Missoni for migraines
July 3, 2013 7:02 PM   Subscribe

Why are zigzags the common factor in so many migraine auras?

I know not all people experience aura as a zigzag visual disturbance, but from what I understand scintillating scotoma frequently includes zigzags, whether color or black and white.

What I am wondering is why so many people would default to seeing zigzag shapes as opposed to rounder waves, or having their brain fill in some space with some other visual disturbance (like an image of a cat or something).
posted by donut_princess to Science & Nature (10 answers total) 9 users marked this as a favorite
Your eyes move in tandem.
posted by Cool Papa Bell at 7:23 PM on July 3, 2013

A scotoma I once had was conveniently centered in the fovial area where I could observe it more clearly than usual.

It seemed that there was an irregular region in the field of vision that was somewhat moon-shaped, like an island that contained a cove. This region was not functioning properly in vision, so it was maybe something like grayed-out.

The zigzags seemed as if they were being generated as a way to map that island shape, shifting around until it was "stitched up." This notion was supported by the observation that the zigzags alternated with larger radially sweeping fronts with the same rainbow coloration as the zigzags. Normally I would not have been able to separate the two different modes.

I was able to watch this process closely for quite some time and it went like this: first the island would be radially scanned by several independent sweeping wipers that traversed the whole shape, then it would switch to the zigzags that stitched up the space contained in the island and the zigzag path adjusted as a springy connected form until it best fit the irregular island without going outside it.

So there appeared to be a coordinated gross and fine analysis occurring in alternation in an attempt to best tile the irregular shape with the triangles formed by the zigzag fitted inside it.

This was so clear to me only that one time, but other more off-centered scotomas that could not be so closely observed have subsequently been seen to plausibly conform perhaps to this alternating scanning process.

So 1) the radar-like sweeping lines map the overall shape, then 2) the zigzags use those findings to fill the shape in, in more detail.

What struck me about this is that it was similar to a two-part scanning algorithm I had once developed to classify shape geometries. I suspected that this development experience of mine was somehow informing my perception of what I was seeing. However I was not aware that zigzags are a common thing in this regard, if they are then that reinforces by belief that I was watching an analyzer using a system of coordinated processes to fill in or map an irregular "hole" that had appeared in my vision.
posted by StickyCarpet at 7:40 PM on July 3, 2013 [2 favorites]

Your eyes move in tandem.
posted by Cool Papa Bell

The process I've just described only happens in one eye at a time, although it can be hard to appreciate it as such when it is all lit up in the perceived visual field. The effect moves with eye movement and that's why it is so hard to observe if it's not directly in the center of vision, as it scoots away when you look at it.
posted by StickyCarpet at 7:56 PM on July 3, 2013

I've had recurring ocular migraines. (All the aura, but blessedly none of the headache.)

In my experience, calling it a zigzag isn't... entirely accurate. The images I've seen represent what a moment in the aura is like, but the whole thing isn't like one constant zigzag. I mean not like a test pattern on a TV or anything. It's more like a series of triangles made of stained glass, all spinning in place.
posted by Andrhia at 8:08 PM on July 3, 2013 [3 favorites]

I'm not entirely convinced that it's in the eye (or even the retina). My hypothesis would be that the aberrant neural activity associated with migraines, similar to other epileptic-like network disorders, may just be mucking the visual cortex around in addition to everything else.

The brain has a bajilion neurons constantly firing at and responding to each other. Network disorders could be overwhelming or de-synchs the pattern that your visual cortex runs at. The streaking might be a version of "buffer" and when the signals synch again the visual cortex fills in the gaps making it seem like zig zags. The brain can work "in reverse time," or rather you think that it was in the past like in chronostasis.
posted by porpoise at 8:09 PM on July 3, 2013

I'm not entirely convinced that it's in the eye (or even the retina).

When they move together with the movement of the eye and occur in only one eye at a time, then it certainly looks/feels as if it's glued to the retina. I expect that the retina has a lot of parallel processing feature detecting and isolating widgets built into it, as it would be more efficient to do some of that there and pass on the results. But as you say, porpoise, who knows where and how and even if any of this "really" happens.
posted by StickyCarpet at 8:35 PM on July 3, 2013

I think it's generally accepted that these zigzag patterns (most often called fortification illusions) are produced when the pattern of vasoconstriction in one area of the brain and vasodilation in another which is one of the essential characteristics of migraines causes swelling which makes the visual cortex (in the occipital region of the brain) impinge on the skull (not to mention the membranes and all that, of course) which in turn causes the neurons in the visual cortex to fire as if they'd received input from the retina, giving rise to a perception of light.

Then from that small area of impingement, the activation spreads out in a slowly moving wave. The best pictures and description of the evolution of that wave I was able to find bear quite a striking resemblance to StickyCarpet's observations, in my opinion:
Cortical spreading depression (SD) has been suggested to underlie migraine aura. Despite a precise match in speed, the spatio-temporal patterns of SD observed in animal cortex and aura symptoms mapped to the cortical surface ordinarily differ in aspects of size and shape. We show that this mismatch is reconciled by utilizing that both pattern types bifurcate from an instability point of generic reaction-diffusion models. To classify these spatio-temporal pattern we suggest a susceptibility scale having the value sigma = 1 at the instability point. We predict that human cortex is only weakly susceptible to SD, and support this prediction by directly matching visual aura symptoms with anatomical landmarks using fMRI retinotopic mapping. Moreover, we use retinal SD to give a proof of concept of the existence of this instability point and describe how cortical susceptibility to SD must be adjusted for migraine drug testing. Close to the instability point at sigma = 1 the dynamical repertoire of cortical tissue is increased. As a consequence, the picture of an engulfing SD that became paradigmatic for migraine with aura needs to be modified in most cases towards a more spatially confined pattern that remains within the originating major gyrus or sulcus.
posted by jamjam at 12:05 AM on July 4, 2013 [1 favorite]

Out visual system is retinotopically mapped, meaning that neurons in the V1 and other visual areas in the brain actually form a map of the retina; so two receptive fields close together in the V2 will correspond to areas that are close together in the visual field. This explains why something happening in the brain would still "track" with your eyes.

This article offers an excellent theory, with evidence, as to what is "causing" these hallucinations.
posted by ancient star at 12:08 AM on July 4, 2013 [3 favorites]

Andrhia: As weird as this may be for you to read, that sounds like a gorgeous aura.

Ancient Star: The section on comparing epileptic hallucinations to visual auras was super informative! Thanks for the article link!
posted by donut_princess at 5:39 AM on July 4, 2013

Oh yeah, it's definitely pretty, in a weird slightly-hallucinogenic kind of way. Wouldn't mind it at all, except for the part where my vision gets so smeary I can't drive or read. But there are definitely worse problems one can have.
posted by Andrhia at 10:17 AM on July 4, 2013

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