Electricity

Where Biological Signals Come From

Before you can monitor biological signals, you need to understand what produces them. When a neuron fires in your client's brain or a muscle fiber contracts in their forearm, charged particles called ions flow across cell membranes. These ion movements create tiny electrical currents that your biofeedback equipment detects. To grasp how this works, we need to start with the basic building blocks of matter.

Atoms and Their Charged Particles

Matter is anything that takes up space and has mass, including the solid, liquid, and gaseous components of your client's body. All matter is made of atoms, which are the smallest units that retain the properties of an element. Think of an atom as having two main parts: a dense central core called the nucleus, surrounded by smaller particles that orbit around it.

The nucleus contains two types of particles. Protons carry a positive electrical charge, while neutrons carry no charge at all. Orbiting the nucleus are electrons, which carry a negative charge equal in strength but opposite in sign to a proton's positive charge. In a normal atom, the number of electrons equals the number of protons, so the overall electrical charge is zero.

Why does this matter for biofeedback? The biological signals you monitor occur because atoms can gain or lose electrons, becoming electrically charged. When an atom gains or loses electrons, it becomes an ion. Ions carry either positive or negative charges, and their movement through body tissues creates the electrical signals that your equipment detects.

Elements and Why They Matter for Physiology

Elements are pure substances made of identical atoms that cannot be broken down by ordinary chemical reactions. Each element has a unique atomic number, which is simply the count of protons in its nucleus. Carbon has 6 protons, sodium has 11, and potassium has 19. The atomic weight approximately equals the number of protons plus neutrons.

Several elements are critical to generating the signals you will monitor. Calcium (Ca), chloride (Cl), potassium (K), and sodium (Na) are the key players. When these elements gain or lose electrons, they become the ions that flow across nerve and muscle cell membranes. For example, when a cortical neuron fires during brain activity, sodium ions rush into the cell while potassium ions flow out, creating the voltage changes that appear on an EEG display.

Recent research by Quigley et al. (2024) in guidelines published by the Society for Psychophysiological Research has emphasized the importance of understanding these ionic mechanisms for proper interpretation of psychophysiological recordings. The authors note that heart rate variability, for instance, reflects complex interactions between sympathetic and parasympathetic neural control systems that operate through ion channel activity. This same ionic foundation underlies the electrodermal, electromyographic, and electroencephalographic signals you will monitor in clinical practice.

How Signals Travel From Body to Equipment

Now that you understand where biological signals originate, you need to know how they travel from your client's body into your biofeedback equipment. This journey involves two different types of electrical movement: the flow of ions inside the body and the flow of electrons through your equipment. Understanding both is essential because problems at any point in this pathway can produce invalid readings.

Current: The Movement of Charged Particles

Current (I) refers to the movement of charged particles through a material. In your equipment, current consists of electrons flowing through wires. The classic model explains this movement through the attraction and repulsion of electrical charges. Opposite charges attract each other, so electrons (negative) flow toward positive regions. Identical charges repel each other, so electrons move away from negative regions.

Materials differ in how easily they allow current to flow. Conductors like copper wires allow electrons to move freely, which is why your electrode cables use copper to carry signals from sensors to your equipment. Insulators like rubber and plastic resist electron movement, which is why your cables are coated in protective material that prevents signals from leaking out or interference from leaking in.

Volume Conduction: How Signals Reach the Skin Surface

Inside your client's body, signals do not travel through wires. Instead, ion currents move through interstitial fluid, the liquid that fills the spaces between cells. This process is called volume conduction, and it explains why you can detect brain activity from the scalp or muscle activity from the skin surface rather than having to insert electrodes directly into the tissue.

Think of volume conduction like ripples spreading across a pond. When a stone drops into water, waves spread outward in all directions. Similarly, when a neuron fires or a muscle contracts, the resulting electrical disturbance spreads through the interstitial fluid until it reaches the skin surface. Your electrodes detect this disturbance much like a buoy detects passing waves.

Research by Chowdhury et al. (2023) has demonstrated that volume conduction creates significant challenges for EMG signal interpretation. In their systematic review of noise and artifact reduction techniques, the authors found that as much as 30% of a signal detected directly over an active muscle can appear at inactive recording sites through volume conduction (Chowdhury et al., 2023). This phenomenon, called crosstalk, occurs when electrical activity from one muscle contaminates recordings from another muscle, potentially leading to incorrect conclusions about which muscles are active during a movement.

Volume conduction also explains why ECG artifact can appear in SEMG recordings. When the heart muscle contracts, it generates strong electrical signals that spread throughout the body via volume conduction. If you place SEMG electrodes on the torso or upper back, these heart signals may appear as sharp vertical spikes (R-spikes) superimposed on the muscle activity you are trying to measure. Recognizing this artifact prevents you from misinterpreting heart signals as muscle tension.

Electrodes: Converting Ions to Electrons

Electrodes are specialized conductors that serve as the bridge between the body's ion currents and your equipment's electron currents. They convert the ionic signal at the skin surface into an electronic signal that can travel through your cables and into your amplifier. Without this conversion, your equipment would have nothing to display.

For EEG monitoring, electrodes function like tiny antennas picking up the weak signals that macrocolumns of cortical neurons broadcast to the scalp surface. The signals arrive as currents of ions that have volume conducted through the skull and scalp tissues. The electrode captures these signals and converts them to electron currents that travel down the cable to your electroencephalograph.

Here is why skin preparation matters so much. The outer layer of skin (the epidermis), along with oils, dead cells, and body fat, all act as insulators that interfere with ion current flow. These barriers significantly reduce the signal that reaches your electrodes, potentially burying valid biological data in electrical noise. When you clean, abrade, and apply conductive gel to a client's skin, you are reducing this insulating barrier so more of the biological signal can reach your electrodes.

Understanding Current Measurement

Current is measured in amperes (A). One ampere equals one coulomb of charge (approximately 6.24 x 10¹⁸ electrons) passing a point in one second. You will rarely work with full amperes in biofeedback since biological signals involve much smaller currents, typically measured in milliamperes (mA, thousandths of an ampere) or microamperes (μA, millionths of an ampere).

Two Types of Current: DC and AC

Electricity travels in two fundamental patterns, and understanding both is essential because different biofeedback modalities measure different types of signals. Direct current (DC) flows in one direction only, like water flowing downhill. Alternating current (AC) periodically reverses direction, like water sloshing back and forth in a bathtub.

Many biofeedback modalities measure DC signals. Blood volume pulse, skin temperature, and respiratory signals are all DC because they represent quantities that change gradually in one direction before reversing. When you monitor hand warming in temperature biofeedback, you are tracking a DC signal that slowly rises as blood vessels dilate and slowly falls as they constrict.

Other modalities measure AC signals. The EEG, ECG, and SEMG all contain waveforms that oscillate above and below a baseline many times per second. The frequency of an AC signal, measured in hertz (Hz), tells you how many complete cycles occur each second. When you train a client to increase alpha waves in neurofeedback, you are working with AC signals that oscillate at 8 to 12 cycles per second (8-12 Hz).

EMG signals contain frequencies ranging from 2 to 10,000 Hz, but surface electrodes can only detect frequencies up to about 1,000 Hz. Higher frequencies are absorbed by the fat and skin layers before reaching the surface, which is why needle electrodes inserted directly into muscle can detect a wider frequency range than surface electrodes.

What Pushes Current Through a Circuit

For current to flow, there must be a difference in electrical potential between two points. This difference, called electromotive force (EMF), provides the "push" that drives electrons through a circuit. In a flashlight battery, the negative terminal has excess electrons that repel each other, while the positive terminal has a deficit of electrons that attracts them. This difference creates the push that makes current flow.

Biological signals work on the same principle. When a neuron fires, the inside of the cell briefly becomes more positive than the outside, creating a potential difference. This voltage difference drives ion current through the surrounding tissue. When that current reaches your electrodes, the electrodes convert it to electron current that flows through your equipment because of the potential differences created by your amplifier.

A Modern Understanding of Electrical Energy

The classic model of electrons physically traveling through wires is actually a simplification. Electrons do not race from a power plant to your light bulb or from a battery to your flashlight. Instead, electromagnetic fields carry energy through space, causing electrons to move slightly in their local positions. This is true for all electrical systems, including the biological signals you monitor. Watch the YouTube video The Big Misconception About Electricity for an excellent explanation of this concept.

Measuring Signal Strength and Opposition

When you perform biofeedback, you need to know whether your equipment is receiving an adequate signal. Two measurements help you evaluate this: voltage (which indicates signal strength) and impedance (which indicates how much opposition the signal encounters). Understanding these concepts and their relationship through Ohm's law will help you troubleshoot problems and ensure valid recordings.

Voltage: Measuring Signal Strength

The strength of an electrical signal is measured in volts (V), with voltage abbreviated as E in equations. Think of voltage as the "pressure" pushing electrons through a circuit, similar to water pressure pushing water through a pipe. A typical flashlight battery produces 1.5 volts. One volt is the potential difference needed to make one coulomb of charge perform one joule of work.

Biological signals are measured in much smaller units. EEG and SEMG amplitudes are typically less than 100 microvolts (μV), where one microvolt is one millionth of a volt. ECG signals are somewhat larger, measured in millivolts (mV), where one millivolt is one thousandth of a volt. When you see SEMG readings of 5 μV on your display, you are looking at an extremely weak signal that your equipment has amplified thousands of times.

Power: Voltage and Current Combined

The overall power of an electrical signal depends on both how much current flows and how much voltage drives it. Power is measured in watts (W) and calculated by multiplying amperes by volts. For example, an appliance drawing 10 amperes at 115 volts consumes 1,150 watts.

In quantitative EEG (qEEG) analysis, clinicians and researchers increasingly express signal strength in picowatts, which are trillionths of a watt. The qEEG is digitized statistical brain mapping that uses at least 19 channels to measure EEG amplitude within specific frequency bands. Expressing results in power units helps standardize comparisons across different recording conditions.

Resistance: Opposition in DC Circuits

Resistance (R) is the opposition that electrons encounter as they move through a material. Resistance is measured in ohms (Ω). Every conductor has some resistance, even excellent conductors like copper wire. The amount of resistance depends on the material's atomic structure. Atoms with more electrons in their outer energy level hold onto those electrons more tightly, creating greater resistance.

Resistance is also a biological signal in its own right. Skin resistance level (SRL) measures how much the skin opposes current flow, expressed in kilohms per square centimeter (Kohms/cm²). Typical values range from 0 to 500 Kohms/cm². When your client experiences emotional arousal, increased sweat gland activity adds moisture to the skin, which reduces resistance. Monitoring this change is the basis of skin conductance biofeedback.

Recent research has expanded our understanding of skin electrical properties. Schach et al. (2022) found that electrodermal activity biofeedback can alter functional brain networks in people with epilepsy, demonstrating the close relationship between peripheral sympathetic activity and central nervous system function. Their work showed that training individuals to control their skin conductance levels may influence cortical excitability, highlighting how the electrical properties you measure at the skin surface reflect deeper neurophysiological processes.

Conductance: The Opposite of Resistance

Conductance (G) is the flip side of resistance. While resistance measures how much a material opposes current flow, conductance measures how easily current flows through it. Mathematically, conductance is the reciprocal of resistance (G = 1/R). Conductance is measured in siemens (previously called mhos, which is ohm spelled backward).

Some biofeedback systems measure electrodermal activity as skin conductance rather than skin resistance. Skin conductance level (SCL) is expressed in microsiemens (μS). When sweat gland activity increases, skin conductance goes up because moisture improves the skin's ability to conduct electricity. Whether your system measures resistance or conductance, you are monitoring the same underlying phenomenon from opposite perspectives.

Ohm's Law: The Fundamental Relationship

Ohm's law describes the mathematical relationship between voltage, current, and resistance: Voltage (E) = Current (I) × Resistance (R). This simple equation has profound practical implications for biofeedback. If you know any two values, you can calculate the third. More importantly, you can use this relationship to understand why certain procedures improve signal quality.

Also, check out the YouTube video MAKE Presents: Ohms Law.

You can also rearrange Ohm's law to understand skin preparation. If Current (I) = Voltage (E) / Resistance (R), then reducing resistance increases current. When you clean and abrade a client's skin, you reduce the resistance that biological signals must overcome. This allows more current to reach your electrodes, improving signal quality. The same principle explains why applying conductive gel helps: the gel provides a low-resistance pathway between skin and electrode.

Dry electrodes, like those in some modern EEG systems, do not require skin preparation or conductive paste. These electrodes use design innovations to achieve adequate signal quality without the traditional preparation steps. They save setup time but may not achieve the same signal quality as properly prepared wet electrodes.

Impedance: Opposition in AC Circuits

Impedance (Z) is the opposition that AC signals encounter in a circuit, measured in ohms. Impedance is more complex than simple resistance because it includes the effects of frequency. When current reverses direction many times per second, it encounters additional opposition from circuit components that store and release energy. For practical purposes in biofeedback, you can think of impedance as the AC equivalent of resistance.

Murphy et al. (2021) conducted a landmark study examining how the skin-electrode interface impedance changes over time under different skin treatments. Their research compared mechanical abrasion, chemical exfoliation, microporation, and no treatment in healthy human subjects over 24 hours. The findings revealed that mechanical abrasion yields the lowest initial impedance, while other treatments provide only modest improvement compared to untreated skin (Murphy et al., 2021). This research confirms why the traditional practice of mild abrasion before electrode placement remains the gold standard for achieving low, stable impedance in clinical biofeedback applications.

You can measure impedance manually using an impedance meter (for AC) or voltohmmeter (for DC), or you can use software integrated with your biofeedback system. Many modern systems include automatic impedance checking that displays values for each electrode site.

For SEMG biofeedback, follow this practical guideline: impedance should not exceed 10 kilohms (10 Kohms), and the impedance at each electrode site should be balanced within 5 Kohms. If one electrode has much higher impedance than another, the imbalance can cause artifacts that distort your readings. When impedance is too high or unbalanced, clean and abrade the skin again, apply fresh conductive gel, and retest.

For AC circuits, Ohm's law becomes: voltage = current × impedance (e = i × z), using lowercase letters to indicate AC values. The relationship works the same way as for DC circuits. Higher impedance reduces current flow for any given voltage.

Signal Processing and Equipment Troubleshooting

Your biofeedback equipment does far more than simply display raw biological signals. It processes those signals through multiple stages: dropping them across input impedance, amplifying them, filtering out unwanted frequencies, and converting them into visual or auditory feedback. Understanding this signal chain helps you troubleshoot problems when your displays do not match what your client is actually doing.

Recording Methods: Monopolar and Bipolar

Monopolar recording uses one active electrode and one reference electrode to produce a single signal. This method captures activity at one location relative to a neutral reference point. Bipolar recording uses two active electrodes and a shared reference, producing two signals that can be compared or subtracted from each other. Bipolar recording helps reject noise that appears equally at both active sites.

When SEMG signals enter your electromyograph, they are dropped across an input impedance (a network of resistors), then amplified, filtered to remove unwanted frequencies, rectified to convert the AC signal to DC, integrated over time, measured by a level detector, and finally displayed. Problems at any stage can produce invalid readings that do not reflect your client's actual muscle activity.

Recent advances in wearable technology have introduced new challenges for signal processing. Böttcher et al. (2022) evaluated data quality in wearable neurophysiological monitoring and found that all recording modalities, including accelerometry, electrodermal activity, and blood volume pulse, are affected by artifacts. Their study of over 127,000 hours of recordings revealed that data loss was significantly higher when using data streaming compared to onboard device storage (Böttcher et al., 2022). For biofeedback practitioners, this research underscores the importance of understanding how your equipment processes and transmits data, as signal processing choices can substantially affect the quality of the feedback you provide to clients.

Open Circuits: When the Signal Path Breaks

A closed circuit provides a complete path for current to flow from source to destination and back. An open circuit has a break somewhere in this path that stops current flow entirely. The most common cause of open circuits in biofeedback is a broken electrode cable. If the wire inside the cable has fractured, electrons cannot travel from your electrodes to your amplifier, and no signal will appear on your display.

You can detect a broken cable by performing a continuity test. Send a test signal down the cable and measure the opposition to current flow. If the cable is intact (continuous), impedance will be very low because current flows easily through the conductor. If the cable is broken (open), impedance will be infinite because current cannot flow across empty space. Replacing a broken cable immediately solves this problem.

Listen to a mini-lecture on Open and Closed Circuits

A blown fuse is another example of an open circuit. Fuses contain a thin filament that melts when current exceeds safe levels, intentionally breaking the circuit to prevent damage. If your equipment suddenly stops working after a power surge, a blown fuse may be the cause.

Behavioral Tests: Verifying the Entire System

A continuity test only tells you whether current can flow through a cable. It does not tell you whether your entire system is accurately tracking your client's physiological activity. For that, you need a behavioral test, also called a tracking test. This test verifies that the complete signal chain (sensor, preamplifier, cable, encoder, and computer) responds appropriately to known physiological changes.

Always perform a behavioral test before beginning training with a client. This simple check catches problems that impedance testing alone would miss, such as software configuration errors, display calibration issues, or sensor malfunctions. If your display does not track voluntary muscle activity, do not proceed with training until you identify and correct the problem.

Short Circuits: When Current Takes a Wrong Turn

A short circuit occurs when an unintended connection creates a new pathway for current to flow. This new pathway typically has much lower resistance than the intended circuit, so it draws excessive current away from the proper route. On an impedance meter, a short circuit reads close to 0 Kohms because current flows very easily through the unintended pathway.

Listen to a mini-lecture on Short Circuits

Short circuits are dangerous because the reduced resistance allows excessive current to flow. According to Ohm's law, if resistance decreases while voltage stays constant, current must increase. This increased current can generate enough heat to melt wires, damage components, or cause electrical burns. Imagine a bare wire inside an electroencephalograph touching its metal case. AC from the power outlet could leak through the case and shock anyone who touches the equipment.

Maintaining Signal Quality

The biological signals you monitor are extremely weak compared to the electromagnetic noise that surrounds them. Power lines, fluorescent lights, computer monitors, and cell phones all generate electrical fields that can contaminate your recordings. Your equipment amplifies the biological signal to make it distinguishable from this noise, but amplification works on everything: it boosts the signal you want while also boosting any noise that got through.

This is why connection quality matters at every point in the signal chain. The skin surface must be properly prepared to reduce impedance. The conductive gel or paste must provide good contact between skin and electrode. The sensors must be firmly attached and properly positioned. The connecting wires must be intact and properly shielded. Problems at any of these points degrade signal quality and can produce displays that mislead both you and your client.

Protecting Clients From Electrical Hazards

Every time you connect a client to biofeedback equipment, you become responsible for their electrical safety. Modern equipment includes multiple protective features, but understanding these safeguards helps you use them properly and recognize when something might be wrong. The stakes are real: electrical current at surprisingly low levels can cause injury or death.

Understanding Dangerous Current Levels

The numbers are sobering. A 1-second exposure to just 5 milliamperes (5 mA) can cause injury. At 18 mA, current can interfere with breathing. At 50 mA, current can trigger ventricular fibrillation, a life-threatening condition where the heart's lower chambers quiver chaotically instead of pumping blood. To put this in perspective, a typical household outlet can deliver thousands of milliamperes.

Biomedical engineers have developed multiple strategies to protect clients from these hazards. Modern biofeedback equipment uses ground fault interrupt circuits, optical isolation, fiber optic connections, and wireless telemetry to prevent dangerous current from reaching clients. Understanding how each protection works helps you verify that safety systems are functioning properly.

The IEC 60601 standard, published by the International Electrotechnical Commission, provides the foundational framework for electrical safety in medical devices worldwide. This series of standards covers general requirements for basic safety and essential performance of medical electrical equipment, including the biofeedback devices you use in clinical practice. The standard specifies allowable leakage current limits, which is the small amount of current that can flow through insulation or along unintended paths even in properly functioning equipment. Understanding that these international standards govern your equipment's design helps you appreciate the multiple layers of protection built into modern biofeedback systems.

Ground Fault Interrupt Circuits

A ground fault interrupt circuit (GFIC) monitors the current flowing through equipment and automatically shuts off power when it detects a problem. Specifically, it watches for current "leaking" from the circuit to ground, which indicates that electricity is flowing through an unintended pathway. When leakage exceeds 5 mA, the GFIC triggers a circuit breaker that cuts power immediately.

Montgomery (2004) recommended plugging your entire biofeedback system into the same power outlet. This creates a common ground reference so that any current leakage in your equipment will trigger the GFIC protection. If you spread equipment across multiple outlets on different circuits, some leakage might not be detected because it returns through a different ground path.

Optical Isolation

Optical isolation protects clients from equipment that receives AC power by converting electrical signals to light and back again. Inside an optical isolator, the biological signal drives a light-emitting component. The light crosses a physical gap (which is actually an open circuit that blocks electron flow), and a light-sensitive component on the other side converts the light back into an electrical signal. Current cannot flow across the gap, so a malfunction in the powered equipment cannot send dangerous current to the client.

Fiber Optic Connections

Fiber optic cables carry signals as pulses of light through thin, flexible glass or plastic fibers. Because light consists of photons rather than electrons, fiber optic cables cannot conduct electrical current. When your biofeedback system uses fiber optic connections between the electrodes and the computer, it becomes physically impossible for electrical current to leak from the computer to your client.

Fiber optic connections provide an additional benefit: they are immune to electromagnetic interference. Power line noise, radio waves, and other electrical disturbances cannot contaminate a light signal the way they can contaminate an electrical signal. This means cleaner recordings with less artifact.

Wireless Telemetry

Telemetry transmits physiological data wirelessly from a battery-powered encoder unit worn by the client to a computer several meters away. Because there is no physical electrical connection between the client and the computer, current surges cannot reach the client even if the computer malfunctions. The encoder unit uses its own batteries, so it is not connected to building power.

Battery-powered encoder boxes are standard in modern biofeedback systems. Even when these units connect to computers via cables rather than wirelessly, the battery power isolates clients from building electrical systems. Combined with optical isolation and fiber optic connections, multiple layers of protection stand between your client and potential electrical hazards.

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