E-Field Lab

Electrodes are selected on a 3D head model, assigned as anodes or cathodes, and a total current is set; the tool then shows the resulting electric field on the cortical surface. The computation is immediate: no job queue, no waiting.

It is meant for the moment before the experiment, when electrode placement is being decided and two montages need to be compared without preparing a full simulation for each one.

Open E-Field Lab

The foundation is a leadfield: the electric field solution for each electrode separately, computed once with finite elements and stored. Because the problem is quasi-static and ohmic, the field of any combination of electrodes is the weighted sum of those individual solutions. That sum is not an approximation: it is exact for the model that was solved.

• — Solved with SimNIBS 4.6 on the Ernie head model from its example dataset
• — EEG 10-10 cap (Jurak 2007): 76 electrodes, with Cz as the reference
• — Round electrodes 20 mm in diameter with 4 mm of gel, meshed onto the scalp
• — Field interpolated on the middle gray matter surface
• — Normalised to V/m per mA, so the current you choose scales linearly
• — Surface reduced from 308,791 to 20,003 vertices so it fits in a browser

Magnitude is the field strength, regardless of its direction.

The normal component is the projection of the field onto the perpendicular to the cortical surface. It is positive when the field points into the cortex and negative when it points out of it. It is the component most closely related to neuronal polarisation, and therefore the one usually reported.

The three metrics reported with each montage are deliberately conservative:

• — Peak: 99th percentile of the magnitude, not the absolute maximum. The maximum is dominated by a handful of vertices and is unstable across meshes
• — Median: the typical field across the whole cortex
• — Focality: share of cortical surface above half the peak. Smaller means a more focal montage

Figures export as PNG with the caption already written: montage, current, model, reference electrode and electrode geometry. The on-screen value overlay can also be switched off.

• — A single head model. Values are indicative, not subject-specific. Another skull, another bone thickness or another anatomy gives different figures
• — Electrode geometry is fixed at compute time. A montage with 5×5 cm sponges spreads the same current over eight times the area and is not reproduced by this model
• — The surface is decimated from about 309,000 to 20,000 vertices. Fine sulcal detail is smoothed
• — Cortical surface only. There are no values for deep structures
• — 16-bit float storage, which introduces small but non-zero rounding

It is not a medical device and does not replace a subject-specific simulation. For a real protocol, the right thing is still to run SimNIBS on that person's MRI.

To cite the tool in publishable work, this temporary format may be used while it has no DOI of its own:

Jones, S. R. (2026). E-Field Lab: browser-based simulation of the electric field in transcranial electric stimulation. Omega Dynamics. https://omegadym.com/en/efield

SimNIBS, the software that solves the physics behind the tool, should also be cited: Thielscher, A., Antunes, A. and Saturnino, G. B. (2015). Field modeling for transcranial magnetic stimulation: a useful tool to understand the physiological effects of the brain? IEEE EMBC.

The code and the computation scripts will be released as a separate repository. To reproduce the leadfield before then, or if a model with a different electrode geometry is required, write to contact@omegadym.com.