The "Cocombola Study": A Physical Phantom Model for tDCS-Induced Electric Field Distribution

Transcranial direct current stimulation (tDCS)-induced electric fields (EFs) acting on brain tissues are hardly controllable. Among physical models used in neuroscience research, watermelons are known as head-like phantoms for their dielectric properties. In this study, we aimed to define an inexpensive and reliable method to qualitatively define the spatial distribution of tDCS-induced EFs based on the use of watermelons.

After creating the eight cranial foramina and identifying the location of the 21 EEG scalp electrodes on the peel of a watermelon, voltage differences during stimulation were recorded in each of the 21 scalp electrode positions, one at a time, at four different depths. The recordings were graphically represented by using polar coordinates with the watermelon approximated to a perfect sphere.

To validate the model, we performed three experiments in well-known montages. The results obtained were in line with the expected behavior of the EFs.

Watermelon might be a cheap and feasible phantom head model to characterize the EFs induced by tDCS and, potentially, even other non-invasive brain stimulation techniques.