Reading topographic maps of the EEG may seem straightforward and relatively simple. Z-score maps highlight areas that deviate from typical values compared to normative databases adjusted for age and sometimes gender and handedness. When an area on the map shows excessive activity in a particular EEG frequency, targeted sensor placement and effective client training can help normalize these levels. Conversely, reduced activity in an area may result in efforts to enhance it.
However, the reality is more complex. EEG recordings often exhibit significant artifacts from multiple sources, such as environmental interference (e.g., 50- or 60-Hz electrical noise) and physiological factors like eye blinks, movements, heartbeats, and muscle contractions. We must clean EEG data to ensure its integrity. This requires distinguishing between genuine EEG activity and transient phenomena such as drowsiness, sleep, or normal variants that do not signify pathology. While important for an accurate EEG report, these EEG features should not factor into statistical analyses.
Creating topographic EEG maps should be considered one of the last stages of a clinical assessment, not its primary focus.
Therefore, adhering to a careful progression from data collection to comprehensive analysis is essential instead of relying on automated artifact rejection algorithms and immediately generating maps.
Focusing neurofeedback training on areas associated with problematic symptoms is important for effective intervention. Simply targeting any abnormality detected in the EEG may not address the underlying issues and could lead to unintended consequences.
Atypical EEG findings can arise from various factors, including exceptional skills, compensatory changes due to illness or injury, developmental differences, or unique characteristics that do not necessarily indicate pathology. Therefore, careful clinical assessment and interpretation are necessary to determine whether observed deviations require correction.
By focusing on symptom-based training associated with an understanding of the clinical picture, clinicians can ensure that neurofeedback protocols address specific concerns and optimize outcomes for individuals undergoing training.
The following is an example of an EEG recording of a 76-year-old male with complaints of "brain fog," memory problems, lack of energy, slow cognitive processing, and difficulty sleeping.
Eyes-Closed Linked Ears (ECLE) Montage
This is a 50-uV scale, 10-sec display. Note the persistent ECG artifact in multiple channels but most clearly seen in reference channels (red outline at the bottom). The peak alpha frequency is approximately 8 Hz, the amplitude is 12-25 uV at the parietal sensors, and there is a small electrode pop in the F4 sensor (blue outline in this and subsequent montages).
Eyes-closed Longitudinal Bipolar (ECLBP) Montage
This is a 50-uV scale, 10-second display. The alpha frequency is 8 Hz, and the amplitude is 10-30 uV in parietal-occipital derivations. Note that the rhythmic activity (frontal alpha) seen in the linked ears montage is not present in prefrontal–frontal derivations in this bipolar montage, indicating it was the result of reference contamination in the previous montage.
Eyes-Closed Average Reference (ECAVE) Montage
This is a 50-uV scale, 10-second display. The alpha frequency is 8-9 Hz, and the amplitude is 5-15 uV at the occipital and 10-18 uV at the parietal sensors. Note the delta activity at the parietal sensors (green outline) and EMG artifact at the occipital sensors
Eyes-Closed Laplacian (ECLP) Montage
This is a 400-microampere (uA) scale, 10-second display. The alpha frequency is 8-9 Hz, with the highest current density in parietal sensors. EMG artifact continues in occipital sensors, and delta is more pronounced in the parietal area. Electrode pop in F4 sensor. The lack of prefrontal and frontal alpha activity suggests reference contamination in the linked ears montage above.
Eyes-Closed Linked Ears (ECLE) Montage
A linked ears montage in NeuroGuide with FFT absolute power spectral display at the top right with a line indicating the highest amplitude at 9 Hz is at the P4 electrode.
The same image showing the maximum power at 8 Hz is at the C4 electrode.
The same linked ears montage image shows the peak activity at 7.5 Hz, which is generally 1-3 SD greater than typical values at multiple locations (see z-score indicators on the left side of tracings next to electrode location labels).
Eyes-Closed Longitudinal Bipolar (ECLBP) Montage
This spectral display shows a longitudinal bipolar montage indicating the maximum z-scores at 2.5 Hz.
Eyes-closed longitudinal bipolar montage showing standard deviations at 8.5 Hz.
Eyes-Closed Average Reference (ECAVE) Montage
Average reference montage showing deviations at 2.5 Hz
Eyes-Closed Laplacian (ECLP) Montage
Laplacian montage showing current source density (CSD) z-scores at 3 Hz.
Eyes-Closed Linked Ears (ECLE) Montage
These absolute power topographic maps represent 1 minute and 30 seconds of a recording from an eyes-closed linked ears montage. Each small head map represents a virtual view of the top of the head with the nose at the top. The absolute power (microvolts squared) values correspond to the colored scale below each map. Red represents the greatest value, and blue is the lowest value for each 1 Hz frequency bin. The P4 electrode shows a power value of 36 in the 9 Hz frequency bin. Each bin has its own scale.
Z-score absolute power, 1-Hz frequency bin maps showing maximum to minimum deviations compared to the NeuroGuide normative database for a linked ears montage. The scale is from -3 (blue) to +3 (red) standard deviations (SD). The excess activity at 1 Hz is likely due to the ECG artifact noted earlier. The heart beats at about 1 beat per second, which equals 1 Hz. Excess activity is seen at 7-9 Hz.
Frequency band maps (like delta, theta, alpha, and beta) show the entire frequency band and lack the resolution of the individual 1-Hz frequency bin maps. The delta map generally shows excess delta activity, which misidentifies the ECG artifact that was seen at 1 Hz in the frequency bin maps and the visual inspection of the EEG.
These are relative power topographic maps representing 1 minute and 30 seconds of data from an eyes-closed linked ears montage. They show relative power values (percentages) that compare the value in each 1 Hz bin to the broadband EEG (0.5 – 30 Hz). This information is displayed as a colored scale below each map, with red representing the highest percentage and blue representing the lowest percentage for each 1-Hz frequency bin. Note that 9 Hz contains 33 percent of the total EEG power at the P4 electrode location. Each head map has its own scale.
These are z-score relative power, 1-Hz frequency bin maps showing maximum to minimum deviations compared to the NeuroGuide normative database using a linked ears montage. The scale is from -3 (blue) to +3 (red) standard deviations (SD). Note that this page represents relative power, showing the relative value of each 1 Hz bin compared to the EEG as a whole. This can result in areas showing incorrect abnormal z-score values at some frequencies because other frequencies are abnormal in the opposite direction. For example, frontal, central, and parietal activity at 8 Hz is excessive in the image below, causing apparent deficient activity in the same areas at multiple frequencies because 8 Hz takes up too much of the percentage "pie."
The previous examples show the progression of EEG analysis from viewing the recorded EEG through spectral analysis to topographic maps representing absolute and relative power and z-score maps showing deviations from expected values when client results are compared to a normative database.
The raw tracings and the spectral displays show examples from multiple montages (sensor comparisons) and help to highlight that what is seen is highly dependent upon which comparisons are used. So far, The topographic map examples have all used the linked ears montage and only showed one perspective on the EEG results. Now, we will look at the same data presented as topographic maps using different montages.
Eyes-Closed Average Montage
Eyes-closed average montage absolute power topographic map for 1-20 Hz, eyes-closed linked ears absolute power topographic map for 1-20 Hz, and eyes-closed Laplacian absolute power topographic map for 1-20 Hz.
Eyes-closed average z-score absolute power topographic map for 1-20 Hz, eyes-closed linked ears z-score absolute power topographic map for 1-20 Hz, and eyes-closed Laplacian montage z-score absolute power topographic map for 1-20 Hz. Notice the differences between the average, Laplacian, and linked ears reference maps. Which one should we follow when completing our assessment?
The average reference and Laplacian montages show fairly similar results and correspond to some of the other indicators, and may generally represent the client’s presenting issues of brain fog, memory issues, and slow cognitive processing.
Other posts and the Neurofeedback Tutor program have addressed the issue of montage selection. An excellent resource for more in-depth information is Nunez and Srinivasan's (2006) Electrical Fields of the Brain (2nd ed.).
For this example, we are confronted with significant differences between the linked ears montage results and both average reference and Laplacian montage results, particularly with respect to the presence of excess activity in the 2-6 Hz range of delta and theta.
The Laplacian montage uses an average of the current flow from electrodes immediately surrounding the electrode of interest as a type of localized average reference using current rather than voltage as its metric. This has been described as more accurate in identifying local activity while minimizing general effects such as those from medication, drowsiness, and others. This can help highlight local abnormalities, which can be lost or masked by other montages.
The average reference montage uses an average of all scalp electrodes to serve as the reference for each individual electrode, thus eliminating the ear or mastoid reference. This helps remove contributions from these reference electrodes, which are common to all electrode pairings when using the linked ears or linked mastoid reference montage.
Each of these montage choices has benefits and limitations. The benefits have been mentioned above, but what are the limitations? The average reference montage, like all montages, is subject to the differential amplifier's common mode rejection phenomenon. Sources (electrical activity such as EEG, ECG, EMG, EOG, EMF) that are the same in frequency and, to a lesser extent, in amplitude are rejected, while sources that are different are retained. If there are multiple sources (electrode locations) of delta activity contributing to the average, and this average is then compared to a location that does not show delta activity, then there is a difference between the signals, and that difference is retained and displayed in the topographic maps displays and of course in the EEG tracings, resulting in apparently abnormal delta activity where it does not exist. The same is true of the linked ears/mastoid reference and, to a lesser extent, of the Laplacian montage.
Therefore, we look for agreement among multiple montages, being particularly attentive to the various bipolar montages that allow revealing comparisons when viewing the EEG tracings. In the present example, though earlier we mentioned that the delta activity was confined to the 1 Hz effect from the ECG (heartbeat) artifact, we can see from the Laplacian and average montages that there is substantial agreement on a broader frequency distribution of delta/theta activity that exceeds statistical significance. Thus, the recommendation is to begin training by focusing on these excesses.
Summary
Given the alignment observed among the findings from both the average reference and Laplacian absolute power z-score topographic maps, alongside the individual's reported symptoms and the established EEG patterns corresponding to those symptoms, an optimal starting point for neurofeedback intervention would seem to involve training aimed at reducing 8-10 Hz alpha activity in central and parietal regions. Addressing the excess 2-5 Hz activity, particularly at the C4 electrode site, may offer additional benefits.
It's worth noting that the individual has a significant background in Tai Chi practice and has been engaged in meditative activities for over five decades. Consequently, the increased amplitude of alpha activity and the slower alpha frequencies compared to normative databases likely reflect the cumulative effects of these long-standing practices rather than indicating pathological conditions.
In this context, it may be more appropriate to focus neurofeedback efforts on addressing the excess delta and low-frequency theta activity. Subsequent reevaluation of symptoms and EEG patterns following several sessions could provide valuable insights into whether to maintain the initial neurofeedback approach or adjust the protocol accordingly.
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.
amplitude: the energy or power contained within the EEG signal measured in microvolts or picowatts.
artifact: false signals like 50/60Hz noise produced by line current.
average reference montage: EEG recording configuration using the average of all scalp electrodes as the reference for each individual electrode. The ear or mastoid reference electrodes are excluded from this average.
bipolar transverse montage: EEG recording configuration chaining adjacent electrodes from left to right (Fp1 to Fp2, F7 to F8, A1 to A2, T5 to T6, and O1 to O2).
channel: an EEG amplifier output resulting from scalp electrical activity from three electrode/sensor connections to the scalp. circular (circumferential) bipolar montage: EEG recording configuration involving the counterclockwise chaining of electrodes around the head's circumference, starting at Fp1 and ending at Fp2.
common mode rejection: a differential amplifier's ability to suppress signals common to its + and - inputs.
derivation: assigning two electrodes to an amplifier's inputs 1 and 2.
differential amplifier (balanced amplifier): a device that boosts the difference between two inputs: the active (input 1) and reference (input 2). EEG topography: displaying the qEEG on a cortical surface map to show the spatial distribution of EEG activity.
electrode: a specialized conductor that converts biological signals like the EEG into currents of electrons.
exogenous artifacts: noncerebral electrical activity generated by movement, 50/60 Hz and field effect, bridging, and electrode (electrode “pop" and impedance) artifacts.
field: EEG signal weakening with increasing electrode distance from its source.
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 the localization of electrical potentials.
International 10-20 system: a standardized procedure for placing 21 recording and 1 ground electrode on adults on adults to provide a total of 19 channels. This system is used for typical 19-channel qEEG recordings, using 19 "active" electrodes, "reference" electrodes at A1 and A2, and a ground electrode.
linked ears (LinkEar) montage: EEG recording configuration in which individual electrode potentials are compared to voltages detected at two linked earlobe references (-). This montage is vulnerable to reference contamination.
linked-mastoid montage: EEG recording configuration that compares individual electrode potentials to voltages detected at two linked mastoid references (-). This montage is vulnerable to reference contamination.
longitudinal bipolar montage or double banana: EEG recording configuration involving the anterior-to-posterior chaining of adjacent electrodes in two lines on each side (Fp1 to O1 and Fp2 to O2) and connecting the midline electrodes (Fz to Pz).
mastoid bone: the bony prominence behind the ear.
montage: EEG recording configuration that groups electrodes (combines derivations) to monitor EEG activity.
phase: the degree to which the peaks and valleys of two waveforms coincide.
phase reversal: reverse polarity observed in voltages from contiguous electrodes that can signal spike epileptogenic foci.
Quantitative EEG (qEEG): the statistical description and analysis of EEG features based on the digitization of analog EEG activity obtained using at least a 19-channel montage.
reference electrode: an electrode placed on the scalp, earlobe, or mastoid.
referential (monopolar) montage: EEG recording configuration with an active (+) electrode (A) on the scalp and a "neutral" reference (-) electrode (R) and ground (G) on the ear or mastoid.
sequential (bipolar) montage: EEG recording configuration using a sequence of comparisons of positive (+) and negative (-) electrodes (often called active and reference) that are attached to sites on the scalp. A sequential montage 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.
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.
References
Collura, T. F. (2014). Technical foundations of neurofeedback. Taylor & Francis.
Demos, J. N. (2019). Getting started with neurofeedback (2nd ed.). W. W. Norton & Company.
Libenson, M. H. (2010). Practical approach to electroencephalography. Saunders Elsevier.
Nunez, P. L., & Srinivasan, R. (2006). Electric fields of the brain (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.
Learn More
Support Our Friends at ISNR and AAPB
Comments