Electrodes and Acquisition Systems

Learning Objectives

After completing this section, you will be able to:

Electrodes and Acquisition Systems

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Electrodes detect biological signals. They are also transducers since they convert energy from one form to another. Four types of EEG electrodes are shown below: gold cup, gold flat, silver cup, and silver/silver-chloride ring.

Common electrode materials for EEG recording include gold-plated, silver, silver/silver-chloride, and tin. Sintered silver/silver-chloride electrodes are used for recording slow cortical potentials. All electrodes in a single EEG recording must be made of the same material (e.g., all tin, all silver, all gold, all silver/silver-chloride, or all sintered silver/silver-chloride).

Sintered means the silver and silver-chloride particles are fused together under heat and pressure into a solid electrode material, rather than being plated or coated onto a base metal. This produces a more stable, lower-noise electrode surface, which is why sintered silver/silver-chloride is preferred for recording slow cortical potentials.

Electrode materials may be flat or formed into rings or cups, some of which have holes at the top. Electrodes are sometimes formed into disposable pellets, with or without a housing, onto which a cable can be snapped. When the earlobe is used as a reference or ground site, electrodes are mounted in a snap or spring-loaded clip.

Here are examples of tin and silver/silver-chloride electrodes. Tin cup electrodes and tin earclip electrodes are shown below.

Bio-Medical Instruments provided this photograph.

Silver/silver-chloride electrodes are shown below.

Bio-Medical Instruments provided this photograph.

Sintered silver/silver-chloride electrodes are shown below.

Bio-Medical Instruments provided these photographs.

Tin disposable pellet electrodes are shown below.

Bio-Medical Instruments provided this photograph.

Electrode Caps

Rather than positioning individual electrodes one at a time, most multichannel recordings use an electrode cap. Caps come in two broad designs, wet and dry, and the choice between them shapes preparation time, client comfort, and signal quality.

Wet Cap Design

An electrode cap is a stretchable garment, usually made of an elastic spandex or Lycra fabric, that holds recording sensors at fixed scalp locations. Manufacturers position the electrode holders according to the international 10-20 system, so that once you fit the cap to your client's head, every sensor lands at a standardized site. In a wet cap, each holder seats a silver/silver chloride (Ag/AgCl) electrode or a tin electrode, and you fill the small well with a conductive gel or paste using a blunt-tipped syringe. That gel bridges the gap between the scalp and the metal, carrying the brain's ionic signals to the amplifier while you gently abrade the skin to lower resistance. Technicians typically aim for an impedance below 5 to 10 kΩ before recording begins (Bayat et al., 2025).

Advantages and Disadvantages of Wet Caps

Wet caps remain the gold standard for a good reason. The gel forms a stable electrochemical bridge that produces low, uniform impedance and an excellent signal-to-noise ratio, which is why clinical and research laboratories have trusted them for decades (Hinrichs et al., 2020). When Kam and colleagues (2019) compared a wet system against a dry alternative, the wet electrodes held a small advantage in single-trial classification, reflecting their cleaner signal. The gel also flows around hair and conforms to the scalp, so it reaches skin that rigid contacts struggle to touch. For long clinical montages where data fidelity is paramount, this reliability is hard to beat.

The costs of that fidelity show up in time and comfort. Preparing a full wet montage is slow and messy, requires a trained technician, and involves scrubbing each site, injecting gel, and rechecking impedance one electrode at a time (Kam et al., 2019). Your client leaves with gel in their hair and must wash it out, which discourages repeated or at-home sessions. Over long recordings the gel slowly dries, so impedance climbs and signal quality degrades after several hours unless someone refreshes each site (Hinrichs et al., 2020). Tight chin straps and the weight of a gelled cap can also become uncomfortable, which many clients find limiting during multi-hour sessions.

Dry Cap Systems

Dry caps were engineered to remove the gel from the equation. Instead of a gel-filled well, each site carries a dry electrode built as a cluster of fingers or pins, often coated with gold, silver, or Ag/AgCl, that push through hair to touch the scalp directly. Many designs are spring-loaded, so the pins retract and adjust as they slide past hair to reach skin (Kam et al., 2019).

Dry electrodes rely on the scalp's natural moisture to establish an adequate electrical connection. Some time may pass before scalp impedance falls to acceptable levels.

Some are passive electrodes that send the raw signal down a wire, while others are active electrodes with a tiny pre-amplifier at the site that buffers the high-impedance contact before noise can creep in (Di Flumeri et al., 2019). Because they need no gel, no abrasion, and no trained technician, dry caps set up in a fraction of the time and suit home monitoring, ambulatory recording, and real-world settings such as sports and military field use.

The tradeoff is a noisier interface. Without gel, dry contacts show higher and more variable impedance, which makes them more susceptible to motion artifact from head movement, cable sway, and shifting pins (Bayat et al., 2025). Sharp or firmly sprung pins can also press uncomfortably on the scalp during longer wear.

In addition, the scalp is richly supplied with superficial arteries—including branches of the supraorbital, superficial temporal, and occipital vessels—that run in the subcutaneous layer close to the skin surface. When the pins or fingers of a dry electrode press firmly over one of these vessels, the electrode can move slightly with each arterial pulsation and introduce pulse artifact at that site. Practitioners report this most often at frontal and temporal locations, though the risk follows an individual client's vascular anatomy rather than any fixed electrode label.

Even so, controlled comparisons are encouraging, because Kam and colleagues (2019) found that dry and wet systems produced comparable resting spectra and P3b responses, with metrics correlating strongly across the two (r = 0.54 to 0.89), and Hinrichs and colleagues (2020) reached similar conclusions in a clinical sample. The practical lesson for your practice is to match the tool to the task, reaching for a wet cap when you need maximum fidelity for a diagnostic or research montage, and a dry cap when speed, comfort, and mobility matter more than the last decibel of signal.

EEG Electrode Operation

Consider how EEG electrodes work in practice. In response to chemical and electrical synaptic messages, the dendrites of cortical pyramidal neurons develop excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). These potentials travel a short distance, on the order of 1 to 2 centimeters, as a current of ions through the cortex, interstitial and cerebrospinal fluid, glial cells, meninges, skull, and scalp to electrodes on the surface—a process called volume conduction. The electrodes then transform this ionic current into an electronic current that flows through the cable into the electroencephalograph's input jack.

The EEG signal is substantially attenuated during volume conduction, which is why the signal that reaches scalp electrodes is measured in microvolts—millionths of a volt. When an EEG electrode is filled with conductive gel or paste, the electrode metal donates ions to the electrolyte while the electrolyte contributes ions to the metal surface. This creates a DC voltage between the electrode metal and the gel or paste, and signal conduction succeeds as long as electrode and electrolyte ions are freely exchanged.

Recording Problems

Two key problems can degrade electrode performance: polarization and bias potentials. Both stem from the half-cell potentials introduced earlier, which stay stable and cancel out only when the two electrodes are made of the same material and remain in good condition.

Polarization disturbs a half-cell potential over time, whereas a bias potential reflects a mismatch between two electrodes, and either one adds a false voltage to the recording. Understanding these issues helps clinicians choose the right electrode materials and recognize when electrodes need replacement.

Conduction breaks down during polarization, which occurs when chemical reactions produce separate regions of positive and negative charge at the junction between electrode and gel. DC flows across this connection, carrying positive ions to the more negative region and negative ions to the more positive region.

This ion buildup polarizes the electrode, favoring current flow in one direction and resisting it in the other—reducing ion exchange, increasing impedance, and weakening the signal. Electrode manufacturers control this problem by using silver/silver-chloride or gold electrodes that resist polarization.

Bias potentials are a second recording problem, resulting from the exchange of metal ions between electrodes and electrolytes in the absence of a biological current. These spurious voltages can be prevented by using electrodes with intact surfaces and identical materials—for example, all gold or all silver—so that no artificial voltage difference exists between recording sites.

Recording the EEG with Three Leads

When recording a single EEG channel (montage or derivation), three electrodes are used. Understanding the role of each electrode is essential for accurate recordings and effective troubleshooting.

Scalp electrical activity is recorded using three electrodes: active, reference, and ground. In the so-called monopolar (referential) montage, the active electrode is placed over a scalp site that is a known EEG voltage source. The reference electrode is located at a neutral site (minimally active electrically) like the earlobe. The ground electrode can be placed anywhere, but is commonly placed on an earlobe or mastoid process (Demos, 2019).

Active and reference sensors are identical in construction and serve as balanced inputs—they are interchangeable. However, some technologies require that you designate a specific sensor as the reference, for example, in a linked-ears reference configuration.

In the so-called bipolar, or sequential, montage, two electrodes are placed on the scalp.

In both montage types, the recording measures the difference in electrical activity between two electrodes, excluding the ground electrode. The monopolar montage therefore provides a measure of the electrical activity beneath the active electrode, whereas the bipolar montage provides a measure that represents the difference between the two scalp electrodes.

In the graphic below, which shows a bipolar montage, the active (+) is red, the reference (-) is black, and the ground electrode (Gnd/Ref) is white. The voltages of the active and reference inputs are measured relative to the ground.

The graphic below of a monopolar montage shows two earlobe references and an active electrode at P3. A ground electrode is not shown.

Basic Set-up and Operation of EEG Equipment

BCIA's Neurofeedback Essential Skills List identifies several competencies in EEG equipment setup and operation (Biofeedback Certification International Alliance, 2020). These include measuring the scalp and locating each International 10-20 system placement site; correctly preparing the scalp and ears and attaching electrodes to assessment sites or attaching an electrode cap for a full-cap qEEG; applying conductive gel to the sensor; making correct hardware connections to the computer; and making correct electrode connections to the EEG amplifier.

They also include performing all steps required for multi-channel or qEEG recording, such as checking impedance, performing a tracking test, detecting and removing artifact, and obtaining eyes-open and eyes-closed data; performing a continuity test to ensure electrode integrity; identifying and removing or controlling sources of common artifacts that contaminate the raw EEG signal; and troubleshooting common equipment failures following the manufacturer's recommendation.

Finally, they include demonstrating a basic understanding of multi-channel and qEEG assessment reports as well as components of qEEG databases (absolute power, relative power, phase, coherence, comodulation, and z-score comparisons); showing how to start the software, set NFT protocol parameters, and run basic feedback functions; demonstrating how to set initial training thresholds and adjust as needed; and describing how to mitigate the transmission of infection, including scalp measurement, skin preparation, electrode attachment, application of gel, and cleaning of cables, sensors, caps, and exposed surfaces.

Proper Electrode Attachment and Location of 10-20 Sites

Review of 10-20 Site Locations

The International 10-20 system is a standardized procedure for placing electrodes at reproducible sites across the scalp. The classic array locates 19 recording electrodes on the scalp plus two auricular reference electrodes, for 21 standard positions (Acharya et al., 2016), with a separate ground. A scalp-detectable signal requires at least about 6 cm² of cortex to be synchronously active, so no electrode reports a neat patch of its own: because current spreads through brain, cerebrospinal fluid, skull, and scalp before it reaches the sensor, adjacent electrodes sample overlapping cortical territory. That spread, called volume conduction, is also why the site where a signal is largest can lie some distance from the tissue that generated it.

The International 10-20 system calculates the distance from the nasion to the inion and from the left preauricular notch to the right preauricular notch. The 19 active electrode positions are found taking either 10% or 20% of these distances. Check out the YouTube video The International 10-20 System. Four crucial landmarks are the nasion, inion, preauricular points, and vertex.

The nasion is the depression at the bridge of the nose.

The inion is the external occipital protuberance, the bony prominence in the midline at the back of the skull along the occipital ridge.

The left and right preauricular points are slight depressions located in front of the ears and above the earlobe. The flap at the opening of the ear is called the tragus.

The vertex (Cz) intersects imaginary lines drawn from the nasion to inion and between the two preauricular points. Cz is 50% of the total distance between the nasion and inion and 50% between the two preauricular points.

The 10-20 system received its name because electrode sites are separated by 10% or 20% of the distance between two corresponding anatomical landmarks. In the graphic below, each midline site is 10% or 20% of the distance from the nasion to the inion.

Each horizontal axis site is 10% or 20% of the distance from the two preauricular points.

Each circumferential site is 10% of the total circumference, excluding Fpz or Oz.

Intermediate sites are halfway between sets of adjacent sites.

The graphic below shows the correspondence between 10-20 sites and Brodmann areas.

The 10-20 system assigns recording electrodes a letter and subscript. The letters represent the underlying region and include Fp (frontopolar or prefrontal), F (frontal), C (central), P (parietal), O (occipital), and A (auricular). A subscript of z represents a midline (central axis from nasion to inion) placement.

Numerical subscripts range from 1-8 and increase with distance from the midline. The 10-20 system assigns odd-numbered recording electrodes on the left and even-numbered electrodes on the right side of the head. Two reference electrodes are usually placed on the earlobe.

As EEG technology advanced, clinicians extended the classic system into the denser 10-10 array, known as the Modified Combinatorial Nomenclature. This update renames four temporal sites so their labels match their true positions: T3 and T4 became T7 and T8, and T5 and T6 became P7 and P8 (Acharya et al., 2016). You will still meet the older labels in many clinics, so it helps to recognize both. The Signal Acquisition unit describes the 10-10 system in more detail.

10-20 System Electrode Placements

Short Cuts for Electrode Placement

Most sites do not require all the steps outlined below. Calculate Cz by marking 50% of the nasion-inion distance and 50% of the left-right preauricular distance. You can obtain Fz, Pz, C3, C4, T3, and T4 from these two measurements. The Biofeedback Foundation of Europe generously provided the site location, preparation instructions, and graphics.

Step-By-Step Guide for Electrode Placement

Consistently mark perpendicularly on the same side of the measuring tape.

Nasion to Inion – Anterior to Posterior

1. Measure from nasion to inion. Note the total and keep the tape measure on this line.
2. Mark 50% (or halfway). This is Cz.
3. Mark 20% forward from Cz. This is Fz.
4. Mark 20% forward from Fz. This is FPz. This should be 10% up from the nasion.
5. Mark 20% back from Cz. This is Pz.
6. Mark 20% back from Pz. This is Oz. This should be 10% up from the inion.

Ear to Ear (Pre-Auricular)

1. Measure from pre-auricular notch to pre-auricular notch. Find the tragus (the flap at the opening of the ear). Move forward to the indention between the skull and jaw. Place the end of the tape measure at this notch and pass it over Cz and to the pre-auricular notch on the opposite ear. Record this measurement.
2. Mark 50% (or halfway). This should intersect with Cz and form a “+.”
3. From Cz, mark 20% on each side toward the tragus. These will be C3 and C4.
4. From C3 and C4, mark 20% toward the tragus. These will be T3 and T4. These sites will be 10 percent up from the pre-auricular notch, directly above the ear.

Circumferential Measurements

1. Place the tape measure on FPz, T3, Oz, T4, and back to FPz. Record the total circumference.
2. From FPz, mark 5% on either side. These will be FP1 and FP2.
3. From Oz, mark 5% on either side. These will be O1 and O2.
4. Mark at 10% increments from FP1 and FP2. These will be F7 and F8.
5. Mark at 10% increments from O1 and O2. These will be T5 and T6.

Final Measurements

1. Measure from Fz to F7: 50% between these points is F3.
2. Measure from Fz to F8: 50% between these points is F4.
3. Measure from Pz to T5: 50% between these points is P3.
4. Measure from Pz to T6: 50% between these points is P4.

Choosing Your Electrodes: Separate Sensors or a Full Cap

Before you prepare a single site, decide how you will record, because two setups dominate neurofeedback practice. You can attach three separate electrodes for a focused, single-channel recording, or you can fit a full cap that holds nineteen or more electrodes for a whole-head qEEG. The choice should follow your goal rather than your convenience.

Every basic recording uses three electrodes, each with a distinct job. The active electrode sits over the site of interest, such as O1 for occipital alpha or Cz for a vertex protocol. The reference electrode, usually on an earlobe or mastoid, is the comparison point for the measurement. The ground electrode, sometimes called the bias electrode, gives the amplifier's input stage a common voltage reference and stabilizes the connection between client and equipment (Collura, 2014). The cancelling of shared noise is done by the differential amplifier itself, through common-mode rejection; the ground makes that rejection possible rather than performing it.

Three separate electrodes are ideal for teaching, for single-site training, and for demonstrating artifacts, because they are quick to place and easy to inspect. A full cap is the right tool when you need a montage of many sites at once, as in qEEG assessment, where nineteen scalp electrodes plus references and ground are recorded together. The American Clinical Neurophysiology Society specifies that “sixteen channels of simultaneous recording are considered the minimum number required to show the areas producing most normal and abnormal EEG patterns,” and separately that all 21 electrode positions of the 10-20 system should be used (Sinha et al., 2016). Channels and electrodes are different quantities, and conflating them understates what a routine clinical EEG requires. More electrodes mean more setup time but a far richer picture of the brain.

Both separate electrodes and caps come in wet and dry versions. A wet electrode uses conductive gel or conductive paste to bridge scalp and sensor, which lowers and steadies impedance but takes longer and leaves a mess.

A dry electrode uses pins, combs, or firm pads that press through the hair to touch the scalp, so it skips the gel and speeds up setup, but its impedance runs higher and less stable and comfort becomes the limiting factor.

Choose wet for the cleanest signal and research-grade data, and dry for speed and convenience when the task tolerates a noisier trace. Note that needle electrodes are not recommended for routine clinical use, because of patient discomfort, risk of injury to personnel, and higher impedance that raises noise; the guideline makes a narrow exception for subdermal needle or wire electrodes in stuporous or comatose patients when cup electrodes are not feasible (Sinha et al., 2016).

Site Preparation

Infection control is now part of site preparation, and the level of protection should follow a risk assessment rather than a fixed routine. For routine training on intact skin, hand hygiene before and after client contact is the essential measure; gloves are indicated when you anticipate contact with non-intact skin, bodily fluids, or contaminated equipment, such as handling used sensors or abraded sites. Masks and eye protection are indicated when you work within two metres of a client with respiratory symptoms or when splashes are possible, and disposable masks should be available for clients who want them. Applying gloves, a mask, and a face shield to every contact regardless of risk is not what current guidance recommends, and indiscriminate glove use has itself been linked to pathogen transmission. See the unit on Aseptic Techniques for a more comprehensive review of infection mitigation.

The steps below prepare a site for a wet cup electrode filled with conductive paste, the setup you will use most often for single-channel and three-electrode recordings. Dry separate electrodes follow at the end of this section, and full caps have their own section.

Once a site is identified and marked, site preparation and sensor placement when using individual sensors follow these steps:

1. Instruct clients to wash their hair and not use any conditioner or hair-styling products. A recent haircut is helpful (particularly for children), and hair must be brushed or combed.
2. Prepare the scalp by cleaning with alcohol. Let the alcohol dry before applying the electrodes.
3. The following is a typical recommendation when using older amplifiers. Modern amplifiers with high input impedance do not require this step. Additionally, the following step is controversial because the old standard of achieving skin-electrode impedance below 5 Kohms has been challenged as unnecessary and risks infection transmission (Ferree et al., 2001; Kappenman & Luck, 2010). The authors of this work do not recommend this step, especially during the COVID-19 pandemic. Still, we included it because it remains an accepted standard in the field of electroencephalography: slightly abrade the skin with a blunted needle that you must discard after use to remove dead skin, dirt, and oil that can weaken the EEG signal.
4. Ask your client to remove jewelry. If your client has pierced ears, do not place the electrode over the hole.
5. Scoop up a small quantity of NuPrep™ skin preparation gel or a similar product on a cotton swab or tissue.

   

6. Be careful to avoid contaminating the gel tube by avoiding direct contact with the Q-tips®.

   

7. With the thumb and index finger of one hand, separate the hair around the electrode site that was previously found and marked.

   

8. Run the gel in the direction of the natural line formed along the scalp by the split hair. Some light force must be used, enough to redden the scalp slightly (again, not recommended but an accepted standard), but not enough to break the skin.

   

9. Wipe away the excess prepping gel with a dry, lint-free cloth. Take care to keep the hair parted and keep track of the site after wiping clean.

   

10. Use a popsicle or craft stick to cover the electrode with a 1/4-inch layer of Ten20™ conductive paste. Gently press the electrode onto the skin surface until the paste flattens out under the electrode.

    

11. Secure the electrode cables to the client and ensure strain relief.

    

12. Fill the electrode cup with Ten20™ conductive paste so that no air bubbles exist in the cup.

    

13. Add more Ten20™ conductive paste onto the cup electrode, just enough to form a ball on the cup, not so much that it spills over the edge. The ideal amount of paste is shown.

    

14. Place the cup face down on the landing pad previously prepared. Gently push the electrode down to fix it to the scalp. A little bit of paste should run out along the edge of the cup to form a thin ring around it. Place the electrode so that the direction of the cable does not place undue stress on the cup, so that it does not get pulled, lifted, or twisted off. The cable should hang naturally and towards the plastic clip, and enough slack should be left to allow for comfortable head movement.

    

15. Repeat the above preparation and placement steps on the earlobes. Do not put too much paste on the ear clip electrode, but ensure that the gold disc is completely covered.

    

16. Leave enough slack in the cable to allow the patient to turn their head easily, but not too much that it can get caught. Think about the angle of the cable from the ear clip to the neck clip so that no extra tension is placed on the ear clip. Note the position of the clip, the direction of the cables, and the slack left to provide mobility. The final configuration should look like the photograph below when viewed from behind.

    

Use enough paste to complete the path from scalp to sensor, but no more. Excess paste that spreads from one electrode to the next forms a salt bridge, an unintended conductive path that merges two channels into one and makes their data useless. This is the same mechanism behind the bridging artifact, so wipe away any excess before it reaches a neighboring site (Sinha et al., 2016).

This movie is a 19-channel BioTrace+ /NeXus-32 display of EEG recording © John S. Anderson.

Applying Three Dry Electrodes

Dry electrodes skip the gel, so the setup is faster but the mechanics matter more. With no electrolyte to steady the interface, a dry electrode depends on direct contact and gentle mechanical pressure, which means hair, motion, and fit have a larger effect on the signal. Prepare three dry electrodes as follows.

1. Confirm the client's hair is dry and free of styling product.
2. Choose the sensor shape for each site: a flat pad for the forehead, earlobe, or mastoid, and a pin or comb sensor for hair-covered scalp.
3. Tell the client to expect gentle pressure, never sharp pain.
4. Place the active dry electrode at the marked site, parting the hair so the contact points reach skin rather than resting on hair.
5. Place the reference and ground electrodes on the earlobe, mastoid, or forehead.
6. Secure the sensors with a headband, elastic wrap, adhesive holder, or spring frame.
7. Increase pressure gradually until the trace is stable and the client stays comfortable, and reposition at once if there is pinching, headache, or lingering discomfort.

Do not hold a dry electrode to the impedance target you use for wet electrodes. Dry contacts read much higher and less stable impedances because no electrolyte stabilizes the interface, so judge them by the manufacturer's contact-quality scale and the live trace rather than a wet-electrode pass or fail number.

Applying a Full Electrode Cap

When you need many sites at once, a full electrode cap replaces dozens of separate placements with a single fitted garment. A well-fitted cap holds each electrode close to its 10-20 target, so sizing is not a step to rush. An ill-fitting cap shifts electrodes away from the regions they are meant to record and quietly undermines every site.

Fitting a Wet Cap

1. Measure head circumference with a flexible tape around the widest part of the head, passing just above the eyebrows and over the inion.
2. Match the measurement to the manufacturer's sizing chart, and when a client falls between sizes, choose the snugger cap if it stays comfortable.
3. Have the client hold the front edge against the forehead while you stretch the cap back over the head.
4. Align Cz at the intersection of the nasion-to-inion and preauricular-to-preauricular midpoints, exactly as you would for a single vertex placement.
5. Check symmetry so that Fp1 and Fp2 are level and equidistant from the midline, and secure any chin strap snugly without restricting the jaw or airway.

With the cap seated, prepare each site through its electrode well. Part the hair with a blunt applicator until the scalp shows, then fill the well with conductive gel so a solid column connects scalp to sensor with no air gap. Fill just enough to make contact. Overfilling lets gel spread from one well to the next and form a salt bridge that merges neighboring channels, the same problem you guard against with separate electrodes.

Fitting a Dry Cap

A dry cap carries the same 10-20 logic but no gel. Its electrodes reach the scalp through pins or spring-loaded contacts that part the hair under gentle tension, so fit and pressure do the work that gel does in a wet system. Size and seat the cap the same way, aligning Cz and checking symmetry, then adjust the tension until the trace is stable and the client stays comfortable. Expect higher and less stable impedances than a wet cap, and judge contact by the system's own quality scale rather than a wet-electrode target.

Checking Impedance and Signal Quality

Whatever electrodes you use, check impedance before you record. For clinical EEG, aim for balanced impedances; modern digital amplifiers accept values up to about 10 Kohms when they are balanced, although values below 100 ohms usually signal a short or a salt bridge and should be corrected (Sinha et al., 2016). Neurofeedback practice is more lenient, accepting values under 20 Kohms for general clinical sessions while reserving the stricter targets for research (Demos, 2019). Balance matters more than any single number, because unbalanced impedances weaken the amplifier's ability to reject shared noise.

Impedance tells you the connection is sound; the live trace tells you the recording is usable. Seat the client comfortably, ask them to relax the jaw, and watch the raw signal while you confirm the setup works. With an occipital site, alpha should rise when the eyes close and fall when they open, a blink should produce a large slow frontal deflection, a jaw clench should add fast muscle activity, and touching a cable should produce a movement artifact. Reproducing these on purpose, as you learned earlier, is the fastest way to prove your electrodes are reporting brain activity rather than noise.

Elimination of Artifact from EEG Recording

BCIA's Neurofeedback Essential Skills List requires that applicants identify and remove artifact sources appearing in EEG recordings (Biofeedback Certification International Alliance, 2020). Review the Signal Acquisition unit to recognize normal EEG patterns and identify and correct noncerebral origin signals like bridging artifacts. Once you understand the mechanics and appearance of common artifacts, Peper et al. (2008) recommend that you intentionally reproduce them to recognize and prevent them.

Brain map validity depends on the integrity of the raw EEG, and the published requirements are more demanding than clinical habit sometimes assumes. The International QEEG Certification Board and the EEG and Clinical Neuroscience Society specify a minimum of 1 minute of artifact-free data for database comparison, ideally 2 to 5 minutes, with no selected segment shorter than 1 second; they assume a raw record of roughly 10 minutes each of eyes-open and eyes-closed recording (International QEEG Certification Board, n.d.; Sinha et al., 2016). Select that clean data separately for each condition rather than pooling the two. Realize that longer recordings risk increased drowsiness artifacts and sleep. Therefore, it is helpful to speak to the client occasionally to help them maintain alertness, and simply saying how much time is left every 1-2 minutes is usually enough to accomplish this.

Stage 1 sleep is a subtle drowsy state that clients often do not recognize. This state change is seen as a decrease in alpha and an increase in theta amplitudes. There will be slow eye-rolling movements and decreased EMG and beta. Note the increased theta amplitudes in the spectral displays for channels 1 and 2.

The less frequent the artifact, the shorter the required recording period. Disable low-pass and high-pass filters before editing to better visualize electro-ocular and SEMG artifacts.

Worst case, as with a hyperactive child, none of the EEG channels may contain usable data, and you will need to repeat the assessment. Where artifact only contaminates a few channels, you may base assessment on the clean channels (Demos, 2019).

Strategies to Reduce Artifact

Demos (2019) recommends several precautions to reduce artifact in raw EEG recordings. Demonstrate how to create artifacts for your clients using screen displays while they clench their teeth, move their eyes, blink, swallow, and fidget. Confirm the cap fits properly, and use reclining chairs with negligible neck cushioning that can force the head downward to minimize SEMG artifact. Limit eyelid movement with cotton balls gently touching the closed eyelids, secured by a loose sleep mask, flexible band, or tape in the eyes-closed recording, ensuring there is no pressure against the eyes.

Ensure that impedance values or DC offset values are appropriate for your amplifier, since values under 5 Kohms are expected for publishable research, while values of less than 20 Kohms are acceptable for general clinical sessions and do not require excessive skin abrasion. Only record qEEG data when the raw waveforms appear clean.

EEG Apparatus

An electroencephalograph consists of several stages that work in sequence: a differential amplifier, gain amplifier, analog-to-digital converter, digital and FFT filters, and optical isolator. Each stage performs a specific function in the signal chain, and a problem at any stage can compromise the quality of your recording.

Signal Amplification

The biological signals monitored in biofeedback are extremely weak—EEG signals, for example, are measured in microvolts (millionths of a volt). These signals must be amplified over several stages to isolate the signal of interest and then drive visual or auditory displays. Think of a stereo amplifier that boosts an audio signal above the noise floor to levels that can power loudspeakers—your EEG amplifier performs essentially the same task.

The display of the EEG waveform is affected by the amplifier's input sensitivity setting. Sensitivity is measured as microvolts of amplitude per millimeter of deflection (µV/mm). A lower sensitivity setting, such as 3 µV/mm, produces larger-appearing EEG waves than a higher setting, such as 10 µV/mm.

In the graphic below, the top frame shows the lower setting, in which fewer microvolts of amplitude are needed to move the tracing 1 mm. The bottom frame shows the higher setting, in which more microvolts of amplitude are required to move the tracing the same distance, so the waveform appears smaller. Do not confuse this display sensitivity, expressed in µV/mm, with an amplifier's input voltage range, which is the largest input the amplifier can accept without clipping; some manufacturers use the word sensitivity for that specification as well.

Gain is an amplifier's ability to increase the magnitude of an input signal, expressed as the ratio of output to input. An amplifier that produces a 1-mV output from a 1-μV input has a gain of 1,000.

Differential Amplifiers

Differential amplifiers help to separate genuine EEG signals from artifacts—one of the most critical functions in the entire recording chain. The concepts of common-mode rejection and differential input impedance directly affect the accuracy of every neurofeedback session.

The EEG signal is first boosted by a differential amplifier (also called a balanced amplifier) and then by a gain amplifier. The differential amplifier amplifies the difference between its two inputs: the active (input 1) and reference (input 2).

One input is non-inverting and the other is inverting, so a voltage common to both inputs is subtracted away while a voltage that differs between them is preserved and amplified.

The portion of the signal that appears with the same amplitude and timing at both inputs is the common-mode signal, typically shared artifact such as power line noise, and the portion that differs between the inputs is the differential-mode signal, which carries the genuine EEG of interest.

Three signal properties determine what a differential amplifier retains or rejects. Frequency is the number of cycles per second (Hz). Amplitude is the signal voltage or power, measured in microvolts or picowatts. Phase is the similarity in timing of the waves at two locations—signals that are 180° out of phase peak when the other reaches its trough.

How does a differential amplifier use these EEG features to reduce artifact? When no EEG activity is present, identical noise signals reach each amplifier input. The differential amplifier subtracts these signals, canceling out the artifact—the output of a perfect differential amplifier would be zero.

All four tracings below occur simultaneously. On the left portion of the top three traces, the first trace shows low amplitude activity at Fp1 (referenced to linked ears), the second shows high amplitude activity at O2 (also referenced to linked ears), and the third shows the difference between Fp1 and O2 when they are referenced to each other, so that shared activity is subtracted out. Toward the right side of the tracings, the third trace again shows the difference between Fp1 and O2.

The first tracing (Fp1-LE) shows the Fp1 electrode referenced to linked ears with an event circled in red. The second tracing (O2-LE) shows the O2 electrode, also referenced to linked ears, with a distinct EEG event. In the third tracing (Fp1-O2), in which these two electrodes are referenced to each other, the differences are retained—demonstrating common-mode rejection. Last, the fourth tracing (LE-LE) shows linked ears compared to each other, resulting in complete rejection of the identical signals.

The Effect of Electrode Location on Common Mode Rejection

Brain activity is more similar when electrodes are placed close together and less similar when they are farther apart. This means that a differential amplifier may inadvertently reject actual EEG voltages detected by adjacent electrodes—a clinical pitfall worth remembering when choosing montages. In the recording below, sensors were placed at essentially the same anatomical location (labeled Fp1-Fp1) so that both inputs saw nearly identical activity, producing the near-flat line that demonstrates almost complete signal subtraction.

Differential Input Impedance

An amplifier's differential input impedance further reduces the effect of unequal impedances at the skin-electrode interface. As EEG signals enter the amplifier, they are dropped across a network of resistors presenting a differential input impedance in the Gohm (billion ohms) range, with state-of-the-art instruments now exceeding 10 Gohms. The differential input impedance must be at least 100 times the skin-electrode impedance so that 99% or more of the signal reaches the electroencephalograph.

Why is this important? Stronger signals help the amplifier differentiate genuine EEG activity from noise, producing more accurate feedback—which is the entire point of your recording system.

The Challenges of Recording Infra-Slow EEG Activity

Recording infra-slow (0-1 Hz) EEG activity pushes amplifier technology to its limits and introduces unique artifact challenges that clinicians must understand before attempting this type of recording.

AC amplifiers have severe limitations when recording infra-slow signals because client movement, eye movement, sweat, and transient field artifacts produce significant voltage changes that the amplifier cannot distinguish from genuine cortical activity. Long time constants over 80 seconds are recommended to integrate artifact-induced voltages over 2-4-minute periods, but persistent artifacts like eye movement will consistently degrade the signal-to-noise ratio.

Infra-slow recording requires DC-coupled amplifiers with a large dynamic range produced by 24-bit A/D converters to prevent saturation by slow drifts in baseline voltage. Standard EEG electrodes made of gold, steel, or tin are poor choices because they are comparatively polarizable: charge accumulates at the metal-electrolyte junction so the interface behaves like a capacitor, blocking the lowest frequencies and producing unstable baseline drift. Sintered silver/silver-chloride electrodes are preferred because they are only minimally polarizable, passing near-DC potentials with a comparatively stable half-cell potential. No electrode is entirely nonpolarizable.

The clinician must also distinguish slow artifacts from genuine infra-slow signals. Eccrine sweat glands produce standing millivolt-range potentials, and while partial skin puncturing can eliminate these, this practice risks infection transmission. Eye blink and eye movement artifacts can be identified by their characteristic location, while body tilt, cough and strain, hyperventilation, and tongue movements produce high-amplitude diffuse very slow potentials (Miller et al., 2007).

Common-Mode Rejection

A differential amplifier's effectiveness at separating signal from artifact is quantified by the common-mode rejection ratio (CMRR). Since differential amplifiers cancel noise imperfectly, both signal and some noise will be boosted. The CMRR compares how much a differential amplifier boosts the signal (differential gain) versus artifact (common-mode gain): CMRR = differential gain / common-mode gain.

CMRR should be measured at 50/60 Hz, where the strongest artifacts (like power line noise) are found. CMRR is usually expressed in decibels (dB), a logarithmic unit for comparing two amplitude values in which each 20-dB step represents a tenfold difference in voltage.

A widely cited floor for biosignal amplifiers is 80 dB, a 10,000:1 ratio, and neurofeedback sources commonly recommend at least 100 dB, which equals a 100,000:1 ratio and means the signal is boosted 100,000 times more than competing noise. Contemporary commercial EEG amplifiers typically specify roughly 100 to 120 dB at 50/60 Hz, and the best instrumentation-amplifier designs reach about 130 dB. Treat far larger published figures with caution, because the ratio actually achieved in a recording is limited by impedance mismatch between electrodes rather than by the amplifier alone, so a superb amplifier specification cannot rescue a poorly prepared montage.

You can take nine practical steps to maximize common-mode rejection: (1) ensure that skin-electrode impedances are balanced within 1-3 Kohm, since imbalance will make the signals look different and prevent complete subtraction of noise; (2) active electrodes should be equidistant from the artifact source; (3) active, reference, and ground sensors should be the same distance from each other; (4) when using two or more channels, the ground and each active should be the same distance apart; (5) ensure that there is a good ground connection, since a deficient ground lets the common-mode voltage appear unequally at the two inputs, so shared noise is no longer identical and can no longer be subtracted away; (6) identify artifact sources by using a portable electroencephalograph or electromyograph as you would a Geiger counter, moving the unit around the room with EEG sensors connected but held in your hand; (7) remove the artifact sources you find, for example, fluorescent lights can be replaced with fixtures that produce less 50/60 Hz noise; (8) remove unused sensor cables from the encoder so they do not function as an antenna for 50/60 Hz artifact; (9) position the electroencephalograph and electrode cable to reduce artifact reception, using the location and angle that yield the lowest readings when not attached to a patient (Thompson & Thompson, 2015).

Sampling the EEG Signal

This section covers the conversion of analog EEG signals into digital data. Sampling rate and resolution determine how faithfully your digital recording represents the actual brain activity, and choosing the wrong settings can introduce errors that are invisible but clinically significant.

An analog-to-digital (A/D) converter samples the EEG signal at a fixed interval, and the sampling rate—the number of measurements taken per second—must be high enough to represent the signal accurately. According to the Nyquist-Shannon sampling theorem, an A/D converter's sampling rate should be at least twice the highest frequency component you intend to sample.

The theorem sets a floor, not a practical target. An anti-aliasing filter, the low-pass filter that removes frequencies too high to be sampled correctly, does not cut off perfectly at its stated frequency but rolls off gradually, and sampling only slightly above the minimum can leave waveform peaks poorly defined. Guidelines therefore call for an extra margin.

The American Clinical Neurophysiology Society (ACNS) recommends a sampling rate of more than three times the high-frequency filter setting. Assuming a typical 70-Hz high-frequency filter, this yields a minimum of 256 samples per second (sps), with 512 sps preferable (Halford et al., 2016). In practice, 256 sps is the common clinical baseline, and rates of 500-1,000 sps or higher are preferred for detailed analysis of fast activity.

Sampling at rates that are too slow results in aliasing, where an analog signal appears to have a lower frequency than it actually does—producing "phantom" slow activity from too few samples per second. The graphic below illustrates this problem: an 11-Hz signal sampled at 12 sps produces the aliasing signal shown in black, while the same signal sampled at 200 sps is accurately reproduced.

Resolution Depends on Bit Depth

An A/D converter's resolution is limited by the smallest amplitude difference it can represent. The bit number refers to the number of voltage levels an A/D converter can discern. The ACNS (Halford et al., 2016) recommends a resolution of at least 16 bits per sample, which discriminates among 65,536 voltage levels; 24-bit converters are common in modern amplifiers. How fine a resolution this yields in microvolts depends on the amplifier's input range, since the available levels are divided across that range, and 16 bits permits fine amplitude resolution while still recording potentials of several millivolts without clipping. Lower resolutions coarsen the voltage steps, so small genuine changes may be lost or exaggerated by quantization, distorting the clinical picture.

Signal Properties

EEG signals are described by their frequency and amplitude. A/D conversion utilizes digital filters to decompose the complex EEG into its component frequencies—much like a prism separating white light into its constituent colors.

The movie below repeats the BioTrace+ /NeXus-32 display of EEG activity from 1-64 Hz broken into component delta, theta, alpha, and beta frequency bands by digital filters © John S. Anderson.

Recall that frequency is the number of cycles completed each second (Hz)—the longer the wavelength, the slower the frequency. The delta, theta, alpha, and beta bands can be defined by wave frequency, wave shape (morphology), and context. Amplitude, meanwhile, represents signal voltage or power and is measured in microvolts or picowatts.

The next graphic illustrates the inverse relationship between wavelength and frequency. The time scale on the horizontal axis is in milliseconds, and the amplitude scales differ for the two tracings.

EEG frequency is measured in cycles per second or Hz, and you can verify it by hand from any tracing on your screen. The zero-crossing method counts how many times the waveform crosses the zero-voltage line within a 1-second interval and divides that count by 2, since every complete cycle crosses zero twice—once ascending and once descending (Anderson, 2025). Counting the peaks that rise above the zero line during the same second yields the same value. Both procedures take only a few seconds and are worth performing whenever a display, a threshold, or an assessment value looks implausible, because they let you confirm the frequency of the activity you are rewarding independently of what the software reports.

The slower the waves, the lower the EEG frequency.

The EEG signal is sent to an integrator to measure signal amplitude in microvolts (μV) or picowatts, and integrators use four methods to calculate the voltage. The peak-to-peak method provides the largest estimate—the voltage difference between the positive and negative maximum values of the original AC waveform, which is 2 times the peak value. Peak voltage is 0.5 of the peak-to-peak value, root mean square (RMS) voltage is 0.707 of the peak value, and average voltage is 0.637 of the peak value.

Conversion among these methods is straightforward. If the peak-to-peak voltage is 20 μV, peak voltage is 10 μV, root mean square voltage is 7.07 μV, and average voltage is 6.37 μV. Knowing which method your equipment uses is essential when comparing readings across different systems.

Two cautions apply to these conversions. First, the factors 0.707 (which equals 1 divided by the square root of 2) and 0.637 hold exactly only for a pure sine wave, so they are approximations for the complex, multi-frequency EEG rather than precise values. Second, the RMS value is the measure tied to power, because it represents the steady DC voltage that would deliver the same average power into the same resistance as the alternating signal, which is why RMS is preferred when amplitude is later converted to power.

Keeping Frequency and Amplitude Straight at the Keyboard

Practitioners who grasp frequency and amplitude in the abstract still stumble when they must apply them while a client waits, partly because our field describes one signal with a bewildering number of overlapping terms (Anderson, 2025). A working formulation keeps the two dimensions separate and independent of each other: frequency is how often the wave repeats within 1 second, and amplitude is the amount of electrical activity within that frequency band during that same period (Anderson, 2025). Every other value your software reports—magnitude, power, relative power, ratios, z-scores—is a transformation of one or both of these two measurements, so when a number confuses you, the first question to ask is which of the two it describes and what was done to it.

Assignment

Now that you have completed this unit, trace a single EEG signal from a cortical pyramidal neuron to a number on your screen. Name each stage it passes through, and at each stage identify one thing that could go wrong and how you would detect it. Which of those failures would still be visible in the finished topographic map, and which would be invisible?

Glossary

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