At some point in my mid-twenties, I refused a doctor’s offer of free and regular Botox. My problem wasn’t age, or fear of ageing, but one long unending migraine. Botox, the doctor explained, might offer some relief by relaxing the muscles around the head and neck, and by blocking some of the nerve signals volleying between those muscles and the brain. I sensed, if you can believe it, that I was being fobbed off. This was not, as I saw it, the kind of headache you get from working or frowning or clenching too much. The migraine seemed like something so deep in my brain, stemming from something so deep in my mind, perhaps even my soul, that I instilled in myself a faith that the cure (there had to be one) would have to emerge from something similarly deep. Chronic daily migraines, as I had gleaned from elsewhere, come from a joy-wreckingly heightened sensitivity in the central nervous system – not merely from tense muscles sending signals along the peripheral nerves.…