In the neon-lit life of the average American, it can seem sometimes as if lights are flashing even in our own homes – from the personal computer, the television, and the kids' video games – in a fluorescent race for our attention.
For the vast majority of people, this cacophony of light and color is no more than the “visual noise” of a wired world, barely noticeable amongst the clutter of competing stimuli.
But for a small segment of the general population – and a somewhat larger segment of children – high frequency flashing lights can be the trigger for serious seizures.
And experts say that while the numbers of affected individuals are not large, the ubiquity of provocative stimuli in the environment makes photosensitive epilepsy (PSE) a matter of concern to neurologists, public health officials, and manufacturers of entertainment that may contain material provocative to those at risk for PSE.
“Photosensitivity is a public health problem in the United States that we have not adequately addressed,” said Robert Fisher, MD, PhD, the Maslah Saul, MD, Professor of Neurology at Stanford University School of Medicine and Director of the Stanford Epilepsy Center in California.
Dr. Fisher said potentially provocative visual stimuli include bright flashes of light and color – especially red – that fill the visual field in frequencies between 5 and 30 cycles per second. These stimuli are embedded in various visual venues to which contemporary children and adults are exposed: movies, DVDs, video games, discos, and rock concerts.
POKÉMON INCIDENT IN JAPAN
But by far the most ubiquitous vehicle for provocative material is the television. In 1997, a TV cartoon, pokémon, in Japan sent 565 children to the hospital with seizures. The incident was traced to the so-called “rocket launch” sequence in the cartoon, no more than a few seconds in duration, in which alternating flashes of deep red and blue occurred at 12.5 Hz, a relatively low frequency.
“Most people believe the seizures in the pokémon incident were triggered by alternating deep-red and blue flashes,” Jaime Parra, MD, PhD, of the Dutch Epilepsy Clinics Foundation in the Netherlands, told Neurology Today.
In a March 8th paper in Neurology, Dr. Parra and colleagues described a technical modification to the pokémon video that appears to have significantly reduced the potential for photosensitivity-induced seizure, without noticeably detracting from the quality of the video.
Dr. Parra explained why he and colleagues believe the seizures occurred in the first place. “The highly saturated red flashes provoked a stimulation of the red cones, which are most numerous in the retina; the stimulation appears to elicit seizures in susceptible people, even at low intensity.”
Dr. Parra said the prevalence of photosensitive epilepsy in the general population is low. “However, its incidence in young people between 7 and 19 years reaches 5.7 per 100 annually,” he said. “We lack appropriate studies about how many apparently normal people are really susceptible to develop seizures triggered by flashing lights or patterns. Some studies suggest that non-epileptic photosensitivity – that is, susceptible children who might be unaware of the risk of having seizures induced by light stimuli – may affect between 4 and 8.9 percent of the children.”
He said photosensitivity is most often detected around puberty, although a medical history often reveals that it may have been present for some years before being recognized. Two-thirds of patients are female, suggesting some hormonal influence.
Dr. Parra said population studies by Graham Harding, Head of the Clinical Neurophysiology Unit at Aston University in Birmingham, UK, have shown that photosensitivity would eventually disappear with age in about one third of the patients. But, he added, “We don't have any way to predict in which patients the trait would disappear.”
Dr. Fisher said photosensitivity can be measured by EEG showing abnormal brain response to flashing light in the lab. But he cautioned that the prevalence of children who might test positive under those conditions is far higher than the actual number who might be considered at risk for a photosensitive seizure.
“That is a test tendency and it is important not to mistakenly suggest that a few percent of children will have a seizure to flashing light,” he said.
But Dr. Parra said episodes such as the pokémon incident suggest that the genetic susceptibility to visually induced seizures may be larger than previously thought, and that an important segment of the infantile population remains latent until a provocative stimulus uncovers the condition.
Drs. Parra and Fisher noted that the United Kingdom and Japan have regulatory guidelines requiring all material that is broadcast to the public to be screened for photosensitivity. An article on “effectiveness of broadcasting guidelines for photosensitive seizure prevention,” by Takahashi and colleagues, outlining the effect of the Japanese guidelines, appeared in Neurology last year (2004;62:990–993).
No such guidelines exist in the United States, Dr. Fisher said, where significant free speech and artistic issues come into play. “Some individual broadcasters and manufacturers of video games have processes they apply, but it is entirely voluntary,” he told Neurology Today.
In the paper published in the March 8th issue of Neurology, Dr. Parra and colleagues said that in test conditions in which the original pokémon video and the modified video were shown to 25 photosensitive patients using two televisions of different signal frequencies (100 Hz and 50 Hz), the modified sequences triggered significantly fewer and less severe photoparoxysmal responses.
“The idea is to change slightly the color modulation, which in this case consists of decreasing the difference between red and blue by means of digital filtering,” Dr. Parra explained “This method is applied in the pixels exhibiting a potentially dangerous frequency, leaving the rest of the image intact. When the patients looked at the different scenes and were asked afterwards to describes differences, many did not notice any. If you watch it carefully you can notice that the red-blue flicker event gets a slightly purple tone. This simple change is enough to decrease the risk of triggering seizures with this scene.”
Selim R. Benbadis, MD, Associate Professor of Neurology and Director of the Comprehensive Epilepsy Program at the University of South Florida and Tampa General Hospital, said the technical fix may be useful to manufacturers of video products. “This is a valuable finding,” Dr. Benbadis said. “In the past we have only been able to remove sensitivity through colored glasses or by altering the visual material in a way that would also distort the content.”
THE ‘FIX’: LIMITED IN SCOPE
But Dr. Benbadis underscored the relatively circumscribed nature of PSE – emphasizing that visually induced seizures do not “cause” epilepsy, and that people with PSE constitute a small proportion of the population with epilepsy – as well as the relatively limited application of the technical fix described in the paper.
Similarly, Dr. Fisher agreed that the method outlined by Dr. Parra and colleagues appears to be a successful strategy that could be narrowly applied by producers and manufacturers of similar formats.
“Speculatively, this technology might have reduced the number of seizures resulting from the pokémon broadcast,” Dr. Fisher said.
But Dr. Fisher said the risk of seizure for individuals with PSE viewing potentially provocative material in a variety of other contexts remains a problem to be addressed in a more comprehensive way by regulatory bodies, physicians, and public health agencies.
An August 2004 consensus conference on the subject of PSE, convened by the Epilepsy Foundation, sought to summarize the medical knowledge and derive voluntary guidelines for industry and recommendations for individuals. Those recommendations will be forthcoming on the Foundation's Web site (www.epilepsyfoundation.org), said Dr. Fisher who participated in the conference.
He reported that the voluntary guidelines for industry will reflect, in modified form, the regulations now in place in the UK and Japan. And Dr. Fisher added that discussions are also ongoing in an international telecommunications division of the United Nations with the goal of developing international guidelines.
Meanwhile, he said physicians should advise parents that children who are watching videos and video games on television should sit at least two meters from the set so that flashes and light patterns will only be a small part of the visual field. Children should also not watch television in the dark and should play video games for a relatively short time with rest periods.
ARTICLE IN BRIEF
✓ The article discusses the role that color and flashes of light from video games play in causing epileptogenic sequences – and a suggested technical fix that could eliminate the seizures.