Nige I started to read this and I can’t help wondering could it help stroke survivors in some way. Non invasive. https://theconversation.com/could-electric-brain-stimulation-lead-to-better-maths-skills-260134
Andrew Better maths skills mean that one should be able to think more clearly and hence cognition is aided. Both can be affected by a stroke. So the first thought is, yes, go for it.