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Browse courses and booksModule 6
Chapter 6 · 2 h · 8 quiz items · pass at 80%
This module completes IQCB Domain III (Technical), 15% of the exam. Accurate 10-20 placement and a defensible acquisition protocol are the technologist's core competency, and montage choice changes every downstream metric and database comparison. The quiz confirms the learner can place electrodes correctly and choose and document an acquisition that a normative database can accept.
Where you put the electrode determines what you measure, and how you reference it determines what the brain looks like once you do. A site a centimeter off sits over different cortex. A montage chosen differently makes the same brain read as a different brain. This is the chapter where the electronics of the last one become a map on a real scalp, and where several skills separate competent Diplomates from the rest: measuring a head accurately by the International 10-20 system, running the protocol that yields data a database can score, and understanding how the choice of reference shapes every metric you will later report. All of it is testable, and most of it is something you do with your hands and your protocol before any software runs.
This chapter covers the 10-20 system in full, the landmarks, the arithmetic, the naming convention; the expansion to the extended 10-10 and 10-5 systems and the 64- and 128-channel arrays built on them; the standard QEEG acquisition protocol of eyes-closed, eyes-open, and task conditions; the recording environment and duration the field requires; the montages (linked ears, average reference, bipolar, Laplacian) with an honest account of what each does to your QEEG metrics; and the documentation of acquisition parameters that makes a record interpretable, reproducible, and defensible. The IQCB exam weights this domain heavily, and it tests the referencing logic in particular at a depth that catches candidates who have only ever clicked a default.
The problem the 10-20 system solves is reproducibility. Heads differ in size and shape, so a placement measured in fixed centimeters would land on different cortex in different people, and a recording made at one clinic would not correspond to a recording made at another or to the normative databases everything is compared against. The system's answer, introduced by Herbert Jasper for the international EEG federation in 1958, is to place electrodes at fixed percentages of the distance between bony landmarks rather than at fixed absolute distances. Because the percentages scale with each head, a site lands over the same cortical region regardless of head size, and the name 10-20 refers to those percentages: electrodes sit at 10 percent or 20 percent intervals along the measured lines.
Four bony landmarks anchor the whole system, and finding them accurately by palpation is the foundational manual skill.
From those four landmarks the placement is built by measurement.
The anteroposterior (front-to-back) line. Measure the distance from nasion to inion straight over the top of the head along the midline. This total is divided into the canonical intervals. Stepping back from the nasion: the frontal-pole midline (Fpz, the reference point for Fp1 and Fp2) sits at 10 percent of the nasion-inion distance up from the nasion. Then 20 percent intervals carry you to Fz, Cz, and Pz in turn, and the occipital-pole midline (Oz) sits at 10 percent up from the inion. The pattern over the midline is therefore 10-20-20-20-20-10, summing to the full 100 percent. Cz, the vertex, lands at exactly 50 percent of the nasion-inion line, the midpoint.
The coronal (ear-to-ear) line. Measure from the left preauricular point to the right preauricular point straight over the top, and this line must cross the midline measurement exactly at Cz. The intersection of the two 50-percent midpoints defines the vertex, and getting Cz right is the linchpin of the whole placement: an error in Cz throws off every other site. Along this coronal line the lateral sites step out at the same 10 and 20 percent intervals, placing T3 (now also called T7) and T4 (T8) over the temporal regions and C3 and C4 over the sensorimotor strip, with Cz between them.
The circumferential line. A horizontal circle around the head through Fpz, the temporal sites, and Oz carries the remaining lateral electrodes (Fp1/Fp2, F7/F8, T3/T4, T5/T6, O1/O2) at their 10 and 20 percent positions around the circumference. The intermediate frontal and parietal sites (F3/F4, P3/P4) are then placed to fall evenly between the midline and the lateral chains.
Throughout, symmetry is verified: each left-right pair must sit equidistant from the midline, because an asymmetric placement manufactures a laterality finding that is pure measurement error, not brain, and laterality is exactly what QEEG asymmetry metrics quantify. The required accuracy for clinical QEEG is placement within about 5 millimeters of the correct position. Soft caps and variable anatomy make perfect placement impossible, but the discipline of measuring carefully, anchoring on a correct Cz, and checking symmetry keeps you inside the tolerance the databases assume. Common errors follow directly from the procedure: a Cz placed too far forward or back skews every other site. Asymmetric placement invents laterality. An electrode shifted up or down, the temporal sites especially, changes the cortex it is over; and a temporal site pushed too far down picks up muscle artifact from the temporalis.
The 10-20 labels are not arbitrary. They encode location, and reading them fluently is exam-essential.
A nomenclature note the exam may probe: the older labels T3, T4, T5, T6 were partially revised in the modified combinatorial system to T7, T8, P7, P8 to make the spacing logic consistent when higher-density arrays are added. You will see both conventions in the literature and in software. T3 and T7 name the same site, as do T4 and T8, while the old posterior-temporal T5 and T6 become P7 and P8 under the relabeling. For routine clinical work either label set is understood. Be consistent within a recording, and document which you used.
The 19-channel 10-20 montage is the clinical workhorse, but the same logic scales to far denser arrays, and the IQCB exam expects you to understand how the extension is built and what it buys. As you add electrodes, you cannot keep using 10 and 20 percent steps alone, because there are not enough labeled positions. You subdivide the intervals and name the new sites systematically.
The 10-10 system (the modified combinatorial system). Halving the 10-20 intervals so that electrodes sit at every 10 percent step (rather than at 10 and 20 percent) roughly doubles the site count to the range that supports a 64-channel array. The 10-10 system fills in the intermediate positions, the row of electrodes between frontal and central (the FC line: FC1, FC2, FC5, FC6, and so on), between central and parietal (CP), between frontal-pole and frontal (AF), between parietal and occipital (PO), and so on, with a consistent naming logic so that any site's label still tells you its region and side. This is the same combinatorial revision that produced the T7/T8/P7/P8 relabeling. Renaming the temporal sites was necessary precisely so the denser grid would be self-consistent. A 64-channel layout is essentially a well-populated 10-10 montage.
The 10-5 system. Subdividing again, to 5 percent steps, yields the several-hundred positions that support 128- and 256-channel arrays. The 10-5 system extends the same naming scheme to an even finer grid, and a 128-channel cap is laid out on this denser scaffold. The principle does not change as the numbers grow: percentage-based spacing between the same four landmarks, scaled to the head, with a systematic label encoding region and side at every site.
What the density buys, and what it costs. Adding electrodes buys spatial resolution and, with it, better source localization: more sampling points on the scalp constrain a source model more tightly, which is why high-density arrays are used in epilepsy presurgical mapping and cognitive-neuroscience research where fine spatial detail is the point. The costs are concrete and matter for clinical QEEG. Setup time rises with every electrode, since each needs preparation and an impedance that passes. The artifact burden grows, because more electrodes mean more sites that can go bad and more channels for a movement or muscle event to contaminate. Patient discomfort increases with heavier caps and longer preparation; and, decisively for QEEG, the normative databases you compare against are overwhelmingly built on 19 channels, so a 64- or 128-channel recording must be reduced to the database montage for z-scoring anyway. For routine clinical QEEG the 19-channel array remains the standard precisely because it balances coverage against setup, tolerance, and database compatibility; the high-density systems add complexity without proportional clinical value for the everyday assessment, and earn their cost only when source localization or research resolution is genuinely required.
The geometry tells you where the electrodes go. The protocol tells you what to record once they are on. A QEEG is not a single recording but a small set of standardized conditions, each chosen because it reveals something the others do not, and run the same way every time so the result is comparable across patients, across sessions, and against the database.
The two foundational conditions are resting baselines with the eyes closed and the eyes open. They are not redundant: eyes-closed (EC) emphasizes internal processing and the thalamocortical alpha rhythm, while eyes-open (EO) emphasizes visual processing and vigilance, and the difference between them is itself a finding. The central thing the pair reveals is alpha reactivity, the normal blocking of the posterior alpha rhythm when the eyes open. Intact reactivity tells you the posterior cortex is responding normally to visual input, and a failure to block carries clinical meaning that neither condition alone would show. Some patients show a normal EC pattern and an atypical EO pattern, or the reverse, which is exactly why you record both.
The 2025 IQCB technical requirements set the field's minimum (Collura et al., 2025): at least 10 minutes of eyes-closed and 10 minutes of eyes-open recording, the duration measured as clean data surviving artifact rejection, captured on at least 19 channels of the 10-20 system in a controlled, low-interference environment with active artifact minimization during acquisition. The procedural detail belongs to the companion QEEG Field Guide, but the conditions matter here because the protocol is what makes the data scorable.
Eyes closed. The patient sits upright, relaxed, eyes gently closed (not squeezed shut, which produces facial-muscle artifact), with no specific mental task. You coach toward a quiet, awake state, because the two failure modes are drowsiness, which raises theta and delta and mimics underarousal, and active rumination, which is not a resting baseline. Watch for the head nods and slow rolling eye movements that signal sleep onset, and for forehead and jaw tension.
Eyes open. The patient opens the eyes and rests a soft gaze on a neutral fixation point, a simple dot or cross at eye level, three to six feet away, blinking naturally but minimizing deliberate eye movement. The fixation target exists to prevent the two EO failure modes: visual scanning, which produces large saccade artifact, and a tense unblinking stare, which produces muscle artifact and is not a relaxed baseline. An interesting picture or a visible screen in the field of view invites scanning and flicker, so the target is deliberately dull.
Beyond the resting pair, the protocol may add task conditions when a clinical question calls for them: eyes-open reading or a continuous-performance demand for sustained attention, mental arithmetic for working memory and executive load, auditory tasks for receptive processing, memory tasks for encoding and retrieval. Tasks reveal state-dependent patterns that do not appear at rest and, when paired with a performance measure, let you relate EEG to behavior. They are not routine: each condition lengthens the recording and the patient's burden, and the resting EC/EO baselines remain the core of the QEEG and the conditions most normative databases are built on. Provocations from clinical EEG, hyperventilation and photic stimulation, sit at the boundary of QEEG and clinical EEG. They test seizure susceptibility and visual response and belong to a clinical-EEG workup more than to a routine quantitative baseline.
A note on consistency that the protocol exists to serve: time of day, the patient's caffeine and medication state, and sleep the night before all shift the baseline, so you standardize what you can (Chapter 3 of the QEEG Field Guide catalogs these state factors), record under stated conditions, and document any deviation. The point of a protocol is that the next recording, on this patient or another, was made the same way.
The environment is part of the instrument. The two things it must control are electrical interference and patient state. A full electrically shielded room (a Faraday cage) is the research ideal and is not required for clinical QEEG. What is required is a controlled, low-interference setting with active artifact minimization during acquisition (Collura et al., 2025). In practice that means a quiet room with the door closed and minimal external stimulation, free of the obvious line-noise sources, fluorescent fixtures (especially older ballast types), monitors and electronics near the patient, fans and heaters, and phones and wireless devices, with the amplifier positioned away from power sources and, where possible, running on battery to avoid a mains ground loop. LED lighting is preferable to fluorescent for its lower interference. The noise floor of the room is not a slogan but something you verify: record a brief baseline with the patient still and inspect the spectrum for a 60 Hz (or 50 Hz) peak, and if it is present, find and fix the source before the formal recording rather than relying on a notch filter to mask it afterward (Chapter 5). The seating supports a relaxed, stable posture, an armchair with back support and feet flat, so the patient is not actively holding a position and generating muscle tension, and the operator is positioned to watch both the patient and the live trace without sitting in the patient's visual field.
Duration is governed by a single principle: spectral estimates become more reliable as more clean data accumulates, and slower frequencies need more of it because each second of recording contains only a few cycles of a delta or theta wave. The 2025 IQCB standard sets 10 minutes per condition, eyes-closed and eyes-open, measured as clean data after artifacting (Collura et al., 2025), and the reason for that length is statistical: recording well past the minimum you intend to analyze ensures that even aggressive artifact rejection leaves enough epochs of usable data for stable spectral and z-score estimates, and it protects against transient contamination and moment-to-moment state fluctuation.
Underneath that standard sits the older, narrower rule the exam may still reference: the minimum of about two minutes of artifact-free data required for a reliable spectral analysis. The logic is the floor under the whole protocol. The fast Fourier transform that produces the power spectrum needs a sufficient number of artifact-free epochs to yield a stable estimate, and below roughly two minutes of clean data the estimate's confidence interval widens to the point that band-power values, and the z-scores computed from them, become unreliable, particularly at the slow frequencies that contribute few cycles per second. The 10-minute recording is, in effect, the practical way to guarantee that the two-minute clean-data floor is comfortably cleared after the inevitable losses to blinks, movement, muscle, and drowsiness.
Two operational consequences follow. First, the relationship between recording time and analyzable data is not one-to-one: a 10-minute recording with heavy artifact may yield only minutes of clean data, while a calm patient yields nearly all of it, which is why you record generously and select clean epochs afterward rather than trusting the wall-clock duration. Second, some patients, children, anxious patients, those with movement disorders, cannot tolerate a continuous 10 minutes, and the standard accommodates this by combining shorter well-tolerated segments across the recording window. Up to 20 to 30 minutes per condition may be needed in a highly artifactual patient to extract sufficient clean data. Recordings made under older 3-to-5-minute protocols are not invalidated, but they sit below the current standard and carry wider statistical confidence, and they should be flagged in the technical section of the report and interpreted with that limitation in view. Whatever you collect, document the actual clean-data yield, not merely the recording length, because the yield is what the analysis rests on.
Here is the conceptual core of the chapter, and the part most likely to trip a candidate who has only ever clicked a default. Every EEG measurement is differential: you never record the absolute voltage at a site, only the difference between that site and something else. That something else is the reference, and the arrangement of active sites and references across a recording is the montage. The same brain activity looks different under different montages because the reference is part of the measurement, not a neutral backdrop, and re-referencing a recording can make a finding appear or vanish. For QEEG the stakes are specific: absolute power, relative power, asymmetry, and above all coherence are each computed from referenced data, so the montage is built into every number you report. This is why the field requires reviewing data across multiple montages before trusting any pattern, and why the 2025 standard mandates that a qualified clinician visually inspect the raw record across linked ears, longitudinal bipolar, transverse bipolar, average reference, and Laplacian montages before any quantitative analysis (Collura et al., 2025).
The linked-ears reference takes the two earlobes (or mastoids), A1 on the left and A2 on the right, and uses their average as the reference for every channel. It has been the traditional default for clinical QEEG and is the reference most normative databases were built on, which is its great practical virtue: recording in linked ears keeps you compatible with the databases you will compare against. The ears are reasonably electrically neutral because they sit far from the cortical generators, so they add relatively little brain signal of their own to the reference.
The cost is twofold, and both halves touch your metrics. First, physically linking the two ears creates an electrical bridge between the hemispheres, which can smear genuinely lateralized activity and distort the asymmetry measures that depend on a true left-right difference, and in some configurations manufacture artificial symmetry. Second, the ears are not perfectly neutral: strongly lateralized cortical activity spreads to the nearer ear, biasing the reference, and a contracting temporalis or jaw muscle near an ear contaminates the reference and, through it, every channel at once. Linked ears remains a sound default for database-compatible recording, but you read it knowing it bridges the hemispheres and that anything contaminating an ear contaminates the whole record, including the reference contribution to every coherence value.
The average reference computes the mean of all the electrodes and uses that average as the reference for each channel. Its appeal is that it commits to no single physical site as neutral, sidestepping the question of where a truly quiet reference could even be, and with adequate whole-head coverage the average approximates a reference free of any one electrode's local bias, which is why it is often preferred for source-oriented work.
Its characteristic weaknesses follow from the averaging itself and feed directly into the QEEG metrics. Because every channel feeds the average, strong global activity gets subtracted from all channels, attenuating widespread rhythms and lowering their apparent power across the map. More dangerously, a single bad electrode contaminates the average and therefore every channel: a jaw-clench artifact at one temporal site, fed into the average reference, smears across the whole montage as a spurious diffuse increase that can read as a real, widespread power elevation. The average reference is also coverage-dependent, working best with the full array and degrading when sites are missing or unevenly distributed, which is a particular caution when you have dropped a bad channel. Use it for the perspective it gives and for source analysis, but exclude or repair bad channels first, because the average inherits their problems and so does every z-score computed from it.
A bipolar montage abandons a common reference entirely. Each channel is the difference between two neighboring electrodes, and the electrodes are arranged in chains. A sequential (longitudinal) bipolar montage runs front-to-back chains (for example Fp1-F3, F3-C3, C3-P3, P3-O1 down the left side), the arrangement old EEG hands call the double banana. A transverse bipolar montage runs the chains left-to-right across the head. Because each derivation measures the gradient between two adjacent sites, bipolar montages emphasize local differences and suppress activity shared across the whole head, the opposite bias from a referential montage.
What this buys you is localization. A focal generator, a real epileptiform spike or a focal artifact, produces a phase reversal in a bipolar chain: the electrode directly over the source reads as the maximum, and the derivations on either side show opposite-polarity deflections, so the polarity reverses at the source site, pinpointing it. This is why bipolar montages are the standard clinical-EEG tool for detecting focal slowing, asymmetries, and epileptiform activity, and why they are central to localizing artifact during the visual review that precedes quantification. What they suppress is large-scale shared activity and long-range connectivity information, and they are harder to read than referential montages because every channel is a difference between two moving sites rather than a site against a fixed reference. Bipolar montages are used more in clinical-EEG review than in routine QEEG spectral analysis, and crucially the standard normative databases are not built on bipolar derivations, so you do not z-score a bipolar montage against them; you inspect in bipolar montages to catch focal features and phase reversals invisible to a referential view, then quantify in the referential montage the database expects.
The surface Laplacian, also called current source density, references each electrode to a weighted average of its immediate neighbors. Mathematically it is a spatial high-pass filter: it emphasizes activity that is local and unique to a site and removes activity shared with the surrounding electrodes. The benefit is sharpened spatial resolution and a strong reduction of the volume-conduction blur that smears the surface EEG, so superficial, focal cortical generators stand out clearly and a site's signal is freed from contributions spreading in from elsewhere. It is a strong choice for high-frequency work like gamma, where reducing volume-conducted spread improves the estimate, and a useful cross-check on whether a putative focal finding survives spatial sharpening. The cost is the mirror image, and it is a metric-level cost: because the Laplacian removes the broadly shared component, it discards large-scale and long-range connectivity information, which makes it the wrong montage for coherence between distant sites, where the very quantity of interest is the shared activity the Laplacian strips away. It deblurs and localizes superbly; it is not a connectivity montage.
The honest summary is that no single montage is correct, because each is a different question put to the same data. Linked ears keeps you database-compatible and shows global power patterns, while bridging the hemispheres and inheriting any ear contamination. Average reference gives a whole-head perspective free of one electrode's bias, at the price of spreading any bad channel everywhere. Bipolar montages localize focal features and reveal phase reversals while suppressing shared activity, and are an inspection tool rather than a database-scoring one. The Laplacian sharpens local sources and deblurs the surface while discarding long-range connectivity. The doctrine, codified in the 2025 standard, is to inspect across all of these before quantifying (Collura et al., 2025), because a pattern that appears under one reference and vanishes under another is reference-dependent and suspect, while a pattern that persists across montages, though it looks different in each, is more likely to be real brain activity. You record in one reference, commonly linked ears for database compatibility or Cz for simplicity, then re-reference mathematically afterward to confirm a finding survives the change.
One consequence deserves singling out because it bites every practitioner who reads QEEG reports: connectivity and coherence are acutely sensitive to the reference. A linked-ears reference that bridges the hemispheres can inflate apparent interhemispheric coherence; an average reference that subtracts a shared component can move coherence in either direction; the Laplacian, by stripping the shared activity, suppresses long-range coherence almost by construction. The same pair of electrodes can therefore yield meaningfully different coherence values under different montages, none of them wrong but none interchangeable, which is why the field also developed lagged and phase-based connectivity measures that reduce the zero-lag volume-conduction component (Chapter 10). When you compute or read a coherence finding, you must know which reference produced it, because the number is partly a statement about the montage and only partly a statement about the brain. This is the single most important reason a Diplomate cannot treat montage as a cosmetic default, and the reason montage choice belongs in the documentation of every map.
Before the first condition records, the impedance check is the gate the recording must pass, and it belongs to the protocol as firmly as the conditions do. The sequence, drawing on the equipment principles of Chapter 5, is fixed. Apply the electrodes with conductive gel. Run the system's built-in impedance check, which injects a tiny harmless current and reports each site's impedance, typically color-coded (green below 5 kilohms, yellow 5 to 10, red above). Review every value, holding individual electrodes below 5 kilohms where possible, the reference and ground to the stricter low standard because a fault there contaminates every channel, and all sites balanced within roughly 2 to 3 kilohms of one another, because imbalance, not just absolute level, defeats the differential amplifier's noise rejection. Correct every high site before recording by adding gel (insufficient gel is the most common cause), parting hair to the scalp, gently abrading the dead-cell layer without breaking skin, and checking the electrode and cable. A site that stays stubbornly high may be a scalp lesion or anatomy, to be documented and possibly omitted rather than recorded bad. Never proceed with impedances above 20 kilohms. And because impedance drifts as gel dries and electrodes loosen over a long session, re-check when you pause between conditions or after any movement, so the data in the second half of the recording was acquired under the same interface quality as the first.
A QEEG record is only as interpretable as its documentation, and the IQCB exam treats acquisition documentation as a professional obligation rather than an afterthought, because the report a Diplomate signs, and a forensic report especially, can be challenged on exactly these parameters. The record must let another practitioner understand precisely how the data was obtained and reproduce the conditions. At minimum, document:
The rule behind the list is that nothing about how the recording was made should have to be reconstructed from memory or inferred from the data. Documentation is what makes a map reproducible for serial comparison, interpretable when a finding is ambiguous, and defensible when a report is questioned. Chapter 13 carries the full report-writing standard; here the obligation is narrower and prior: capture the acquisition parameters completely and at the time of recording, because they are the foundation every later claim is built on.
When you acquire a QEEG, the sequence is fixed. Palpate the four landmarks (nasion, inion, both preauricular points), measure the nasion-inion midline and the ear-to-ear line, mark their crossing as Cz at the 50 percent midpoint of each because every other site depends on it, step out the 10 and 20 percent intervals, and verify left-right symmetry so you do not invent a laterality finding; place to within about 5 millimeters. Run the impedance pre-check and fix every high site before recording, holding reference and ground to the stricter standard and all sites balanced within 2 to 3 kilohms. Record the standard conditions, at least 10 minutes of eyes-closed and 10 minutes of eyes-open as clean data, in a quiet, low-interference room, adding task conditions only when a clinical question calls for them. Record in a database-compatible reference, then re-reference across montages to confirm any finding survives the change. Document every acquisition parameter at the time of recording.
For the IQCB exam, fix the system, the protocol, and the montage logic. The 10-20 system places electrodes at 10 and 20 percent intervals between nasion, inion, and the preauricular points so sites scale with head size and stay database-compatible; Cz is the 50 percent vertex and the linchpin; naming encodes location (the letter names the region, odd numbers left, even right, z midline); and the extended 10-10 and 10-5 systems subdivide the same intervals to 10 and 5 percent steps to support 64- and 128-channel arrays, buying spatial resolution and source localization at the cost of setup, artifact burden, and a mismatch with the 19-channel databases. The protocol is eyes-closed and eyes-open resting baselines (10 minutes each of clean data, the pair revealing alpha reactivity) plus task conditions when indicated, recorded in a controlled low-interference environment, with the practical floor being at least about two minutes of artifact-free data for a reliable spectrum, which the 10-minute recording exists to guarantee after artifact loss. On montages, every measurement is differential and the reference shapes every metric: linked ears is the database-compatible default that bridges the hemispheres and is vulnerable to ear contamination; average reference gives a whole-head perspective but spreads any bad channel everywhere; bipolar montages take differences between neighbors, localize focal activity, and show phase reversals at a source while suppressing shared activity, and serve inspection rather than database scoring; the Laplacian sharpens local sources and deblurs the surface while discarding long-range connectivity. Coherence values depend on the montage they were computed in, a finding that disappears under re-referencing is suspect, and the reference and filter settings belong in the documentation of every map. Place the electrode correctly, reference it knowingly, record to the standard, and document it fully, and the data you hand to interpretation is data worth scoring against a database.