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Why Do Some Still Images Trigger Seizures?

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By now, most people are aware of strobe lights’ ability to induce photosensitive epileptic seizures. A troll allegedly gave a journalist a seizure with a tweet. An episode of Pokémon sent almost 700 Japanese children to the hospital. But still images can cause seizures, too, and scientists are just now starting to figure out how that happens.

There’s still a ton we don’t know about seizure triggers, since there isn’t a reliable way to test them in humans or animals. A team of researchers from the University Medical Center Utrecht wanted to know what was going on in the brain that might help them better understand why some still images trigger seizures and others don’t.


“One particular type of brain wave...called a gamma oscillation, is particularly strongly driven by certain kinds of visual patterns,” Dora Hermes, lead author of a correspondence published today in Current Biology, told Gizmodo. A black and white bar pattern can induce these gamma oscillations in the brain’s visual cortex, the image processing part of the brain—and in an epileptic brain, the researchers hypothesize that the oscillations are linked with seizures.

Gamma oscillations are a much-discussed brain wave pattern, characterized by neurons firing rhythmically around 50 times per second. The researchers sifted through lots of old research, and found associations between images that caused seizures and those that produced narrow-band gamma oscillations in healthy individuals. Basically, if the image caused the visual cortex’s neurons to clap in unison at the gamma oscillation frequency, it was more likely to cause seizures in epileptic individuals. Those images that caused the brain’s neurons to clap out of sync were less likely to cause seizures.


“We hypothesize that the circuitry that produces these oscillations might also provoke a seizure in patients with photosensitive epilepsy,” said Hermes.

One neurologist, Khalid Hamandi from Cardiff University, pointed out to me that Hermes and her co-authors’ correspondence isn’t presenting new data, although it has nicely summarized information already out there. The link between gamma oscillations and epileptic seizures was observed in a 2003 study (but not with still images) for example.

But another professor, György Buzsáki from New York University, told Gizmodo in an email that he found the paper’s timing important. “Just a year ago, I would not even blink about this paper. However, this year is different.” You may have listened to a recent Radiolab episode about a new study demonstrating that flickering light at these gamma frequencies at mice suffering from Alzheimer’s reduces the level of a kind of plaque found in their diseased brains. This suggests that a future Alzheimer’s treatment might involve inducing these firing patterns in patients, potentially causing seizures.

“I expect that a large number of companies will try to sell their gadgets in the coming years, and many people will attempt flickering light in the hope of decelerating the progression of degenerative diseases,” Buzsáki wrote. “Without understanding first the mechanisms, this may be a dangerous venture. This report points to at least one danger, especially given that a large fraction of Alzheimer’s Disease patients are prone to epileptic activity.”


This paper only mentions black and white bars, but now the researchers want to test whether other still images might induce these gamma oscillations, too—the researchers don’t know, yet. For now, there’s a lot of experimenting ahead of them. “I’d like to emphasize that this is a review of the literature,” said Hermes. “We’re working on testing this by measuring it in various patient populations. This paper is really a hypothesis.”

For a diagram depicting the potentially seizure-inducing pattern and mechanism, click here.


[Current Biology]