A terrifying form of targeted internet bullying involves sending flashing images via Twitter to people with epilepsy. No, this isn’t an episode of Black Mirror: As the BBC reports, the vocal Donald Trump critic Kurt Eichenwald received a malicious tweet that succeeded in triggering the seizure it threatened. While it’s not the first time that online graphics have been implicated in an epileptic seizure, it’s a reminder that anyone susceptible to the shock of bright lights should be extra careful on the internet.

Eichenwald was the recipient of a direct tweet with the caption “you deserve a seizure for your posts.” The tweet, which has since been taken down, included a flashing image that Eichenwald’s wife confirmed had triggered a seizure in her husband. In about 3 percent of people with epilepsy, seizures are triggered by flashing lights at certain intensities or in specific patterns because they are especially sensitive to light. Some people may not always be aware that they have photosensitive epilepsy — sometimes they have no idea until they actually have the seizure — while others respond to flashing lights with different symptoms, such as headaches and nausea, and never actually have a seizure. It’s for this reason that sending seizure-inducing tweets is especially backhanded: The people who open them might not even be aware of how dangerous they could be.

A screenshot of the seizure-inducing tweet sent to Kurt Eichenwald.
A screenshot of the seizure-inducing tweet sent to Kurt Eichenwald.

The most important characteristic of a seizure-triggering graphic is the speed at which the light flashes on and off. While this frequency may differ for everyone, it’s thought that seizures tend to be triggered by lights that flash somewhere between 5 and 30 times per second (Hertz). Other factors that play a role are the brightness of the overall image, the image contrast, the distance between the viewer and the screen, and the wavelength of the light reaching the viewer. It’s not guaranteed that any one of these factors, however optimized, will trigger a seizure. What it comes down to is a combination of these elements — and really unfortunate timing on the viewer’s part.

Even without the trolls, the world can be a dangerous place for people with epilepsy and other illnesses provoked by flashing lights, like migraine headaches. While the Epilepsy Foundation’s professional advisory board helps make situations safer — for example, by recommending that the flashing lights accompanying a fire alarm don’t have a frequency of more than 2 Hertz (that is, they don’t flash more than twice per second) — it’s impossible to safeguard every visual part of society, especially on the internet.

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Something as innocuous and ubiquitous as Rihanna’s video for “This Is What You Came For,” for example, can pose a danger, just as seemingly benign Pepe memes might do as well. Fortunately, there exists some sympathetic, concerned people — like this guy on Reddit warned League of Legends players about flashing lights in the game — who give people a heads up before it’s too late.

People who are especially photosensitive should remain vigilant of internet trolls and be careful about opening files from unknown sources, Aston University’s Stefano Seri, a neurophysiologist and neuropsychiatrist, told the BBC. Because it takes a confluence of multiple characteristics for an image to actually trigger a seizure, people can lower their chances of having one by taking charge of the factors they can control: They should maintain a safe distance from the screen, lower the brightness of the display, and increase the amount of light in their surroundings so that their sense of visual contrast isn’t as stark.