Technical (4 Hours)

Contamination of the EEG by physiological and exogeneous artifacts requires that clinicians take extensive precautions, examine the raw EEG record, and remove contaminated epochs through artifacting. Impedance tests and behavioral tests help ensure the fidelity of EEG recording. Graphic © Medical-R/Shutterstock.com.

IQCB Blueprint Coverage

This unit addresses III. Technical. The competent clinical neurophysiologist must acquire knowledge of electronics and instrumentation related to EEG and EPs.

This unit covers:
A. Topographical representation of the EEG
B. Electrodes and acquisition systems
C. Instrumentation (Acquisition and review parameters/settings)
D.Montages
E. Electrical/clinical safety

The International 10-20 system is a standardized procedure for electrode placement on 19 scalp and reference and ground sites. Electrodes measure electrical activity from a surrounding area the size of a quarter. The site recorded may be distant from the EEG generator due to neural pathways.

The International 10-20 system calculates the distance between the nasion to the inion and 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 essential landmarks are the nasion, inion, preauricular points, and vertex. Graphic © Alila Medical Media/Shutterstock.com.

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

The inion is the bony prominence on the back of the skull in the middle of the inion 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% between the nasion and inion and 50% between the two preauricular points. Minaanandag adapted the diagram below from Fisch (1999).

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 adapted from Fisch (1999) by minaanandag, 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. Graphic adapted from Fisch (1999) by minaanandag.

Each circumferential site is 10% of the total circumference, excluding Fpz or Oz. Graphic adapted from Fisch (1999) by minaanandag.

Intermediate sites are halfway between sets of adjacent sites. Graphic adapted from Fisch (1999) by minaanandag.

The graphic by Dailey (2013) shows the correspondence between 10-20 sites and Brodmann areas, which are 47 numbered cytoarchitectural zones of the cerebral cortex based on Nissl staining.

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. John Balven adapted the diagram below from Fisch (1999).

Modified Combinatorial Nomenclature

The American Clinical Neurophysiology Society published guidelines for expanding the 10-20 system to 75 electrode sites. This system, while more complex, also allows us to precisely define the placement sites for our electrodes.

The expansion of the 10-20 system allows clinicians to define the sites midway between two 10-20 sites commonly used in clinical practice, better localize epileptiform activity, increase EEG spatial resolution, and improve detection of localized evoked potentials. The modified combinatorial system replaces inconsistent designations (T3/T4 and T5/T6) with consistent ones (T7/T8 and P7/P8). Black circles depict these replacement sites with white lettering in the diagram below.

The modified combinatorial system, also called the 10-10 system, locates electrodes at every 10% along medial-lateral contours and adds new contours. Each electrode site intersects a medial-to-lateral coronal line (designated by letters) and a longitudinal sagittal line (designated by numerical subscripts).

As with the 10-20 system, letters represent the underlying region and include: N (nasion), Fp (frontopolar or prefrontal), AF (anterior frontal), F (frontal), FT (frontotemporal), FC (frontocentral), A (auricular), T (temporal), C (central), TP (temporal-posterior temporal), CP (centroparietal), P (parietal), PO (posterior temporo-occipital or parieto-occipital), O (occipital), and I (inion). FT and FC lie along the second intermediate coronal line, TP and CP along the third, and PO along the fourth.

A subscript of z represents a midline (central axis from nasion to inion) placement. Numerical subscripts range from 1-10 and increase with distance from the midline. The modified combinatorial system assigns odd-numbered recording electrodes on the left and even-numbered electrodes on the right side of the head. David Kelsey adapted the diagram below from Fisch (1999).

Comparison of Neuroimaging Techniques

Neuroimaging methods can be thought of as either structural or functional. Structural methods include CT and MRI and present images of brain structures. Functional methods include EEG, MEG, fMRI, PET, and SPECT, each of which constructs images showing the location of differing levels of brain activity.

The different functional neuroimaging methods use different biologic signals as their index of function. EEG and MEG use brain electrical activity, fMRI uses blood oxygen level, PET uses positron-emitting radioisotopes bound to glucose, and SPECT uses gamma-emitting radioisotopes. Therefore, PET and SPECT are more invasive and pose more significant risks to patients and research participants (Breedlove & Watson, 2020).

Each functional neuroimaging method can be rated with respect to how quickly it can detect changes in function (temporal resolution) and over how small an area it detects changes in function (spatial resolution). Whereas EEG and MEG methods detect changes in function most quickly, they are less able to detect the precise area where functional changes occur compared to fMRI. Graphic from Pfister et al. (2012) © Mathematics and Visualization. EM = electron microscope and LM = light microscope.

STRUCTURAL TECHNIQUES

The main structural imaging techniques are computerized axial tomography and magnetic resonance imaging.

Computerized Axial Tomography

Computerized Axial Tomography (CAT or CT) provides medium-resolution images of brain structure by moving an x-ray source along an arc surrounding the head (Breedlove & Watson, 2020). Watch the Blausen CAT Scans animation. Graphic © Tyler Olson/Shutterstock.com.

CT scans allow physicians to visualize structural abnormalities like stroke damage and tumors. Graphic © Triff/Shutterstock.com.

Magnetic Resonance Imaging (MRI)

Magnetic resonance imaging (MRI) constructs higher-resolution images than CT scans. Since MRI scans use powerful magnetic fields and radio wave pulses to construct images of living brains, they are safer than CT scans because there is no radiation exposure. MRI scans allow a detailed examination of brain topography, including the location and volume of specific brain regions. Graphic © Peastock/Shutterstock.com.

MRI scans' superior spatial resolution can detect demyelination in disorders like multiple sclerosis that CT scans would miss (Breedlove & Watson, 2020). Graphic © MriMan/Shutterstock.com.

FUNCTIONAL TECHNIQUES

Functional techniques reviewed in this section include the EEG and qEEG, magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), positron emission tomography (PET), and single-photon computerized emission tomography (SPECT). See Lebby (2013) for an excellent overview of these techniques. Also, consult the McGill brain imaging tool module.

EEG

EEG and qEEG can be conceptualized as functional imaging techniques. A single-channel EEG performs “neuroimaging” by displaying an image of microvolts in adjacent 1-Hz bins or adjacent bands (e.g., a 2D spectrogram (shown below) or with a 3D spectrogram. Graphic © John S. Anderson.

Further, 19-channel qEEG methods show images of activity as it is distributed across the brain’s convexity (i.e., over a 2D 10-20 map) or in 3 dimensions using more advanced qEEG methods (e.g., LORETA). Graphic courtesy of BrainMaster Technologies.

Magnetoencephalography

Magnetoencephalography (MEG) is a noninvasive functional imaging technique that uses SQUIDs (superconducting quantum interference devices) to detect the weak magnetic fields generated by neuronal activity. As with the EEG/qEEG, spatial resolution is inferior (cm compared to mm) to the functional MRI (fMRI) (Breedlove & Watson, 2017). Graphic © Steve Shoup/Shutterstock.com.

MEG's millisecond temporal resolution allows it to measure rapidly shifting patterns of cortical circuit activation (Breedlove & Watson, 2020). Researchers may combine MEG with MRI to better delineate the cortical structures generating the magnetic fields (Lin et al., 2004). The MEG graphic below is courtesy of the University of Montreal MEG Laboratory website.

Functional Magnetic Resonance Imaging (fMRI)

Functional Magnetic Resonance Imaging (fMRI)generates intense magnetic fields to indirectly detect brain regions' oxygen use during specific tasks.

fMRI images represent communications from other neurons and changes in local potentials instead of action potentials. Although the fMRI is limited by significant (100-ms to several-s) delays, it can reveal network contribution to cognitive performance. The fMRI trades spatial resolution for PET's superior temporal resolution (Breedlove & Watson, 2020). A fMRI image © Wikipedia is shown below.

Watch the Blausen MRI animation.

Positron Emission Tomography

Positron emission tomography (PET) is a functional imaging technique that injects radioactive chemicals into the brain's circulation to measure brain activity (Breedlove & Watson, 2020).

PET scans achieve low temporal resolution (seconds to minutes) with moderate spatial resolution. PET images © Yok_onepiece/Shutterstock.com are shown below.

Single-Photon Emission Computerized Tomography

Single-photon emission computerized tomography (SPECT) is a functional imaging technique that uses gamma rays to create three-dimensional images and slice images of cerebral blood flow averaged over several minutes. Graphic © rumruay/Shutterstock.com.

SPECT achieves limited temporal (minutes) and spatial resolution (centimeters).

Using a Limited Number of Electrodes

Peer-reviewed evidence suggests that more EEG channels provide a more accurate assessment and achieve superior clinical or performance outcomes than fewer channels (Lau et al., 2012).

Although some practitioners conduct assessment and training with a single channel, assessments and training methods that use more than one channel have become more available. For example, the cost of a full 19-channel EEG assessment has decreased substantially. Such an assessment can provide not only EEG amplitude data from all sites in the 10-20 system but additionally makes calculations of metrics such as coherence and phase that give information on how well the 10-20 sites communicate with each other. These data from multi-channel methods are beneficial for complex symptom profiles like those associated with Autism Spectrum Disorders, epilepsy, and traumatic brain injury (Thompson & Thompson, 2016). Graphic © Chaikom/Shutterstock.com.

A channel is an EEG amplifier output resulting from scalp electrical activity from three electrode/sensor connections to the scalp. These sensors are commonly known as active, reference, and ground electrodes, though they are more appropriately called positive +, negative - and reference. They are placed on the head in the following manner: an active or positive electrode is placed over a known EEG generator like Cz. A reference or negative electrode may be located on the scalp, earlobe, or mastoid. A ground/reference electrode may also be placed on an earlobe or mastoid (Thompson & Thompson, 2015).

Active and reference sensors are identical balanced inputs and interchangeable. However, some neurofeedback data acquisition systems require the designation of a specific sensor as a "reference," as in a linked-ears reference.

A derivation is the assignment of two electrodes to an amplifier's inputs 1 and 2. For example, Fp1 to O2 means that Fp1 is placed in input 1 and O2 in input 2.

A montage groups electrodes together (combines derivations) to record EEG activity (Thomas, 2007).

All montages compare EEG activity between one or more pairs of electrode sites.

Modern amplifiers record all input sensors in reference to a common sensor - often Cz - and all montage (sensor comparison) changes are performed in the software. Amplifiers no longer require manual switching of electrodes between inputs.

The narrated video below © John S. Anderson displays the same 21-channel recording viewed using different montages with a 60-Hz notch filter on and off.

Montage Options and Their Consequences

Referential (Monopolar) Montage

A referential (monopolar) montage places one active electrode (A) on the scalp and a "neutral" reference (R) and ground (G) on the ear or mastoid. Graphic © John S. Anderson.

A referential montage assumes that the EEG activity seen on the computer screen represents the active (+) site because the reference (-) site is assumed to be neutral (i.e., producing no EEG activity) and because of the subtraction of signals produced by noise and artifacts that are common to both active and reference sites (common-mode rejection). In the photograph below, the blue cable would be used for the active electrode, the yellow cable with an ear clip for reference, and the black cable with an ear clip for the ground.

However, this montage is vulnerable to artifacts from the contraction of facial muscles (Demos, 2019). The ear reference is also known to produce reference contamination, where EEG signals from this electrode are contributed or added to other electrodes via the mechanism of the differential amplifier, where anything different between the "active" and "reference" sensors is retained. This commonly results in alpha activity produced by posterior alpha sources close to the ear.

Sequential (Bipolar) Montage

A sequential (bipolar) montage presents a sequence of comparisons of positive (+) and negative (-) electrodes (often called ‘active’ and ‘reference’) that are attached to sites on the scalp and therefore considers the reference electrode to be a second active electrode. The ground (G) electrode is attached to the scalp, to an earlobe, or over the mastoid process. Graphic © John S. Anderson.

The sequential montage detects the difference in EEG between the positive and the negative electrodes (active and reference), as the referential montage does. Still, now the signal for the channel represents the difference between two sources of EEG activity. In cases when 19-channels are used, this montage is usually presented with electrode pairs shown in sequence. Note that only the black cable for the ground has an ear clip in the photograph below.

When used as only a single channel, this montage does not detect localized EEG activity well because it shows only the difference between the A and R signals. However, when used as part of a 19-channel assessment, it localizes EEG events related to epilepsy. This montage can also reduce artifacts when the A and R electrodes are relatively close.

A sequential montage is frequently used in neurofeedback and trains the difference between EEG activity at the A and R electrodes. However, when neurofeedback training produces a change, it remains uncertain whether it is because of a change in EEG at the A electrode, the R electrode, or both electrodes.

The EEG record appears more similar when sensors are closer together and less similar when they are farther apart (Fp1-O2). When electrodes are spaced close together (Fp1-Fp2), this montage may reject actual EEG activity (Fp1-Fp1). Graphic © John S. Anderson.

Recognizing and Correcting Signals of Noncerebral Origin

EEG artifacts, consisting of noncerebral electrical activity, can be divided into physiological and exogenous artifacts. Physiological artifacts include electromyographic, electro-ocular (eye blink and eye movement), cardiac (pulse), sweat (skin impedance), drowsiness, and evoked potential. Exogenous artifacts include movement, 60 Hz and field effect, and electrode (impedance, bridging, and electrode pop) artifacts.

Please click on the podcast icon below to hear a lecture over the first half of this unit.

Electromyographic (EMG) Artifact

EMG artifact is interference in EEG recording by volume-conducted signals from skeletal muscles. This artifact contains high-frequency activity that resembles a "buzz" of fast activity during a contraction. EMG is seen as fast beta activity in the qEEG. While some frequencies are between 10-70 Hz, most are 70 Hz or higher.

The graphic shows how high-frequency filter (HFF) selection can affect contamination by this artifact. A high-frequency filter (low-pass filter) attenuates frequencies above a cutoff frequency. In the examples below, the cutoffs are 35 Hz and 15 Hz.

All the channels on the left side of the tracing show SEMG artifact admitted by a 35-Hz high-frequency filter. The right tracing is free from SEMG artifact since its 15-Hz high-frequency filter attenuates the higher frequencies that contain this artifact.

The next graphic shows how gum chewing can generate SEMG artifact by contracting the muscles of mastication. Graphics © eegatlas-online.com.

The frequency spectrum for SEMG artifact ranges from 2-1,000 Hz. While strong muscular contraction can contaminate all frequency bands, including 10 Hz, the beta rhythm (at 70 Hz or higher) is most affected by this artifact. EMG artifact may create the appearance of greater beta activity than is present. Graphic © eegatlas-online.com.

Below is a BioGraph ® Infiniti EMG artifact display. Note how the amplitude of the EEG spectrum increases with each contraction.

Thompson and Thompson (2016) observed that EMG artifact is readily detected because it affects one or two channels, particularly at T3 and T4 at the periphery, and less often at O1, O2, Fp1, and Fp2.

You can identify EMG artifact by visually inspecting the raw signal. The next graphic shows SEMG artifact using a 70-Hz high-frequency filter.

Electro-Ocular Artifact

Electro-ocular artifact contaminates EEG recordings with potentials generated by eye blinks, eye flutter, and other eye movements. For example, anxious patient eyelid flutter may cause deflections at Fp1 and Fp2 (Klass, 2008).

This artifact is due to the movement of the eye’s electrical field when the eye rotates and the contraction of the extraocular muscles. The eye creates a dipole that is electropositive at the front and electronegative at the back. Bell’s Phenomenon refers to the upward rotation of the eye when it closes and causes an artifact seen as an apparent increase in EEG.

The next two graphics show eye movement artifacts due to rapid blinking. Graphics © eegatlas-online.com.

The next graphic shows eye blinks, sharp lateral eye movement, and slow lateral eye movement.

Below is a BioGraph ® Infiniti EEG display of eye movement artifact.

Below is a NeXus display of eye blink and EMG © John S. Anderson.

An upward eye movement will create a positive deflection at Fp1, while a downward eye movement may create a negative deflection. In a longitudinal sequential montage, the artifact is typically seen at frontal sites (Fp1- F3 and Fp2-F4). A left movement may produce a positive deflection at F7 and a negative deflection at F8 (Thompson & Thompson, 2015).

Rapid eye flutter may resemble seizure activity. Graphic © John S. Anderson.

Cardiac and Pulse Artifacts

Cardiac artifact occurs when the ECG signal appears in the EEG. This artifact may be produced when electrode impedance is imbalanced or too high or when an ear electrode contacts the neck. The frequency range for ECG artifact is 0.05-80 Hz, and it contaminates the delta through beta bands. Since multiple electrodes detect this artifact, it can create the appearance of greater coherence than is present. Graphic © eegatlas-online.com.

You can detect cardiac artifacts by inspecting chart recorder, data acquisition, or oscilloscope displays of the raw EEG waveform. Cardiac artifact appears as a wave that repeats about once per second (Thompson & Thompson, 2016). Below is a BioGraph ® Infiniti ECG artifact display.

Pulse artifacts are due to the mechanical movement of an electrode in relation to the skin surface due to the pressure wave of each heartbeat. Graphic © eegatlas-online.com.

Sweat (Impedance) Artifact

Sweat artifact results from sweat on the skin changing the conductive properties under and near the electrode sites (i.e., bridging artifact). Sweating reduces electrode contact with the scalp and generates large-scale up and down EEG line movements in several frontal channels. This artifact is often elicited by abrupt, unexpected stimuli and usually appears as isolated 1-2 Hz slow waves of 1-2-s duration at frontal and temporal sites (Thompson & Thompson, 2015). Graphic © eegatlas-online.com.

Bridging Artifact

A short circuit produces bridging artifact between adjacent electrodes due to excessive application of electrode paste or a client who is sweating excessively or who arrives with a wet scalp. Bridging artifacts can cause adjacent electrodes to create a short circuit that produces identical referential EEG recordings or a flat line with a bipolar montage. The Fp1-F3 channel's reduced amplitude and frequency illustrate a bridging artifact.

Drowsiness Artifact

Drowsiness artifact is the appearance of stage 1 or stage 2 sleep in the EEG. Stage 1 and stage 2 of sleep are most likely during eyes-closed recording. Sleep may occur during eye-closed awake recording. Graphic © eegatlas-online.com.

Stage 1 sleep is a subtle drowsy state which clients often do not recognize. Alpha amplitude (especially occipital) may decrease, and theta (especially frontal) may increase. Reductions in EMG and beta amplitude will accompany slow eye-rolling movements. Sleepiness may be accompanied by spike-like transients (vertex or V-waves). The graphic below © John S. Anderson shows increased theta during stage 1 sleep.

When you detect drowsiness artifact during a training session, suspend recording and instruct your clients to move their hands and legs to increase wakefulness. To avoid this artifact, ask them to retire early and sleep for 9 hours if possible (Thompson & Thompson, 2003).

Evoked Potential

Evoked potential artifact (also called event-related potential artifact) consists of somatosensory, auditory, and visual signal processing-related transients that may contaminate multiple channels of an EEG record. While evoked potentials increase recording variability and reduce its reliability, they minimally affect averaged data (Thompson & Thompson, 2015).

The graphic below is courtesy of BPM biosignals' YouTube video EEG: Visually evoked potentials (VEP).

Movement Artifact

Movement artifact is caused by client movement or the movement of electrode wires by other individuals. Most of these artifacts are produced by brief changes in electrode-skin surface connection. Cable movement is called cable sway. The graphic below that illustrates cable sway is courtesy of iMotions.com.

Movement artifacts can produce high-frequency and high-amplitude voltages identical to EEG and EMG signals. While the delta rhythm is most affected by this artifact, it may also contaminate the theta band (Thompson & Thompson, 2016). Graphic © eegatlas-online.com.

The graphic below shows movement artifacts due to head movement (left), respiration (center), and tongue movement (right).

Below is a BioGraph ® Infiniti cable movement artifact display. Note the two voltage spikes at the beginning of the recording.

50/60 Hz and Field Artifacts

Both 50/60 Hz and field artifacts are external artifacts transmitted by nearby electrical sources. While 60-Hz artifact is a risk in North America where AC voltage is transmitted at 60 Hz, 50-Hz artifact is a problem in other locations that generate power at 50 Hz. Their fundamental frequency is 50 or 60 Hz with harmonics at 100/120 Hz, 150/180 Hz, and 200/240 Hz. Imbalanced electrode impedances increase an EEG amplifier's vulnerability to these artifacts. The 60-Hz artifact graphic below © John S. Anderson.

A BioGraph ® Infiniti display of 60-Hz artifact is shown below in red. Note the cyclical voltage fluctuations and 60-Hz peak in the power spectral display.

Radiofrequency Artifact

Radiofrequency (RF) artifact radiates outward like a cone from the front of televisions and computer monitors. Graphic © eegatlas-online.com.

Electrode Artifacts

There are several sources of electrode artifacts. Even with proper care, electrode surfaces can become corroded and the leads and connectors damaged. Using sensors with mismatched electrode metals can cause polarization of amplifier input stages.

Impedance Artifact

Unless skin-electrode impedance is low (under 5 KΩ for research and 20 KΩ for training) and balanced (under 1-3 KΩ ), diverse artifacts like 50/60 Hz and movement can contaminate the EEG signal, as seen in the P3 and Pz electrodes. Graphic © eegatlas-online.com.

Electrode Pop Artifact

Even when the impedance is low and balanced, mechanical disturbance can produce a unique artifact. Electrode pop artifact has a sudden large deflection in at least one channel when an electrode abruptly detaches from the scalp. This may also happen when there is a bubble or other defect in the gel or paste, and a charge builds up and subsequently "jumps" across the gap, resulting in a large electrical discharge. Graphic © John S. Anderson.

Recognizing Normal EEG Patterns

This section covers normal EEG patterns, including the posterior dominant rhythm, differences between eyes open and eyes-closed resting conditions, developmental aspects of the EEG, and diurnal influences on the EEG.

Normal EEG Patterns

The healthy adult EEG is a cerebral symphony comprised of theta, alpha, sensorimotor rhythm, beta, and gamma activity. We will survey the generators, distributions, and behavioral correlates of these rhythms. EEG rhythms correlate with patterns of behavior (level of attentiveness, sleeping, waking, seizures, and coma), occur in distinct frequency ranges, and are characterized by synchrony and desynchrony.

Synchrony means that pools of neurons coordinate their firing due to pacemakers (top) and mutual coordination (bottom).

Desynchrony means that pools of neurons fire independently due to stimulation of specific sensory pathways up to the midbrain and high-frequency stimulation of the reticular formation and nonspecific thalamic projection nuclei.

The desynchronized EEG graphic below © John S. Anderson.

Effect of Eyes Open and Closed Conditions on the EEG

The alpha rhythm is strongly modulated by visual input. Opening the eyes blocks or reduces the occipital alpha rhythm. Hans Berger (1929) originally described this phenomenon. In contrast, eyes-closed alpha is associated with alert wakefulness and reduced visual input (Thompson & Thompson, 2016).

The movie below is a 19-channel BioTrace+ /NeXus-32 display of eyes open and closed EEG © John S. Anderson. Note the appearance of alpha activity with eyes closed at about 14 seconds and alpha-blocking with eyes open at about 45 seconds.

The graphic below illustrates alpha-blocking and is courtesy of iMotions.

Posterior Dominant Rhythm

The posterior dominant rhythm (PDR) is the highest-amplitude frequency detected at the posterior scalp when eyes are closed. A healthy adult PDR is 10 Hz. Values below 9 Hz and above 11 Hz are abnormal, may result from psychoactive drugs, and may be associated with clinical symptoms like anxiety (Demos, 2019). Graphic © eegatlas-online.com. See the PDR at P3-O1.

Developmental Aspects of EEG

Delta is dominant below age 3, theta from 3 to 5, and low alpha from 6 to 8. Alpha frequency increases to a peak around 10 Hz after age 10. Peak frequencies slow during adulthood with aging (Thompson & Thompson, 2016).

The PDR rises with age: 1 year (6 Hz), 8 years (8 Hz), 10-12 years (9 Hz), and 13-14 years (10 Hz) (Demos, 2019).

Diurnal Influences on the EEG

Alpha and theta rhythm amplitudes vary across the day, with the highest values at 11 am, 1 pm, and 3 pm. Fatigue and individual differences (but not eating) influence the magnitude of change and precise peak times. Serial assessments should be conducted at the same time of day to control for diurnal fluctuations (Thompson & Thompson, 2015).

Evaluation of Subject Variables During Acquisition

Subject variables are crucial to the interpretation of EEG measurements. In this section, we will examine the importance of alertness-drowsiness, physical relaxation, and anxiety. We covered the effects of eyes closed/eyes open in the previous section and medication effects in the Psychopharmacology unit.

Alertness-Drowsiness

A client's age determines how drowsiness is expressed in the EEG. Between 6 months and 2 years (and rarely after 12 years), most children present with hypnogogic hypersynchrony (abrupt appearance of low-amplitude spikes that resemble epileptiform activity otherwise normal record). Beta activity between 20-25 Hz also maximally appears centrally and posteriorly. When older children and adults are drowsy or enter stage 1 and 2 sleep, frontocentral beta may be activated (Fisch, 1999).

Adult EEG drowsiness patterns increasingly appear by age 10. Slow lateral eye movements are associated with 1-Hz (or slower) waveforms that can be detected with the greatest amplitude and reverse polarity at F7 and F8. Continued drowsiness may be related to 1-2 Hz slowing of the alpha rhythm. For this reason, it is crucial to confirm client wakefulness when assessing alpha rhythm frequency (Fisch, 1999. Graphic © fizkes/Shutterstock.com.

From ages 10 to 20, drowsiness is often accompanied by rhythmic frontal theta (Fisch, 1999).

Physical Relaxation

The human stress response is multidimensional and involves diverse systems, ranging from the central nervous system to the immune system. Each person uniquely responds to stressors. This is called response stereotypy.

Individuals differ in which systems are involved, the degree of their activation or suppression, and the impact of these changes on their health.

Since clients show widely different response stereotypies, assessment should monitor multiple physiological channels, including blood volume pulse (BVP) for heart rate and heart rate variability, electromyography (EMG), a respirometer for respiration rate and pattern, skin conductance (SC), and skin temperature.

A capnometer, which measures end-tidal CO2, can complement the information provided by a respirometer by detecting CO2 reductions due to overbreathing, which is more subtle than hyperventilation (Khazan, 2019).

Professionals can find the normative values for these measures in Moss and Shaffer's (2019) Physiological Recording Technology in Biofeedback and Neurofeedback.

Stress responses that affect physiologic responses outside the CNS can cause artifacts in EEG recordings. Therefore, when client distress produces excessive artifact, a professional may have to coach the client to relax guided by one or more biofeedback modalities.

Clients should be monitored in a comfortable but upright position.

Anxiety

Anxious clients often present with decreased alpha and increased 19-21 Hz or 20-23 Hz beta activity. Conversely, anxious adults diagnosed with ADHD may show increased alpha activity. Even without perceptible sweating, anxious patients may present with intermittent biphasic slow-wave activity (Picton & Hillyard, 1972). Graphic © Peshkova/Shutterstock.com.

Clients who experience panic may exhibit paroxysmal EEG activity (Thompson & Thompson, 2015).

Glossary

50/60 Hz: external artifacts transmitted by nearby electrical sources.

active electrode: an electrode placed over a site that is a known EEG generator like Cz.

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.

amplitude: the strength of the EEG signal measured in microvolt or picowatts.

artifact: false signals like 50/60Hz noise produced by line current.

asynchronous waves: neurons depolarize and hyperpolarize independently.

bridging artifact: a short circuit between adjacent electrodes due to excessive application of electrode paste or a client who is sweating excessively or who arrives with a wet scalp.

cardiac artifact: the contamination of the EEG by the ECG signal.

channel: an EEG amplifier output that is the result of scalp electrical activity from three electrode/sensor connections to the scalp.

computerized axial tomography (CAT or CT): the creation of medium-resolution images of brain structure by moving an x-ray source along an arc surrounding the head.

derivation: the assignment of two electrodes to an amplifier's inputs 1 and 2.

desynchrony: pools of neurons fire independently due to stimulation of specific sensory pathways up to the midbrain and high-frequency stimulation of the reticular formation and nonspecific thalamic projection nuclei.

differential amplifier (balanced amplifier): a device that boosts the difference between two inputs: the active (input 1) and reference (input 2).

drowsiness artifact: in adults, 1-Hz (or slower) waveforms can be detected with greatest amplitude and reverse polarity at F7 and F8 may progress to 1-2 Hz slowing of the alpha rhythm.

EEG artifacts: noncerebral electrical activity in an EEG recording can be divided into physiological and exogenous artifacts.

electro-ocular artifact: contamination of EEG recordings by potentials generated by eye blinks, eye flutter, and eye movements.

electrode: a specialized conductor that converts biological signals like the EEG into currents of electrons.

electrode pop artifact: sudden large deflections in at least one channel when an electrode abruptly detaches from the scalp.

EMG artifact: interference in EEG recording by volume-conducted signals from skeletal muscles.

evoked potential artifact (event-related potential artifact): somatosensory, auditory, and visual signal processing-related transients that may contaminate multiple channels of an EEG record.

exogenous artifacts: noncerebral electrical activity generated by movement, 50/60 Hz and field effect, bridging, and electrode (electrode “pop" and impedance) artifacts.

field artifacts: external artifacts transmitted by nearby electrical sources.

frequency (Hz): the number of complete cycles that an AC signal completes in a second, usually expressed in hertz.

functional magnetic resonance imaging (fMRI): an imaging technique to detect brain regions' oxygen use during specific tasks indirectly.

ground electrode: a sensor placed on an earlobe, mastoid bone, or the scalp that is grounded to the amplifier.

hertz (Hz): unit of frequency measured in cycles per second.

high-frequency filter (HFF): a filter that attenuates frequencies above a cutoff frequency.

hypnogogic hypersynchrony: the abrupt appearance of low-amplitude spikes that resembles epileptiform activity in an otherwise normal record.

impedance (Z): the 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.

impedance test: automated or manual measurement of skin-electrode impedance.

inion: the bony prominence on the back of the skull.

International 10-10 system: a modified combinatorial system for electrode placement that expands the 10-20 system to 75 electrode sites to increase EEG spatial resolution and improve detection of localized evoked potentials.

International 10-20 system: a standardized procedure for placing 21 recording and one ground electrode on adults.

magnetoencephalography (MEG): a noninvasive functional imaging technique that uses SQUIDs (superconducting quantum interference devices) to detect the weak magnetic fields generated by neuronal activity.

magnetic resonance imaging (MRI): a noninvasive imaging technique that uses strong magnetic fields and bursts of RF energy to construct highly detailed images of the living brain.

mastoid bone: the bony prominence behind the ear.

microvolt (μV): the unit of amplitude (signal strength) that is one-millionth of a volt.

monopolar recording: a recording method that uses one active and one reference electrode.

montage: a grouping of electrodes (combining derivations) to record EEG activity.

motor unit: an alpha motor neuron and the skeletal muscle fibers it innervates.

movement artifact: voltages caused by client movement or the movement of electrode wires by other individuals.

nasion: the depression at the bridge of the nose.

notch filter: a filter that suppresses a narrow band of frequencies, such as those produce by line current at 50/60Hz.

ohm (Ω): the unit of impedance or resistance.

physiological artifacts: noncerebral electrical activity that includes electromyographic, electro-ocular (eye blink and eye movement), cardiac (pulse), sweat (skin impedance), drowsiness, and evoked potential.

polarization: chemical reactions produce separate regions of positive and negative charge where an electrode and electrolyte make contact, reducing ion exchange.

positron emission tomography (PET): a functional imaging technique that injects radioactive chemicals into the brain's circulation to measure brain activity.

posterior dominant rhythm (PDR): the highest-amplitude frequency detected at the posterior scalp when eyes are closed.

preauricular point: the slight depression located in front of the ear and above the earlobe.

pulse artifacts: noncerebral voltages due to mechanical movement of an electrode in relation to the skin surface due to the pressure wave of each heartbeat.

Quantitative EEG (qEEG): digitized statistical brain mapping using at least a 19-channel montage to measure EEG amplitude within specific frequency bins.

reference electrode: an electrode placed on the scalp, earlobe, or mastoid.

referential (monopolar) montage: the placement of one active electrode (A) on the scalp and a neutral reference (R) and ground (G) on the ear or mastoid.

response stereotypy: a person’s unique response pattern to stressors of identical intensity.

sequential (bipolar) montage: placement of active (A) and reference (R) sensors on active scalp sites and the ground (G) to an earlobe or mastoid.

single photon emission computerized tomography (SPECT): a functional imaging technique that uses gamma rays to create three-dimensional and slice images of cerebral blood flow averaged over several minutes.

sweat artifact: changes in the EEG signal when sweat on the skin changes the conductive properties under and near the electrode sites (i.e., bridging artifact).

synchrony: the coordinated firing of pools of neurons due to pacemakers and mutual coordination.

tragus: the flap at the opening of the ear.

vertex (Cz): the intersection of imaginary lines drawn from the nasion to inion and between the two preauricular points in the International 10-10 and 10-20 systems.

TEST YOURSELF ON CLASSMARKER

Click on the ClassMarker logo below to take a 10-question exam over this entire unit.

REVIEW FLASHCARDS ON QUIZLET PLUS

Click on the Quizlet logo to review our chapter flashcards.

Visit the BioSource Software Website

BioSource Software offers Physiological Psychology, which satisfies BCIA's Physiological Psychology requirement, and Neurofeedback100, which provides extensive multiple-choice testing over the Biofeedback Blueprint.

Assignment

Now that you have completed this module, explain why low-and-balanced skin-electrode impedances are important in neurofeedback training. Describe the precautions you take to achieve acceptable impedance values. How do you measure impedance with your neurofeedback system?

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.

Breedlove, S. M., & Watson, N. V. (2020). Behavioral neuroscience (9th ed.). Sinauer Associates, Inc.

Cacioppo, J. T., & Tassinary, L. G. (Eds.). (1990). Principles of psychophysiology. Cambridge University Press.

Collura, T. F. (2014). Technical foundations of neurofeedback. Taylor & Francis.

Demos, J. N. (2019). Getting started with neurofeedback (2nd ed.). W. W. Norton & Company.

Fisch, B. J. (1999). Fisch and Spehlmann's EEG primer (3rd ed.). Elsevier.

Floyd, T. L. (1987). Electronics fundamentals: Circuits, devices, and applications. Columbus: Merrill Publishing Company.

Grant, A. (2015). Four elements earn permanent seats on the periodic table. Science News.

Halford, J. J., Sabau, D., Drislane, F. W., Tsuchida, T. N., & Sinha, S. R. (2016). American Clinical Society Guideline 4: Recording clinical EEG on digital media. Journal of Clinical Neurophysiology, 33(4), 317-319. https://doi.org/10.1080/21646821.2016.1245563

Hugdahl, K. (1995). Psychophysiology: The mind-body perspective. Harvard University Press.

Hughes, J. R. (1994). EEG in clinical practice (2nd ed.). Butterworth-Heinemann.

Khazan, I. Z. (2019). Biofeedback and mindfulness in everyday life: Practical solutions for improving your health and performance. W. W. Norton & Company.

Klass, D. W. (2008). The continuing challenge of artifacts in the EEG. EEG artifacts. American Society of Electroneurodiagnostic Technologists, Inc. https://doi.org/10.1080/00029238.1995.11080524

Kubala, T. (2009). Electricity 1: Devices, circuits, and materials (9th ed.). Cengage Learning.

Lau, T. M., Gwin, J. T., & Ferris, D. P. (2012). How many electrodes are really needed for EEG-based mobile brain imaging? Journal of Behavioral and Brain Science, 2(3), 387-393. https://doi.org/10.4236/jbbs.2012.23044

Lebby, P. C. (2013). Brain imaging: A guide for clinicians. Oxford University Press.

Libenson, M. H. (2010). Practical approach to electroencephalography. Saunders Elsevier.

Lin, F., Witzel, T., Hamalainen, M. S., Dale, A. M., Belliveau, J. W., & Stufflebeam, S. M. (2004). Spectral spatiotemporal imaging of cortical oscillations and interactions in the human brain. NeuroImage, 2(3), 582-595. https://dx.doi.org/10.1016%2Fj.neuroimage.2004.04.027

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.

Picton, T. W., & Hillyard, S. A. (1972). Cephalic skin potentials in electroencephalography. Encephalogr Clin Neurophysiol, 33, 419-424. https://doi.org/10.1016/0013-4694(72)90122-8

Pfister, H., Kaynig, V., Botha, C. P., Bruckner, S., Dercksen, V., & Hege, H.-C. (2012). Visualization in connectomics. Mathematics and Visualization, 37. https://doi.org/10.1007/978-1-4471-6497-5_21

Stern, R. M., Ray, W. J., & Quigley, K. S. (2001). Psychophysiological recording (2nd ed.). Oxford University Press.

Thomas, C. (2007). What is a montage? EEG instrumentation. American Society of Electroneurodiagnostic Technologists, Inc.

Thompson, M., & Thompson, L. (2015). The biofeedback book: An introduction to basic concepts in applied psychophysiology (2nd ed.). Association for Applied Psychophysiology and Biofeedback.