Maybe Mom was right when she said not to sit too close to the TV.

Japan was still trying to figure out what hit it today, after 685 people were taken to hospitals with seizures, convulsions or loss of vision after watching a popular television cartoon on Tuesday. Some 200 victims, mainly children, remained hospitalized today.

Outraged mothers accused television networks of ignoring children's health in the competition for ratings in the multi-billion-dollar animation business. Some called for introduction of an electronic screening device, comparable to the American V-chip, to help parents block out intense animation.

"We are gravely concerned about this escalating race, this competition by the television networks to show ever more stimulating images, targeting even children," the country's largest mothers' organization said in a statement.

The victims, who range in age from 3 to 58 and live throughout the country, suffered attacks during a fast-action cartoon serial called "Pokemon," or "Pocket Monsters." The scene that apparently triggered the neurological episodes involved a bright-white explosion followed by brilliant red, white and blue lights that flashed like a strobe for about five seconds.

In the hour following the show, emergency service telephone lines all over the country began lighting up as people called ambulances. Some families reported that children stopped breathing momentarily; others reported seizures similar to those suffered by epileptics. Newspapers reported that even more people were stricken later in the evening when television news rebroadcast the scenes that made people sick, in an editorial decision not likely to be nominated for any public service awards. The Yomiuri Shimbun newspaper reported today that education officials had identified 12,950 children who suffered at least minor symptoms after watching the show.

The popular program was immediately yanked off the air and a major video rental chain removed taped versions. Shares of Nintendo, which invented the characters on which the show is based, took a nearly 5 percent shellacking on the Tokyo stock market.

And Japan, which produces some of the most creative and distinctive animation in the world, including some of the most violent, was left to ask itself: Is there really such a thing as a killer cartoon?

"It is already well known that television has a tremendous impact on human beings, and in the centuries to come it will become bigger and bigger," said Kikuo Asai, a researcher at the Media Education Development Center in Tokyo. "But in many ways that mechanism has not been fully understood. Perhaps the Pokemon' case will help make it clearer."

A grave Prime Minister Ryutaro Hashimoto offered this Buck Rogers-ish comment: "Rays and lasers have been considered for use as weapons. Their effects have not been fully determined."

The incident has struck almost everyone here as the most bizarre thing that's happened in Tokyo since a bearded guru with a Christ complex ordered his disciples to gas the city's subway system in March 1995, killing 12 people and sending 5,500 others to the hospital.

About the only people who weren't too surprised were neurologists and the makers of video games. As it turns out, there is ample precedent for intense optical stimulation causing epileptic seizures. Illness related to video games has increased in the past 10 years as video games have proliferated and their visual effects have become more intense.

After several teenagers suffered seizures while playing Nintendo games a few years ago, the company now includes a warning label on much of its software, saying that the games could cause a "shigeki," or a strong stimulation, from bright lights, resulting in unconsciousness or convulsions. The warning advises users prone to such episodes to consult a doctor before playing. Sega also places a similar warning on its video games and software.

In Britain in 1994, the Independent Television Commission, which regulates commercial TV, limited the rate of flash to three per second.

The guidelines were instituted after a 1993 incident in which an ad for noodles aired that "had a great number of fast-moving computer graphics" and other quick variations in brightness, said Suzanne Prance, a spokesman for the commission. "A number of people complained about it, and there were three cases of people actually suffering seizures."

But "it's very difficult to eliminate the problem entirely," Prance said. The issue "is flashing images but also quick changes in patterns such as a black-on-white star changing to a white-on-black star. All our broadcasters have to preview programs to ensure they are in compliance with our code." Several Japanese television networks today said they would screen their animated programming to ensure that it does not contain the kinds of visual effects that caused Tuesday's problems.

Animators said the "Pokemon" episode that sickened people contained no special effects or techniques that had not been used hundreds of times before. Animators said the technique known as "paka-paka," or the use of different-colored lights flashing alternately to create tension, is quite common. "We cannot understand why it turned out this way," said Takemoto Mori, the show's producer.

Akinori Hoshika, a neurologist at Tokyo Medical College, said it is well established that optical stimulation can produce dizziness, nausea, fainting, loss of vision, seizures and other symptoms. Hoshika said children are especially susceptible because their brains and central nervous systems are not fully developed.

Hoshika said strong flashes of red and blue, which are at opposite ends of the color spectrum, tend to be particularly hard on the eye and brain and more likely to produce a reaction.

Japanese homes, many of which are very small and have large-screen televisions, may have made the problem worse. Watching TV in a Japanese apartment can be a little like sitting in the first row at a movie theater.

Today's Yomiuri Shimbun reported that one 14-year-old boy who was sitting less than three feet from his big-screen television was unconscious for more than 30 minutes. A fifth-grade girl sitting seven feet from a 35-inch set suffered a seizure and felt, her family said, "as if she were under the spell of a hypnotist."

A pediatrician in Osaka said he treated two children who were eating dinner in front of the television when they collapsed and fell into convulsions. The seizures lasted for about five minutes, but the children were conscious by the time they arrived at the clinic.

Still, having bright lights strobed into your face from 2 1/2 feet away would be enough to make anyone queasy. Having your senses assaulted with a bizarre kaleidoscopic explosion of light in the middle of your sushi and rice has got to be worse. Staff writer John Burgess in London and staff researcher Akiko Yamamoto in Tokyo contributed to this report. CAPTION: A character from Japan's seizure-inducing children's cartoon, "Pocket Monsters."