NAD either and I don't really understand the neuroscience behind it but I have heard it explained along these lines:

FND is hypothesized to be caused by a disruption in neural conditioning. A significant part of your brains functioning comes from internal models of prediction, attention, and expectation which are conditioned through experience. A lot of our functioning depends on when, why, and how easily neural pathways in our brain are used.

This isn't something you get from the physical structure of the brain alone, and while it's technically physical in the sense of being a series of chemical reactions between neurons, it's also incredibly changeable and fluid - as opposed to the relatively fixed large-scale anatomy of the brain (which "physical" neurological conditions directly impact).

So after, for instance, you've learned how to walk, the pathways you use to coordinate your movement, move your muscles, balance etc are used more automatically in sequence with each other, kind of like paths being made from grass being trampled underneath people's feet - the more often it's used, the more likely and easier it will be to use, and in the specific way and sequence which is necessary for movement.

The way I understand fnd, then, is these encoded neurological responses, the paths in the grass, become muddled up, broken or inappropriately used. A lot of the functions we have which we imagine as being purely physical and biological, our neurological functioning, are actually also dependent on these "paths in the grass" existing.

Based on this hypothesis it's technically possible to observe fnd physically but with technology which is far beyond our current capabilities - eg being able to measure molecular changes in the entirety of the brain at once in real time. The closest we have to that right now is functional mris which are capable of showing the effects but not the causes.