Electrogastrography (EGG)

The gastric counterpart to the ECG, and the reason the gastric-network is knowable at all. Four electrode pairs on the abdomen pick up the smooth-muscle depolarizations driven by the interstitial cells of Cajal — the stomach’s pacemaker — at 2–4 cycles per minute.

The clinical use is decades old (gastroparesis, functional dyspepsia; Parkman et al. 2003). The interoception use is recent and different in kind: EGG is recorded concurrently with fMRI, not to diagnose the stomach, but to use the stomach’s rhythm as a reference signal against which brain activity can be phase-aligned.

Where it sits among this wiki’s instruments

Almost every method page here measures a judgment — how many beats did you count, which breath was harder, where do you feel anger. EGG measures none. It is one of a small set of pages (event-related-potentials partly, physiological recording generally) where the participant supplies a body and no opinion.

That has a consequence the field has not fully absorbed. The reliability of interoceptive tasks is contested throughout this wiki — see is-the-heartbeat-counting-task-valid. The reliability of EGG is not: r = .74 across months and settings. What fails to retest is not the visceral measurement but the coupling quantity computed from it and the BOLD signal (PLV-delta, r ≈ 0). Whatever is unstable in brain–body coupling research, it is not the stomach.

The pipeline, and where choices bite

1. Channel selection. Four channels are recorded; one is chosen — the largest spectral peak inside 0.033–0.066 Hz, unless a quality assessment favours the second-largest. A researcher degree of freedom, usually reported briefly.
2. Quality grading. Wolpert et al. (2020) supply three criteria (clear peak in range; peak consistent across channels; peak consistent across runs) and three grades; grade 3 is discarded. This is the source of the ~24% attrition.
3. Band-pass and Hilbert. Filter ±0.015 Hz around the individual peak, then take the analytic signal’s angle to get instantaneous phase. Everything downstream is phase; amplitude is discarded.
4. Downsampling to the BOLD rate before computing PLV against each voxel.

Step 3 is why this method pairs with PLV specifically: what EGG delivers cleanly is when in the gastric cycle you are, not how strong the contraction is.

What it would take to make it a measure of a person

Presently EGG supports group-level claims well and individual-difference claims badly, and the failure is downstream of the recording. Improvements the literature points at: imaging-guided electrode placement, high-density arrays (Gharibans et al. 2019), automated artefact rejection (Gharibans et al. 2018), and — the one that would matter most for this wiki — a perceptual gastric task to sit beside the physiological one, so that the gastric channel could enter is-interoception-domain-general as a third axis with matched psychophysics rather than as an absence.