A charge analysis of non-invasive electrical brain stimulation

The brain is an electric organ. As such, for well over a hundred years, physicists, engineers, biologists, and physicians have used electromagnetic theory to try to understand how the brain works and to diagnose and treat disease. The field of electro-neuroscience is immense with thousands of papers being published each year. In this paper, we provide physics students with an introduction to the field using a conventional model of the head that employs four concentric spheres having different conductivities to represent the scalp, skull, cerebrospinal fluid (CSF), and the brain itself. To ground the discussion, we use the specific case of non-invasive DC electrical stimulation of the brain through electrodes applied to the surface of the scalp, a procedure known as transcranial DC stimulation or tDCS. Currently, tDCS is under clinical investigation for such diseases as depression, anxiety, and chronic pain as well as to enhance the performance of athletes and the training of fighter pilots. We solve the corresponding physics problem from a charge perspective and explain why the charge distributions look the way they do using what we call the "sensing pixel" technique. This paper can introduce students to the ways in which electromagnetic theory is being applied to problems in neuroscience; in this case, the problem of how to noninvasively stimulate the brain to treat disease or improve performance. (c) 2023 Published under an exclusive license by American Association of Physics Teachers.