EEG Simulator

Lead Placement Practice

Practice placing EEG electrodes according to the International 10-20 System. Select electrodes from the list and place them on the head model, then check your accuracy. You can also view different montage configurations to see how electrodes are arranged.

How to Use This Tool

Select an electrode from the list on the left (Fp1, F3, etc.)
Click on the head model where you think it should be placed
Continue until you've placed all the electrodes
Click Check Placement to see your accuracy
Use the montage dropdown to visualize different EEG montages

Select Montage

Difficulty Level

Standard tolerance for correct placement

Practice Mode

Place all leads at your own pace

EEG Leads

Accuracy

0 / 0 correct

Nasion

Inion

Left

Right

Understanding Phase Reversal in EEG

Phase reversal is a key concept in interpreting EEG recordings and localizing the source of electrical activity. It occurs when adjacent electrode pairs in a bipolar montage display deflections in opposite directions.

How Phase Reversals Work

In a bipolar montage, each channel records the voltage difference between two electrodes. When an electrical discharge occurs beneath an electrode:

The voltage at that electrode changes relative to surrounding electrodes

The electrode sees the highest voltage when directly over the source

In bipolar recordings, this creates opposite deflections in adjacent channels

Interactive Phase Reversal Simulator

Fp1

Fp2

Montage:

Voltage Intensity:

120 μV

50 μV 100 μV 150 μV 200 μV

Click or tap on any location on the head to simulate an electrical discharge and observe the resulting phase reversal.

Mathematical Foundation of EEG Montages

Bipolar Montage Calculation

V_channel = V_electrode1 - V_electrode2

Example: For channel F3-C3

V_F3-C3 = V_F3 - V_C3

If V_F3 = +60μV and V_C3 = +100μV, then V_F3-C3 = -40μV (upward deflection)

Referential Average Montage

V_avg = (V₁ + V₂ + ... + V_n) / n
V_channel = V_electrode - V_avg

The average reference is calculated as the mean of all electrode voltages, then each electrode is referenced to this average.

Example: If we have electrodes F3, C3, P3 with voltages +60μV, +100μV, +80μV:

V_avg = (60 + 100 + 80) / 3 = 80μV

V_F3-avg = 60 - 80 = -20μV (upward deflection)

V_C3-avg = 100 - 80 = +20μV (downward deflection)

Phase Reversal Criterion

V_channel1 × V_channel2 < 0

A phase reversal occurs when two adjacent channels that share an electrode have opposite polarities (one positive, one negative).

Example: If F3-C3 = -40μV and C3-P3 = +40μV, then -40 × 40 = -1600 < 0, indicating a phase reversal at C3.

EEG Fundamentals: Resident & Fellow Teaching Guide

1. EEG Physics and Core Principles

Electroencephalography records the summated extracellular potentials generated by synchronous postsynaptic currents in cortical pyramidal neurons. These neurons are oriented perpendicular to the cortical surface, and their aligned dipoles create measurable voltage fields at the scalp.

⚡

Scalp Potentials: Small (tens of microvolts) because the skull acts as a strong resistive barrier and spatial low-pass filter

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Differential Amplification: Measures voltage differences between electrodes; common-mode signals are suppressed

2. Electrode Placement Systems

The International 10–20 system specifies electrode locations as percentages (10% and 20%) of the total nasion–inion and preauricular distances. Nineteen active electrodes (Fp1–O2) form the standard array, with A1 and A2 as reference electrodes.

Numbering Convention:

Odd numbers = Left hemisphere
Even numbers = Right hemisphere
'z' = Midline sites

System	Electrodes	Applications
10–20	19 scalp + 2 references	Standard clinical EEG
10–10	≈64	Improved spatial resolution
10–5	Up to 345	High-density research and source localization

3. Signal Acquisition and Filters

EEG signals are sampled at discrete intervals and digitized for analysis. The Nyquist theorem requires a sampling frequency greater than twice the highest desired signal frequency.

Sampling Rate

256–512 Hz

Preserve waveform fidelity

Resolution

≥16-bit

Capture microvolt changes

Low-frequency Filter

0.5–1 Hz

Reduce drift and slow artifact

High-frequency Filter

70 Hz

Reduce EMG and noise

Sensitivity

7 µV/mm

Standard display scaling

4. Referencing and Montages

EEG is inherently reference-dependent. Each channel represents the potential difference between an active electrode and a reference site. Different montages reveal complementary aspects of the same activity.

Montage	Description	Clinical Use
Referential	Each electrode vs. common reference	Good for field visualization
Bipolar	Adjacent electrode differences	Localizes phase reversals
Average Reference	Subtract mean of all electrodes	Useful with uniform coverage
Laplacian	Center minus weighted neighbors	Enhances focal features
REST	Model-based neutral reference	Reduces reference bias

5. Frequency Bands and Physiologic Rhythms

Delta

<4 Hz

Sleep, focal slowing

Theta

4–7 Hz

Drowsiness, limbic regions

Alpha

8–13 Hz

Posterior dominant rhythm

Beta

13–30 Hz

Frontal, active concentration

Gamma

>30 Hz

Fast oscillations; can be physiologic or artifact

6. Artifact Recognition and Mitigation

EOG

Eye blinks and movements

Use Fp1/Fp2 and EOG channels

EMG

Muscle tension, high-frequency noise

Ask patient to relax jaw

ECG

Cardiac pulsation

Use ECG channel, reposition reference

Sweat

Slow baseline drifts

Cool environment, stable contacts

Movement

Transient baseline shifts

Instruct stillness

7. Clinical Correlation and Localization

EEG interpretation integrates waveform morphology, spatial field, and temporal evolution. Phase reversals in bipolar montages indicate the point of maximal voltage and suggest the cortical generator beneath.

Focal Activity: Spikes or sharp waves reflect transient depolarizations within a localized neuronal population

Generalized Activity: Discharges involve widespread, synchronous networks

Localization: Awareness of reference influence and skull conductivity variations is critical for accurate localization

8. Technical Summary (ACNS Guidelines)

Electrode impedance: ≤5 kΩ preferred; ≤10 kΩ acceptable if balanced

Sampling frequency: ≥256 Hz (512 Hz preferred)

Resolution: ≥16-bit

Low-frequency filter: 0.5–1 Hz

High-frequency filter: 70 Hz

Display sensitivity: 7 µV/mm

Reference: Dedicated neutral electrode (not linked ears)

9. References

Jasper H. (1958). The Ten–Twenty Electrode System of the International Federation. Electroencephalography and Clinical Neurophysiology.
Klem, Lüders, Jasper & Elger (1999). The Ten–Twenty System of the International Federation. Electroencephalography and Clinical Neurophysiology Supplement 52.
American Clinical Neurophysiology Society (ACNS) Guidelines 1 & 4 (2021).
Rowan, A.J., & Tolunsky, E. (2003). Primer of EEG: With a Mini-Atlas.

EEG Waveform Simulator

EEG Lead Placement Tutorial

Welcome to the EEG Lead Placement Practice!

Fp1

Fp2

The International 10-20 System places electrodes at specific locations on the scalp.

This tool will help you learn and practice the International 10-20 System for EEG electrode placement.

How to Place Leads

Drag and drop leads from the list onto the head model. Position them where you think they should go according to the 10-20 system.

Fp1

Each lead has a specific location based on cranial landmarks.

Checking Your Placement

When you've placed all the leads, click the "Check Placement" button to see how accurate you are.

Fp1

Correct placements will turn green, incorrect ones will turn red.

Different Practice Modes

Try different practice modes to challenge yourself:

Standard: Place all leads at your own pace
Timed: Place leads within a time limit
Random: Place specific requested leads
Guided: Follow step-by-step instructions

Visualizing Montages

Use the montage selector to see how different EEG montages connect the electrodes. This helps you understand how the leads relate to each other in clinical practice.

Fp1

Montages show how leads are connected for EEG interpretation.