Electrical properties of the human skull

What caused the electric radiation around the head of this person, included in the mosaic of a floor in the Synagogue Hamat in Tiberias, Israel? 

So far, scientists thought the brain was 80 times more conductive than the skull because the brain holds billions of electrically conductive brain cells. With experiments in 2007, biomedical engineer Bin He showed that the brain is only 18.7 times more conductive than the skull.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1867457/