nitrogen (N), oxygen (0), and phosphorous (P) are most important to human life. Calcium (Ca), chloride (Cl), potassium (K), and sodium (Na) are critical to generating physiological potentials like the EEG. Elements are neutrally charged since their atoms contain an equal number of protons and electrons.
How does a carbon atom differ from a sodium atom? The difference lies in the number of protons located in the nucleus. A carbon atom has 6 protons while a sodium atom has 11. The total number of protons determines the atomic number. The number of protons and neutrons approximates the atomic weight.
Ions are atoms charged by the gain or loss of electrons. The biological potentials produced by cortical neurons (EEG), eccrine sweat glands (EDA), and skeletal muscles (SEMG) are actually currents of ions. The ions most responsible for these signals are chloride (Cl-), potassium (K+), and sodium (Na+).
Current (I) flows because atoms and molecules contain two types of electrical charge: positive and negative. Opposite charges attract while identical charges repel each other. When there is a difference in the overall charge of atoms between two points—for example, between two ends of a wire— negatively-charged electrons will flow toward the positively-charged end of the wire creating electric current.
Orbiting electrons move at varying distances from the nucleus in regions called energy levels. Each energy level contains no more than its maximum number of electrons. The energy level closest to the nucleus holds up to 2 electrons, the second energy level, 8 electrons, and the third level, a maximum of 18 electrons.
Electrons in the outermost energy level are responsible for electricity. When this energy level is incompletely filled, its electrons are bound less tightly and more easily dislodged by collisions. These ricocheting free electrons act like cue balls striking a long line of billiard balls. Each ball dislodges the next until the last ball is jarred free. Dislodged electrons are eventually captured by the atoms they strike. Energetic free electron movement produces an electric current.
Hikers travel more easily across a dry trail than a muddy one. Muddy ground resists movement and steals energy. Electrons are also affected by the materials in their path. Conductors allow electron movement, while insulators oppose movement.
Current travels through conductors, like the copper wiring of an EEG electrode, by the successive collision of free electrons. The secret of conductors is that their farthest energy level contains so few electrons that electrons are readily dislodged. While all metals are good conductors, the best conductors are silver (Ag), copper (Cu), and gold (Au) which have only one electron that can be effortlessly jolted free.
Biological signals like the EEG travel through fluid, instead of metal, conductors. The interstitial fluid surrounding body cells is also an excellent conductor. Signals like the EEG bump their way through body fluids as a current of colliding ions (not electrons) until they reach the skin. This process, called volume conduction, allows us to eavesdrop on cortical potentials from the scalp instead of inserting electrodes inside the brain.
Electrodes are specialized conductors that convert biological signals like the EEG into currents of electrons. Surface EEG electrodes function like an antenna to detect the EEG signals produced by macrocolumns of cortical neurons. Currents of ions volume conduct to the scalp (like an FM radio broadcast) and then electrodes change this signal to a current of electrons.
Insulation from body fat, connective tissue, and the epidermis (outer skin layer) interferes with ion current flow. Insulators like the rubber covering the wiring of a muscle electrode, block the flow of electric currents. In both biological and fabricated insulators, the large number of electrons in their final energy level produces a cohesiveness that resists electron loss due to collision. The best insulators, like rubber, possess the maximum number of outer level electrons.
When we measure current, we learn how much of "x" has passed by a point over a fixed period of time. Utility companies measure currents of water and electrons the same way. A water meter counts the number of gallons that flow through pipes. An electricity meter calculates the number of electrons that travel through wiring over time. The "amount" of electric current is measured in amperes (A). You use 1 ampere of current when 1 coulomb (6.24 x 1018 or 6 billion billion electrons) passes a point in 1 second.
Electricity travels as either a direct current (DC) or alternating current (AC). Direct current (DC) is the flow of electricity in one direction—from negative to positive. A difference in electrical potential pressures electrons to move. The negative end of a wire repels electrons (e-) while the positive end attracts them. A DC current shows a marked drop in amplitude over distance.
Animation © 2000 by Chancie Adams
The movement of a DC current through a wire is analogous to a disconnected garden hose filled with water. When you hold one end of the hose higher than the other (different potential), water spills out. Conversely, when you keep the hose level (equal potential), water stays put.
Direct current functions as both a power source and signal generator in biofeedback. The batteries that run portable biofeedback instruments supply direct current.
Biological signals representing peripheral blood flow (blood volume pulse and skin temperature), respiration, and skin electrical activity are all DC signals. Below is a BioGraph ® Infiniti blood volume pulse (BVP) display.
When we plot these signals against time, they never completely reverse direction over a second's time. The electroencephalogram (EEG) contains both DC (slow cortical potentials) and AC (slow cortical potentials and delta through 40-Hz) waveforms.
In the space of a second, an alternating current (AC) regularly reverses direction (50 or 60 Hz) because of the way it is generated. An AC current is more effective than a DC current in maintaining its amplitude over distance. During each complete cycle, electron flow starts at zero, reaches a maximum value in one direction, drops back to zero, reaches a maximum value in the opposite direction, and then returns to zero. This is how your car's alternator works. An alternator contains a spinning magnet that reverses direction.
Examine an individual magnet and you'll find that its ends (poles) have negative and positive charges. Movement in one direction repels electrons, while movement in the opposite direction attracts them. So, your alternator produces AC power because magnets periodically spin in the opposite direction. The frequency of an alternating current is the number of cycles completed per second or hertz (Hz).
Alternating current also functions as a power source and signal generator. Microcomputers that record data and provide color graphic displays of patient performance are powered by 60-Hz AC current from wall outlets. Below is a BioGraph ® Infiniti 60-Hz artifact display.
Electrical potentials detected from the cerebral cortex (EEG), heart (ECG), and skeletal muscles (SEMG) all contain AC waveforms. Below is a BioGraph ® Infiniti heart rate variability (HRV) display.
When we plot EMG signals against time, their frequencies range from 2-10,000 Hz. EMG frequency range depends on the recording method. Electrodes inserted into skeletal muscles can detect frequencies as high as 10,000 Hz. In contrast, surface electrodes are limited to 1,000 Hz because insulating fat and skin absorb higher frequencies.
Consider how SEMG electrodes work. Muscle fibers must become more positive inside (depolarize) before contracting. This positive shift produces a current of ions (muscle action potentials) that travels through the fluid surrounding body cells.
Since interstitial fluid is a superb conductor, SEMG electrodes can detect potentials from distant motor units. This process is called volume conduction. Electrodes transform the current of ions into an electron current that flows through the cable into an electromyograph’s input jack.
Animation © 2000 by Chancie Adams adapted from Stern, Ray, & Davis (1980)
Volume conduction is also responsible for EKG contamination of SEMG measurements since the depolarization of the heart muscle generates potentials that travel to the skin surface. Below is a BioGraph ® Infiniti SEMG display without its protective filtering to reject EKG artifact. Note the SEMG signal's contamination by the sharp vertical R-waves of the EKG.
What forces electrons to move through a circuit? Consider the mechanics of draining a water bed. If the drainage hose is exactly the same elevation as the mattress, water won't move. Water only flows when the free end of the hose is lowered to the basement. Replace elevation with potential and you have explained how to move electrons through a circuit. Water flows when there is a difference in elevation. Electrons flow when there is a difference in electrical potential or charge.
A flashlight works because its battery contains negative and positive poles. These two regions of opposite charge produce an electrical potential difference called the electromotive force (EMF) that drives the current ahead. The electrical potential difference can be considered the "strength" of the current. A battery's negative pole repels electrons (e-) while its positive pole attracts them resulting in current flow. If the battery's two poles had identical charges, instead, electrons would stay put. No potential difference, no current, and no light.
Batteries and generators are devices that produce electric current to power lights and other appliances. Electric currents also occur in nature—lightning being a dramatic example. The pressure a battery exerts on electrons flowing through a flashlight is measured in volts (E). A typical flashlight battery is rated at 1.5 volts. One volt is the potential difference required to make 1 coulomb (6.24 x 1018 electrons) perform 1 joule of work. Voltage indexes signal power. Many household appliances run on 115 volts, but some require 220 volts. Voltage in power lines that deliver electricity around the country is measured in tens of thousands of volts.
Biofeedback requires a working knowledge of voltage. When using portable instruments, you must be able to install the correct battery and routinely measure its test voltage (battery test) at the start of each session. Further, when monitoring biological signals, you will record signals ranging from microvolts or μV (millionths of a volt) to millivolts or mV (thousandths of a volt). EEG and SEMG amplitudes are measured in microvolts (μV) and are usually less than 100 μV.
You will need to understand when voltages are typical and when they indicate equipment malfunction. For example, skin potential level (SPL) detected from the palm runs from +10 to -70 mV. Values sharply outside this range, for example +50 mV, should be suspect.
An electric current’s overall power depends on the amount of current flowing through a circuit (measured in amperes) and the electric potential driving it (measured in volts). Electric power is measured in watts (W). One watt is equal to one ampere moving at one volt. Multiplying amperes by volts produces the number of watts. For example, an appliance that uses 10 amperes and runs on 115 volts consumes 1150 watts of power. In neurofeedback, clinicians and researchers increasingly express the signal strength of the quantitative EEG (QEEG) in picowatts (trillionths of a watt).
Electrons moving through a conductor encounter opposition which reduces current flow. This phenomenon is called resistance (R) in DC circuits and is measured in ohms (Ω). Resistance depends on the number of electrons found in an atom's outermost energy level. Increasing the numbers of electrons in this level binds these electrons more tightly together. This cohesiveness reduces loss of electrons due to collisions with free electrons.
Resistance is also inversely related to conductor diameter. For example, standard surface electrodes (3 cm diameter) electrodes exhibit lower resistance than miniature (0.5 cm) electrodes. From an electron's perspective, a standard electrode's greater cross-sectional area provides more channels for movement. The greater number of channels means fewer electron traffic jams and lower resistance to current flow.
Resistors are components of an electric circuit that can limit or divide current. In everyday life, we use resistors to protect an electric circuit against excessive current flow and provide large amounts of heat or light.
Resistance is a practical concern in clinical biofeedback. Biological signals compete with stronger false signals for a biofeedback instrument's attention. Clinicians clean, abrade, and apply conductive gel to their patients' skin when monitoring the brain (EEG) and skeletal muscles (SEMG). Since dead skin, oil, and dirt block biological potentials from reaching electrodes, these precautions improve signal reception.
Skin resistance is also a biological signal, in its own right, that reflects emotional and cognitive processes. Clinicians measure skin resistance level (SRL) by running an AC or DC current across the inner surface of the fingers or palm. SRL is expressed in Kohms of resistance per cm2. Typical values range from 0-500 Kohms/cm2. Lower values reflect more intense sweat gland activity since moisture reduces resistance.
Resistance and conductance are mirror images of each other. Resistance is the reciprocal of conductance. Where resistance measures the opposition free electrons encounter, conductance (G) indexes how easily they travel through a conductor like copper or silver. Resistance is expressed in ohms (Ω); conductance is now measured in siemens and used to be measured in mhos (mho is ohm spelled backwards).
Electric current flows easily in some substances but not at all in others. Solids, liquids, and gases that carry electric currents are called conductors. Many metals like gold and silver used in electrodes are good conductors. More than one conductor may be needed to build an electric circuit—a path for electric current to move from one place to another. For example, when monitoring the EEG we may use a copper wire, precious metal sensor, and gel or paste that contains sodium-chloride.
Skin conductance is a biological signal that also reflects emotional and cognitive processes. Clinicians measure skin conductance level (SCL) by passing an AC or DC current across the sites we described earlier on the fingers or palm. SCL is currently expressed in microsiemens (millionths of a siemen) per cm2. Microsiemens is abbreviated μS. Typical values range from 2-100 μS/cm2. Higher values reflect more intense sweat gland activity since moisture increases conductance.
Except for the special circumstance in which a substance becomes a superconductor, all conductors resist the flow of current to some extent. The measurement of a conductor’s resistance to electric current is measured in ohms.
Ohm’s law states that the “amount” of current (I) flowing through a conductor is equal to the voltage (E) (the “push”) divided by the resistance (R). These values are measured in amperes, volts, and ohms, respectively.
Ohm’s law can be used to find any value in a DC circuit:
Voltage (E) = current (I) X resistance (R)
Current (I) = voltage (E) / resistance (R)
Resistance (R) = voltage (E) / current (I)
For example, using actual units, 10 volts = 2 amperes x 5 ohms. The system that delivers water to your kitchen faucet illustrates this relationship.
Visualize a water tower standing hundreds of feet above the ground. Why so high? To generate the pressure needed to move water through miles of pipe. That is voltage. The water that is under pressure flows at a measurable rate. That is current. Finally, water encounters more opposition flowing through narrow pipes than wide pipes. The narrow pipes create a traffic jam for rapidly moving water molecules. That is resistance. Taken together, the water pressure (voltage) = rate of flow (current) x pipe diameter (resistance).
Ohm's law is useful because it describes the relationship between voltage, current, and resistance. We can use this law to show two ways used to detect adequate voltages.
First, if voltage (E) = current (1) x resistance (R), then we can increase voltage by increasing current or resistance. Hardware designers use this relationship to increase the voltage reaching an electromyograph. When muscle action potentials (current) enter an electromyograph's amplifier, they are dropped across a network of resistors (resistance). This large differential input impedance increases the SEMG voltage seen by an electromyograph, which helps separate muscle action potentials signals from artifact.
Second, we can restate Ohm's law from the standpoint of current. If current (I) = voltage (E) / resistance (R), then we can increase current by increasing voltage or reducing resistance. This relationship is the reason clinicians prepare the skin when monitoring muscles. Skin cleaning, abrasion, and application of conductive gel reduce resistance. This increases the current reaching SEMG electrodes which, again, helps an electromyograph select muscle action potentials from artifact.
Ohm's law applies to DC circuits where current only flows in one direction. In AC circuits, current periodically reverses direction. This introduces frequency, the number of complete cycles completed each second. Frequency is measured in hertz (Hz). In AC circuits, current periodically reverses direction.
When an AC current travels through a circuit at a given frequency, it encounters a complex form of opposition called impedance (Z) which is measured in ohms (Ω).
Clinicians perform an impedance test to determine whether they have prepared the skin and attached electrodes properly. They measure impedance, a complex form of resistance, in Kohms (thousands of ohms). Excessive impedance means that a weak biological signal must compete at a disadvantage with false electrical signals (60-Hz artifact). This could contaminate the SEMG signal so badly that the electromyograph displays power line fluctuations instead of skeletal muscle activity.
An impedance test can be manually performed with a separate impedance meter (AC current) or voltohmmeter (DC current).
An impedance test may also be performed by software integrated with a data acquisition system and sensors, as shown below.
A conservative rule of thumb is that impedance should not exceed 10 Kohms and should be balanced within 5 Kohms for each active electrode-reference electrode pair.
We can extend Ohm's law to AC circuits by substituting impedance (z) for resistance and using lower case letters for voltage and current. The revised expression is voltage = current x impedance (e = i x z). This means that voltage is the product of a current flowing across an impedance. In actual units, 50 volts = 10 amperes x 5 ohms.
Biological signals enter the black box via an electrode cable. Monopolar recording produces only one signal, while bipolar recording produces two different signals.
SEMG signals entering an electromyograph are dropped across an input impedance (network of resistors), amplified, filtered, rectified, integrated, measured by a level detector, and finally displayed.
Broken electrode cables are a major cause of equipment malfunction since they prevent electron movement. Clinicians perform a continuity test to check if a cable is damaged. An impedance meter sends an AC signal down the cable to measure opposition to current flow. If there is a break, there is no continuity and the circuit is described as open. Impedance will be infinite since current cannot flow across empty space. If, instead, the cable is free of breaks (continuous), the circuit is described as closed. Impedance will approach 0 Kohms since current can easily travel through the circuit.
Behavioral tests, also called tracking tests, provide an indirect way to check cable continuity. When monitoring the occipital alpha, a clinician can test the performance of the entire data acquisition system (EEG sensor, cable, electroencephalograph encoder, and microcomputer) by asking the patient to close the eyes and then open them and visually focus on an object. If the graphic display mirrors these actions, showing decreased alpha (alpha blocking) during visual focusing, the cable is intact. If the display does not show decreased alpha, any system component may be responsible.
A short circuit results when an abnormal connection is made between two points of a circuit. The new path has lower resistance than the original circuit and should measure close to 0 Kohms on an impedance meter. The reduced resistance draws electrons through the short and may increase current flow to levels that can melt circuitry and injure patients.
Visualize a bare wire inside an electroencephalograph touching its metal case. The AC current powering this equipment could leak through the metal case and injure anyone touching this surface.
Objects such as a bar magnet or a current-carrying wire can influence other magnetic materials without physically contacting them. This is because these objects produce a magnetic field. Magnetic fields are usually represented by magnetic flux lines.
Physiological signals are quite small compared to surrounding electromagnetic “noise.” They need to be amplified to be distinguishable from background noise. Amplification is analogous to attaching a pump to a garden hose to increase the water pressure (voltage).
Physiological monitoring requires high quality connections between the subject and the electronic device. The quality of that connection determines the quality of the signal (information) gathered from that connection. Connections that are of poor quality, for whatever reason, produce poor quality (contaminated) information.
Many factors affect connection quality. These connection points include the skin surface, collodion glue, and sensors and connecting wires.
Clinicians prepare the skin surface to reduce resistance due to dead skin, oil, and dirt. They apply collodion glue (adhesive solution of pyroxylin in ether and ethanol) to the scalp to provide a highly-conductive path between the sensor and skin, and to maintain sensor stability. They inspect the quality of sensors and connecting wires because breaks in wires create open circuits that prevent signal transmission and worn sensor surfaces increase resistance. They prefer shorter wires over longer ones to reduce contamination by artifacts (false signals) between the patient and amplifier.
Biomedical engineers prevent shock hazards through ground fault interrupt circuits, optical isolation, and use of fiber optic connections. A one-second exposure to a current exceeding 5 mA can injure. A 50 mA current can cause fatal ventricular fibrillation in which the heart chambers cannot pump blood (Schwartz & Andrasik, 2003).
A ground fault interrupt circuit is designed into some power outlets to shut down power when a short circuit occurs. This protective circuit monitors current leakage. When harmful leakage is detected (> 5 mA), it triggers a circuit breaker that shuts off power to the equipment, protecting the patient, therapist, and hardware. Montgomery (2004) recommends plugging the entire biofeedback system into the same power outlet to create a common ground, so that current leakage in any of your equipment will trigger the ground fault interrupt circuit.
Optical isolation is a second method that prevents injury from short circuits and power surges. This circuit isolates a patient from hardware receiving AC power. An optical isolator converts a biological signal into a beam of light, the light crosses a gap (open circuit), and a photoreceptor reconverts the light into an electrical signal.
Fiber optic connections between the electrodes and data acquisition system is a third preventive method. Fiber optic cable is designed to transmit photons of light instead of electrical current. This approach also reduces contamination by electrical artifacts like 60-Hz noise.
Computer-based data acquisition systems increase the risk of shock since they expose both the patient and therapist to line-powered equipment. Both therapist and patient should avoid contact with metal surfaces and water leakage should be corrected immediately.
alpha blocking: Replacement of the alpha rhythm by low-amplitude desynchronized beta activity during movement, attention, mental effort like complex problem-solving, and visual processing.
alternating current (AC): An electric current that periodically reverses its direction.
ampere (A): A unit of electrical current or the rate of flow of electrons through a conductor. One volt dropped across one ohm of resistance produces a current flow of one ampere.
artifact: False signals like 60-Hz noise produced by line current.
atom: The basic unit of matter consisting of a central nucleus that contains protons and neutrons and orbiting electrons.
atomic number: The number of protons in the nucleus of an atom that defines an element.
atomic weight: Approximately, the number of protons and neutrons in the nucleus of an atom.
behavioral test (tracking test): Procedure to ensure that a biofeedback instrument accurately detects and displays subject performance.
bipolar recording: Recording method that uses two active electrodes and a common reference.
closed circuit: A complete path that allows electrons to travel from the power source, through the conductor and resistance, and back to the source.
collodion glue: Adhesive solution of pyroxylin in ether and ethanol.
conductor: A material that readily allows electron movement like a copper wire.
conductance (G): The ability of a material like copper or silver to carry an electric current; measured in siemens (formerly mhos).
continuity test: A procedure to ensure that a circuit is closed; that a cable is not broken.
coulomb: Approximately 6.24 x 1018 or 6 billion billion electrons.
current (I): The movement of electrons through a conductor measured in amperes (A).
differential input impedance: The opposition to an AC signal entering a differential amplifier as it is dropped across a network of resistors.
direct current (DC): Electric current that flows in only one direction as in a flashlight.
electron: Negatively-charged particle that rotates around the nucleus at varying distances and participates in chemical reactions.
electrode: Specialized conductor that converts biological signals like the EEG into currents of electrons.
electromotive force (EMF): A difference in electrical potential that "pushes" electrons to move in a circuit.
energy level: One of an electron's possible orbits around a nucleus at a constant distance.
epidermis: Outermost skin layer.
fiber optic: A thin flexible cable that transmits digital signals as pulses of light with the advantages of high-speed data transmission, electrical isolation, and resistance to electromagnetic interference.
frequency (Hz): The number of complete cycles that an AC signal completes in a second, usually expressed in Hertz.
ground fault interrupt circuit: Protective device that opens a circuit—shutting down power—when current leakage exceeds 5 mA.
impedance (Z): Complex opposition to an AC signal measured in Kohms.
impedance meter: Device that uses an AC signal to measures impedance in an electric circuit, such as between active and reference electrodes.
insulator: Material that resists the flow of electricity like glass and rubber.
interstitial fluid: Fluid between cells through which biological signals travel via volume conduction.
ion: An atom or a compound with a positive or negative electrical charge.
mho: Unit of conductance replaced by siemen.
microsiemen (μS): Unit of conductance that is one-millionth of a siemen.
microvolt (μV): Unit of amplitude (signal strength) that is one-millionth of a volt.
milliampere (mA): Unit of electrical current that is one-thousandth of an ampere.
millivolt (mV): Unit of amplitude (signal strength) that is one-thousandth of a volt.
monopolar recording: Recording method that uses one active and one reference electrode.
motor unit: an alpha motor neuron and the skeletal muscle fibers it innervates.
nucleus: Central mass of an atom that contains protons and neutrons.
Ohm's law: Voltage (E) = current (I) X resistance (R). The “amount” of current (I) flowing through a conductor is equal to the voltage (E) or “push” divided by the resistance (R)
open circuit: An incomplete path that prevent electrons movement from the power source, through the conductor and resistance, and back to the source. For example, a broken sensor cable.
optical isolation: Device that converts a biological signal into a beam of light, the light crosses a gap (open circuit), and a photoreceptor reconverts the light into an electrical signal.
power (W): The rate at which energy is transferred, which is proportional to product of current and voltage; measured in watts.
proton: Positively-charged subatomic particle found in the nucleus of an atom.
Quantitative EEG (QEEG): Digitized statistical brain mapping using a 19- or 72-channel montage that measures EEG amplitude within specific frequency bins.
resistance (R): Opposition to a DC signal by a resistor measured in ohms.
resistor: A component in electric circuits that resists current flow.
skin conductance level (SCL): A tonic measurement of how easily an AC or DC current passes through the skin, expressed in microsiemens
skin resistance level (SRL): A tonic (resting) measurement of the opposition to an AC or DC current as it passes through the skin, expressed in Kohms.
superconductor: A material that conducts electricity without resistance.
ventricular fibrillation: Medical emergency in which the lower heart chambers contract in a rapid and unsynchronized fashion and cannot pump blood.
volume conduction: Movement of biological signals through interstitial fluid.
volt (V): Unit of electrical potential difference (electromotive force) that moves electrons in a circuit.
voltage (E): The amount of electrical potential difference (electromotive force).
voltohmmeter: Device that uses a DC signal to measure resistance in an electric circuit, such as between active and reference electrodes.
watt (W): Unit of power used to express signal strength in the QEEG.
Now that you have completed this module, review how you check whether your electrode is intact and measure impedance with your own biofeedback equipment.
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