By the time the 38th episode of the animated children’s series Pokémon, or Pocket Monsters, aired in Japan, it was a bona fide sensation, drawing roughly 4 million viewers weekly. One survey estimated that 55 percent of schoolchildren in Tokyo's Kawasaki school district followed the series. The show—which began airing April 1, 1997, and focused on the adventures of Ash and affable monsters like Pikachu in their attempt to collect one creature from each species to train for combat—was also a comic, a Nintendo video game, a trading card series, and more. The devoted fandom would soon spread to the United States.
But then something peculiar happened—so peculiar that it become the subject of medical journal research. The Pokémon episode that aired at 6:30 p.m. on Tuesday, December 16, 1997 depicted a cataclysmic explosion between thunderbolts thrown by Pikachu and a “vaccine bomb.” Red and blue flashing lights began to pulse onscreen. Though the sequence lasted only a few seconds, hundreds of children were stricken by an immediate and visceral response that ranged from headaches and dizziness to full-blown seizures. Japanese hospitals found themselves treating viewers for epileptic symptoms.
This wave of deleterious effects became international news. Never before had a television program had such a direct and immediate health consequence on its audience. Some people initially dismissed the whole thing as a hoax or possibly some kind of mass hysteria, but the physical reactions were genuine. What had made this episode of Pokémon so dangerous—even among those viewers not diagnosed with epilepsy? And could it happen again?
The potential for a television program to trigger seizures is rooted in how it displays light. Light displayed at frequencies between 10 and 30 hertz, or the number of cycles per second, is known to induce symptoms for a percentage of the population susceptible to them. The color red is also stimulating. When light is shifting from color to black and back again, nerve cells in the brain can fire electrical impulses rapidly, leading to convulsions. This is often referred to as photosensitive epilepsy, where certain visual stimuli can cause a seizure.
As a result, there have been a handful of programs that have prompted medical concern for viewers. In 1993, the UK had three reported seizures as a result of a commercial for pot noodles that used flickering light, prompting the advertiser to pull it from the air. A 2012 animation for the Olympics also triggered adverse effects for a reported 18 viewers. People don’t necessarily need to have epilepsy in order to be affected; they might have an undiagnosed condition, remaining symptom-free until viewing such footage. Others might react even in the absence of epilepsy, suffering headaches or other symptoms as a result of being overly sensitive to flickering light.
In Japanese animation, the strobe effect was obviously not intended to cause distress. Animators considered it a technique, which they dubbed paka paka, and which was intended to communicate to the viewer a sequence of high intensity. In “Denno Shenshi Porigon” (“Electric Soldier Porygon”), the Pokémon episode that became infamous, Pikachu’s attempt to free a monster named Porygon from a digital prison results in his being attacked by computer virus missiles. Throwing his thunderbolt attack, he intercepts the missiles and creates a paka paka explosion augmented by another technique known as flash, which accentuates bright and flashing lights. The frames in the sequence were alternating at 12 hertz—well within the window to cause problems.
The scene, which occurred roughly 21 minutes into the episode, is what prompted individuals with photosensitive epilepsy to react. Statistically, it made sense. It’s believed that one in every 4000 people are vulnerable to the condition. With 4 million people watching, 1000 of them could conceivably have been struck with symptoms. A reported 618 people were hospitalized for treatment. Some even wound up in intensive care with breathing problems.
That such a sizable number were in need of attention did not go unnoticed, particularly since it was the result of a children’s show. The story was covered by the late-evening newscasts in Japan, some of which inexplicably decided to air footage of the episode, which provoked more photosensitive reactions. By Wednesday morning, the Pokémon incident was the talk of Japanese schoolyards, with kids being asked if they had been struck down by the cartoon.
It took science some time to figure out why this sequence had been so particularly consequential, even among those who weren’t epileptic. As it turned out, the typical living environment in some areas of Japan was partly to blame. In small living rooms often dominated by large television screens, kids were confronted with a towering and flickering image. Some even sat close to the screens, compounding the potentially negative effects of the sequence. Children are also more susceptible to epileptic seizures, and kids were Pokémon's target audience. The length of the sequence, which was roughly six seconds, and its heavy emphasis on the color red may have also played a part.
Hospitals who were sent questionnaires by researchers reported that many of the children treated were not diagnosed with epilepsy, though the incident seemed to precede a diagnosis. One letter to the editor published in The New England Journal of Medicine in 2004 stated that of 91 patients evaluated for Pokémon-induced symptoms, 25 had another convulsion within five years. Of 13 patients who were treated and had no history of epilepsy, 10 wound up being diagnosed eventually.
Animators were flummoxed. The paka paka and flash sequences had been used before, though likely not in a program approaching the viewership of Pokémon. Police launched an investigation to make sure Television Tokyo, the broadcast network, was not somehow negligent in airing the program. They weren’t, though the consequence would be the same either way: No one would ever take the risk of airing “Denno Shenshi Porigon” again.
The episode was pulled from the series and was never rebroadcast, save for the news clips. The show itself was taken off the air in Japan entirely, not returning until April 1998 and carrying cautionary warnings. (When Pokémon was imported to America in 1999, the episode was predictably left out.) New broadcast standards in Japan were implemented that mandated the color red could not flash more than three times per second, with no more than five flashes per second of any color, and no flash more than two seconds in length.
That wasn’t quite the end of seizure concerns in popular culture. In 2018, some theaters put up signs cautioning viewers that flashing lights in The Incredibles 2 could be a problem for those with photosensitive epilepsy. Disney later reedited the film in the UK so it complied with the Harding Box test, which sets standards on flash and flicker rates for light and can reduce—though never eliminate—the potential for problems. The company is also issuing a warning for the upcoming December 20 release of Star Wars Episode IX: The Rise of Skywalker, stating the film has “several sequences” utilizing flashing lights.
Owing to the relative rarity of these events, it’s likely productions will continue to use flashing images, though producers of Pokémon would probably prefer to forget the 1997 incident ever happened. The episode has never again been cited and the character of Porygon disappeared, save for one fleeting mention in Japan when Hulu kept a preview for the episode at the conclusion of the previous installment. While it doesn’t contain any of the incendiary sequence, it might be the only surviving footage of the day television really was bad for kids.