The observation that one neuron can alter the activity of a nearby one is old as dirt. Emil du Bois-Reymond observed it in the late 19th century, but I don't know of anyone trying to quantify it until Katz and Schmitt (1940) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1393925/ and Angelique Arvanitaki (1941) https://journals.physiology.org/doi/abs/10.1152/jn.1942.5.2...., who named it. There are some other reports in squid (Ramon & Moore, 1978) https://pubmed.ncbi.nlm.nih.gov/206154/, rat cerebellum (Korn and Axelrad, 1980) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC350252/, and others. This review by Anastassiou et al. (2011) might be a good place to start https://www.nature.com/articles/nn.2727.pdf?origin=ppub or this Scientific American article about a paper by my grad school neighbors (https://www.scientificamerican.com/article/brain-electric-fi...)

In parallel, people have asked whether external electric fields can be used to alter neurons' activity, which is even older: a Roman physician in 46 reportedly cured headaches by applying a live electric fish to patients' heads. The idea of using electricity to improve mental function has waxed and waned ever since, with the most recent peak around ~2015 or so. Terzuolo and Bullock collected some of the first data on this using crayfish axons in 1956 (https://www.pnas.org/content/42/9/687) and subsequent experiments by Deans et al. (2007), Radman et al. (2007-9), Ozen et al. (2010), and Frolich and McCormick (2010) found similar results using in vitro and small animal experiments. In parallel, people went absolutely wild with human studies of transcranial electrical stimulation (TES), a family of techniques including tDCS (w/ direct current) and tACS (alternating current). While some of the results have been exciting, they have not always been reproducible (Horvath et al, 2015ab) and some work suggested that the previous work relied on fields much stronger than those achievable in humans (Voroslakos et al, 2018).

Together with some awesome collaborators, I set up a non-human primate model that let us test tES under conditions that closely match those found in humans: like macaques (and unlike rodents), we have big, convoluted brains in thick bony skulls and comparatively sparse neural networks. We found that tDCS could affect neural circuits (i.e., LFP oscillations) and behavior (Krause et al., 2017) https://www.cell.com/current-biology/pdfExtended/S0960-9822(... and single neurons, even in deep brain areas (Krause, Vieira, et al. 2019) https://www.pnas.org/content/116/12/5747.abstract [0] The fields we used were much weaker than those produced by some parts of the brain itself (~0.3 - 1 V/m vs ~4-8+ V/m), so it suggests that ephaptic mechanisms are probably pretty common.

I'm pretty confident in those results, but--to bring things back to the original topic--our recent experiments suggest that getting tES to do exactly what you want, when and where you want it, will take some cleverness and a lot of simplifying assumptions tend not to hold up.

[0] The missing full references above are in these two articles' bibliographies.