Are scintillating scotomas really caused by cortical spreading depression? If so, how is this known to be true?
The primary visual cortex is topographic, which means that specific parts of that brain region correspond exactly to specific parts of your visual field. In order to prove that a scotoma (literally a blind spot in your vision) is due to dysfunctional neurons, all you need to do is show that the neurons responsible for that spot in your vision are not working but the rest of them are. We can do this with a variety of tools, the most common one these days being fMRI. In addition to showing that the area of neuronal dysfunction maps onto the area subserving vision in that area where the scotoma is located, a spreading depression does exactly that: it spreads, at a predictable rate, from the location of its onset to adjacent brain tissue. fMRI will show you not only the area of brain dysfunction, but will allow you to "follow" the neuronal dysfunction as it moves over time, and show that it corresponds to the location of the scotoma in your visual field, which will also move in correspondence. HTH. See: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC31895/ for first reference to this type of research (more in pubmed with keywords fMRI and migraine aura).