Oliver Sacks at the NYT blog Migraines:
In my own migraine auras, I would sometimes see — vividly with closed eyes, more faintly and transparently if I kept my eyes open — tiny branching lines, like twigs, or geometrical structures covering the entire visual field: lattices, checkerboards, cobwebs, and honeycombs. Sometimes there were more elaborate patterns, like Turkish carpets or complex mosaics; sometimes I saw scrolls and spirals, swirls and eddies; sometimes three-dimensional shapes like tiny pine cones or sea urchins.
Such patterns, I found, were not peculiar to me, and years later, when I worked in a migraine clinic, I discovered that many of my patients habitually saw such patterns. And when I looked back on historical accounts, I found that Sir John Herschel, the astronomer, had given detailed descriptions of his own visual migraines in the 1850s. He wrote to his fellow astronomer and fellow migraineur, George Airy, quoting his own notes: “The fortification pattern twice in my eyes today …. Also a sort of chequer work filling in, in rectangular patches, and a carpet-work pattern over the rest of the visual area.” Herschel wondered whether there might be “a kaleidoscopic power in the sensorium to form regular patterns by the symmetrical combination of casual elements,” a power “working within our own organization [but] distinct from that of our own personality.”