Spreading depression is a wave phenomenon of the central nervous system. It is still debated what its basic mechanistic steps are — but it clearly belongs to the self-organisation phenomena that occur in neural tissue. This is where my physics of migraine began.
If you want to understand why I ever thought physics had anything to say about migraine, this is the place to start.
Spreading depression (SD) is a slow wave that travels through grey matter at a few millimetres per minute. As it passes, it briefly and massively perturbs the brain’s ionic homeostasis — ions that are normally kept carefully separated across cell membranes rush across, activity collapses, and then, minutes later, the tissue recovers. It is not electrical activity in the ordinary sense; it is closer to a chemical wave in an excitable medium. What made it fascinating to me was exactly that: SD does not need a wiring diagram. It is a self-organisation phenomenon, the kind of pattern that physics and chemistry describe in reaction–diffusion systems.
Spirals in the retina
The clearest way to see this is not in the folded cortex but in a flat, transparent sheet of neural tissue: the retina. Working with Stefan C. Müller, I studied spreading depression in the isolated chicken retina, where the wave can be watched optically as it moves.
Under the right conditions these waves do what waves in every other excitable medium do — from the Belousov–Zhabotinsky chemical reaction to cardiac tissue — they curl up into spirals. And they do something stranger still: a single spiral-shaped wave can split itself into two, without any external trigger. This self-induced splitting is a signature of the underlying nonlinear dynamics, not of any particular biological detail. It told us that the same mathematics that describes pattern formation in chemistry was operating in the living nervous system.
That was the moment the project stopped being “biology I happened to measure” and became a physics of the brain: the wave’s shape, speed, and instabilities were governed by the general laws of excitable media.
From retina to migraine aura
Why does this matter for headache? Because the visual disturbance that many people experience before a migraine — the shimmering, expanding arc of a migraine aura — is thought to be the perceptual shadow of spreading depression crossing the visual cortex. If SD is an excitable-medium wave, then the aura is a wave too, and its peculiar geometry becomes something we can predict rather than merely describe. The retinal spirals were the first evidence that this prediction had a solid physical basis.
Everything I have done since — mapping auras onto the cortical surface, treating migraine as a dynamical disease, asking how to steer these waves with stimulation — grew out of these early experiments.
References
- Dahlem MA, Müller SC. Self-induced splitting of spiral-shaped spreading depression waves in chicken retina. Experimental Brain Research 115(2): 319–324 (1997).
- Dahlem MA, Müller SC. Migraine aura dynamics after reverse retinotopic mapping of weak excitation waves in the primary visual cortex. Biological Cybernetics 88(6): 419–424 (2003).
A full publication list is on Google Scholar ↗. The same story, told in German, runs through Statistische Physik und das Gehirn.