How Do Flashing Lights Trigger Epileptic Seizures?

Photo by Fabio Ballasina on Unsplash

In 1997, a Pokémon episode that aired in Japan sent roughly 700 people, mostly children, to the emergency room suffering nausea, headaches, vomiting, and epileptic seizures. The culprit turned out to be light — specifically red and blue flashing lights during a battle scene in the episode. Although many didn’t know it at the time, these children had photosensitive epilepsy — a condition in which flashing lights can trigger seizures.

In response, Japan introduced a set of guidelines for animations that included limiting how long flashing lights lasted and the amount of time colors could flash on screen. To learn more about how flashing lights affect photosensitive epilepsy, BrainFacts.org turned to several experts. Dora Hermes, an epilepsy researcher at the Mayo Clinic, David Burkholder, a neurologist at the Mayo Clinic, and Robert Fisher, director of the Stanford Epilepsy Center explain photosensitive epilepsy and what’s happening in the brain during these types of seizures.

What is epilepsy?

Robert Fisher: Epilepsy is a disease characterized by recurrent seizures.

Dora Hermes: During a seizure, groups of neurons fire excessively, generating uncontrolled electrical signals that spread through the brain.

David Burkholder: The symptoms someone experiences during a seizure depend on the area or areas of the brain involved in the seizure. If motor areas are involved, then a limb may move or jerk. If visual areas are involved, then a person may see shapes or colors in a part of their field of vision. The typical generalized tonic-clonic seizure occurs when a seizure spreads to involve the majority of the brain.

What is photosensitive epilepsy? How do flashing lights trigger seizures in vulnerable people?

RF: Anything that abnormally increases the synchrony of brain cells might provoke seizures in susceptible individuals. Certain patterns of light — flashing bright lights at particular frequencies — synchronize cells within the visual cortex. If the neurons then fire through their networks at too high a level, they can recruit other neurons into a hyper-synchronous discharge. That’s what happens in the brain during a seizure.

DH: The brain shows a strong response to flashes around 20 per second which are also the most likely to trigger seizures. When light hits the eye, signals are sent through the thalamus, a central brain structure that relays brain signals, to the cortical brain areas tasked with processing visual stimuli. These brain areas provide strong inputs to the rest of the brain. In photosensitive epilepsy, the brain responds excessively to certain visual inputs, sometimes so strongly that a seizure is triggered.

What makes people susceptible to photosensitive epilepsy and how common is it?

DH: Photosensitive epilepsy has a prevalence of about one in 10,000 individuals overall. But it’s more common in younger people, affecting about one in 4,000 between the ages of five and 24.

DB: The factors involved in photosensitivity, including age-dependent responses, are complex and not well understood. Genetic studies show photosensitivity can be inherited.

RF: Several genes have been identified as risk factors for photosensitivity, but we haven’t found one single gene to explain the condition. Variants in the genes CHD2 and GABRA1 may be risk factors for photosensitivity. However, having one of these gene mutations does not guarantee photosensitivity (in fact, these variants are quite rare) and not having one does not mean the person will be free from photosensitivity.

What kinds of stimuli are most likely to induce seizures?

RF: Brightness is provocative, particularly the contrast between the flash and the no-flash period. Brightness is important because, for example, modern TV screens or computer screens can get that bright. The image also must occupy enough of the retina. Then there’s the length of the stimuli. For the most part it requires at least a few seconds of flashing to cause a seizure. For most people, the most troublesome frequency range is 10 to 20 flashes per second.

DH: In addition to flashing lights, certain regular patterns can trigger seizures — like high-contrast black and white striped patterns. The first cortical brain area to process visual input is structured in columns that respond to edges or stripes of different orientation. We call these columns orientation columns. Orientation columns responding to the same orientation may inhibit each other. Typically, there’s a lot of inhibition from orientation columns when you see a striped pattern. One hypothesis about pattern-sensitive epilepsy suggests this inhibition is less effective. Without this inhibition, a strong stimulus driving one set of orientation columns may provoke strong, uncontrolled, neuronal activity, e.g. runaway excitation.

How is photosensitive epilepsy treated?

RF: Our treatments for epilepsy, including the light-sensitive variety, are symptomatic — they can suppress seizures, but not cure the epilepsy. Occasionally, surgery to fully remove a seizure onset zone in the brain can cure epilepsy, but epilepsy clinicians think more in terms of treatment than of cure.

If you know that you’re photosensitive, you can avoid the stimuli. Stay away from the discotheque or strobe lights. If you’re playing video games, sit back farther from the screen and play in a well-lit room. The contrast is higher, and riskier, if you play a brightly-lit game in an otherwise dark room.

How do creators of video games, TV shows, and movies know if their content is safe for people who are photosensitive?

RF: There’s technology that will analyze whether a clip has high-contrast flashes in a certain frequency range and tell you whether the flashing is a risk or not. These tools, however, don’t work entirely well for a video game. Unlike playing a fixed clip, a video game can be played so many different ways.

About the Author

Hannah Thomasy

Hannah Thomasy is a freelance science journalist. She received her PhD in neuroscience from the University of Washington, where she studied how concussions affect sleep behavior.