Muscle sensors monitor a middle-aged patient's lower back during a sequence of movements designed to test these antigravity muscles. An
electromyograph detects, boosts, and separates faint surface EMG (SEMG) muscle voltages from
competing energy sources. A physical therapist conducting this muscle evaluation watches a rapidly updated graphic display of these voltages to assess the patient's back pain's muscular component.
Without a basic understanding of electricity and the circuits
used in biofeedback instruments, we might mistakenly accept readings
produced by equipment misuse or breakdown. "Garbage in, garbage out."
BCIA Blueprint Coverage
This unit addresses 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).
This unit covers the Building Blocks of Matter, Electric Current, Measures of Signal Power and Opposition, Signal Processing, and Safety Precautions.
Please click on the podcast icon below to hear a full-length lecture.
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,
whereas 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 or molecules 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 currents of ions. The ions
most responsible for these signals are chloride (Cl-), potassium (K+),
and sodium (Na+).
Electric Current
Current (I) is the movement of electrons through a conductor. The classic model is that current 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 an electric current.
Electrons are also affected by the materials in their path. Conductors like copper allow electron movement, while insulators
that enclose the wires oppose their movement.
Biological signals like the EEG travel through interstitial fluid,
which is the fluid between the cells in our bodies. 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 the scalp's cortical potentials instead of inserting electrodes inside the brain.
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.
Electrodes transform the current of ions into an electron
current that flows through the cable into an electromyograph's input
jack. Below is a
BioGraph ® Infiniti SEMG display.
Volume conduction is also responsible for ECG 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 protective filtering to reject ECG artifacts. Note the SEMG
signal's contamination by the sharp vertical
R-spikes of the ECG.
Insulation from body fat, connective tissue, and the epidermis (outer skin layer) interferes with ion current
flow and significantly reduces surface EMG readings. Insulators, like the rubber
covering the wiring of a muscle electrode, block the flow of electric currents. In biological and
fabricated insulators, a 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 (Nilsson & Riedel, 2008).
Measuring Current
We learn how much "x" has passed by a point over a fixed period when we measure current. The
"amount" of electric current is measured in amperes (A). You have used 1
ampere of current when 1 coulomb (6.24 x 1018 or 6 billion billion electrons) has
passed a point in 1 second (Kubala, 2009).
DC and AC
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 battery's negative terminal repels electrons (e-) while the positive terminal attracts them. Biological signals representing peripheral blood flow (blood volume pulse and skin temperature),
respiration, and skin electrical activity are all DC signals.
When we plot DC signals against time, they never completely reverse direction over a second. The
electroencephalogram (EEG) contains both DC (slow cortical potentials) and AC (slow cortical potentials and delta
through 40-Hz) waveforms. Below is a BioGraph ®
Infiniti blood volume pulse (BVP)
display.
In the space of a second, an alternating current (AC) regularly reverses direction 50
or 60 times. The frequency of an alternating
current is the number of cycles completed per second or hertz (Hz). Electrical potentials detected from the
cerebral cortex (EEG), heart (ECG), and skeletal muscles (SEMG) all contain AC waveforms (Kubala, 2009). Check out the YouTube video AC and DC Differences.
Below is a BioGraph ®
Infiniti 60-Hz artifact display. The software uses an auto-scale feature to keep the fluctuating signal on the
screen.
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. The raw ECG (also called EKG) waveform is shown at the top in pink.
Note the upward-thrusting R-spike. The frequency distribution is shown at the bottom left as a frequency histogram
and the bottom right as a three-dimensional power spectral 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.
Electromotive Force (EMF)
What forces electrons to move through a circuit? 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. There is no potential difference, no current, and no light (Nilsson & Riedel, 2008).
Electromagnetic Fields Carry Energy Instead of Electrons
The classic model of electrons traveling through conductors in two directions is an explanatory fiction. Electrons don't actually travel from battery to light bulb or power plant to your microwave. As the Veritasium video shows, it is electromagnetic fields (shown above) that travel and carry energy in one direction. This is true for sunlight, powerlines, and neurons. Watch the YouTube video, The Big Misconception About Electricity.
Measures of Signal Power and Opposition
Voltage
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 (Nilsson & Riedel, 2008).
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.
Watts
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 (Kubala, 2009).
In neurofeedback, clinicians and researchers increasingly express the quantitative EEG (qEEG) signal strength, digitized statistical brain mapping using at least a 19-channel montage to measure EEG amplitude within specific frequency bins, in picowatts (trillionths of a watt). Below are 21- and 32-channel Mitsar amplifier systems featured on the NovaTech EEG website.
Resistance
Electrons moving through a conductor encounter opposition which reduces current flow. This phenomenon is
called resistance (R) in DC circuits and impedance (Z) in AC circuits and is measured in ohms
(Ω). Resistance depends on the number of electrons found in an atom's
outermost energy level.
Resistance is a practical concern in biofeedback. Biological signals compete with stronger false signals
for a biofeedback instrument's attention. Clinicians clean, abrade, and apply conductive gel to their clients'
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 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 is
expressed in ohms (Ω). Conductance is now measured in Siemens and was previously
measured in mhos (mho is ohm spelled backward). Skin conductance is one index of
eccrine sweat gland activity.
Ohm's Law
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 (Nilsson & Riedel, 2008).
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 (I) 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 helps an electromyograph select muscle action potentials from artifacts.
The picture below illustrates mild abrasion using a cosmetic pad. Abrasion removes dead skin and dirt, which can
weaken the SEMG signal.
Dry electrodes like BrainMaster's Freedom 20R do not require time-consuming skin preparation and the application of conductive paste.
Impedance
In AC circuits, current periodically reverses direction. Reversal of current direction
introduces frequency, the number of complete cycles completed each
second. Frequency is measured in hertz (Hz). When an AC travels
through a circuit at a given frequency, it encounters a complex form of opposition called impedance (Z),
measured in ohms (Ω).
Clinicians perform an impedance test to determine whether they have prepared
the skin adequately and attached electrodes (Andreassi, 2007). Excessive impedance means that a weak biological signal must
compete at a disadvantage with false electrical signals like power line artifact. High impedance could contaminate the SEMG
signal so severely that an electromyograph displays power line fluctuations instead of skeletal muscle activity.
An impedance test may also be performed by software integrated with a
data acquisition system and sensors.
A conservative rule of thumb for SEMG biofeedback is that impedance should not exceed
10 Kohms
and should be balanced within 5 Kohms for each active electrode-reference
electrode pair.
Ohm's Law for AC Circuits
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). Voltage is the product of a current flowing across an impedance. In actual units, 50 volts = 10 amperes x 5
ohms.
Signal Processing
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.
Open and Closed Circuits
Broken electrode cables are a significant 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 space.
If, instead, the cable is free of breaks (continuous), the circuit is described as closed. Impedance will approach 0 Kohms since current can readily travel through the circuit.
For example, when monitoring muscle activity, we can test the entire signal chain's performance (EMG sensor, EMG preamplifier, cable, encoder, and computer) by asking the client to contract a muscle and then relax it three times. See the three blue 100-microvolt SEMG peaks. Since the computer display mirrored these actions, the behavioral test was passed, confirming that the entire system tracks voluntary muscle activity.
Short Circuit
A short circuit results when an unintended connection is made between two circuit locations.
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 clients (Nilsson & Riedel, 2008).
Visualize a bare wire inside an electroencephalograph touching its metal case.
The AC powering this equipment could leak through the metal case
and injure anyone touching this surface.
Preventing Signal Contamination
Physiological signals are quite small compared to surrounding
electromagnetic "noise." They need to be amplified to be distinguishable
from background noise.
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, conductive gel or paste, and sensors, and connecting wires.
Safety Precautions
Like computer-based data acquisition systems, line-powered equipment can expose both a client and practitioner to shock hazards. Clients and clinicians should avoid contact with metal surfaces, and water spills should be immediately cleaned
up.
Encoder boxes that receive sensor cables are battery-powered and connected to computers by fiber optic cables or Bluetooth connections to eliminate shock hazards.
A ground fault interrupt circuitis 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 client, therapist, and hardware.
Montgomery (2004)
recommended 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 isolationprotects a client from hardware receiving AC
power. An optical isolator converts a biological signal into a light beam, the light crosses a gap (open
circuit), and a photoreceptor reconverts the light into an electrical signal.
Graphic retrieved from Autodesk.
Fiber optic connections, thin, flexible cables that transmit digital signals
as pulses of light, transmit photons between the electrodes and data acquisition system. This prevents current
from leaking from a computer to a client since electrons cannot travel through fiber optic cables. This approach
also reduces contamination by electrical artifacts like power line noise.
Telemetry can wirelessly transmit physiological data from a battery-powered encoder
unit to a computer many meters away. This technology protects clients from shock since current surges cannot
travel across a Bluetooth connection (Montgomery, 2004). MindMedia's NeXus-10 featured below communicates wirelessly with a computer for data acquisition.
Glossary
alpha blocking: the 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 flow rate of
electrons through a conductor. One volt dropped across one ohm of
resistance produces a current flow of one ampere.
artifact: false signals like 50/60Hz noise produced by line current.
atom: thebasic 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): a procedure to ensure that a
biofeedback instrument accurately detects and displays subject
performance.
bipolar recording: a recording method that uses two active electrodes
and a shared reference.
closed circuit: acomplete path that allows electrons to
travel from the power source, through the conductor and resistance, and
back to the source.
conductance (G): the ability of a material like copper or silver
to carry an electric current. Conductance is measured in siemens (formerly mhos).
conductor: a material that readily allows electron movement like
a copper wire.
continuity test: a procedure to ensure that a circuit is closed, for example, that a
cable is not broken.
coulomb (C): approximately6.24 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 resistor network. direct current (DC):an electric current that flows in only one
direction, as in a flashlight. dry electrode:electrode that does not require a conductive gel or paste.
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.
electron: negatively charged particle that rotates around the nucleus at varying
distances and participates in chemical reactions.
energy level: one of an electron's possible orbits around a
nucleus at a constant distance.
epidermis: the outermost skin layer.
fiber optic cable: 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: a 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 measure
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 molecule with a positive or negative electrical
charge.
mho: unit of conductance replaced by the siemen.
microsiemen (μS): the unit of conductance
that is one-millionth of a siemen.
microvolt (μV): the unit of amplitude
(signal strength) that is one-millionth of a volt.
milliampere (mA): the 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: a recording method that uses one active and
one reference electrode.
motor unit: an alpha motor neuron and the skeletal muscle fibers
it innervates.
nucleus: the 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 prevents electron movement
from the power source, through the conductor, and back to
the source. For example, a broken sensor cable.
optical isolation: a 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, proportional to current and voltage products. Power is measured in watts and picowatts (trillionths of a watt).
proton: positively charged subatomic particle found in the
nucleus of an atom.
Quantitative EEG (qEEG): digitized statistical brain mapping
using at least a 19-channel montage to measure EEG amplitude within
specific frequency bins.
resistance (R): the opposition to a DC signal by a resistor measured
in ohms.
resistor: the component in an electric circuit that resists current
flow.
skin conductance level (SCL): a tonic measurement of how easily
an AC or DC passes through the skin, expressed in microsiemens.
skin resistance level (SRL): a tonic (resting) measurement of the
opposition to an AC or DC as it passes through the skin,
expressed in Kohms.
superconductor: a material that conducts electricity without
resistance.
ventricular fibrillation: a 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): the unit of electrical potential difference (electromotive
force) that moves electrons in a circuit.
voltage (E): the amount of electrical potential difference
(electromotive force).
voltohmmeter: a device that uses a DC signal to measure resistance
in an electric circuit, such as between active and reference electrodes.
watt (W): the unit of power used to express signal strength in the
qEEG.
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Assignment
Now that you have completed this module, review how you check whether
your electrode is intact and measure impedance with your own biofeedback
equipment.
References
Andreassi, J. L. (2000). Psychophysiology: Human behavior and physiological
response. Lawrence Erlbaum and Associates, Inc.
Basmajian, J. V. (Ed.). (1989). Biofeedback: Principles and practice for
clinicians. Williams & Wilkins.
Cacioppo, J. T., & Tassinary, L. G. (Eds.). (1990). Principles of
psychophysiology. Cambridge University Press.
Floyd, T. L. (1987). Electronics fundamentals: Circuits, devices, and
applications. Merrill Publishing Company.
Grant, A. (2015). Four elements earn permanent seats on the periodic table. Science News.
Kubala, T. (2009). Electricity 1: Devices, circuits, and materials (9th ed.). Cengage Learning.
Montgomery, D. (2004). Introduction to biofeedback. Module 3:
Psychophysiological recording. Association for Applied Psychophysiology and Biofeedback.
Nilsson, J. W., & Riedel, S. A. (2008). Electric circuits (8th ed.). Pearson Prentice-Hall.
Peek, C. J. (2016). A primer of traditional biofeedback instrumentation.
In M. S.
Schwartz, & F. Andrasik (Eds.). (2016). Biofeedback: A practitioner's guide (4th ed.).
The Guilford Press.
Stern, R. M., Ray, W. J., & Quigley, K. S. (2001). Psychophysiological
recording (2nd ed.). Oxford University Press.
