Photosensitive epilepsy is a form of epilepsy that is triggered by visual stimuli, such as flickering or high contrast oscillating patterns, and it's believed that around 3% to 5% of people with epilepsy are susceptible to photosensitive material. Photosensitive epilepsy is usually triggered where the flicker rate is between 16Hz to 25Hz, although it's not uncommon for seizures to be triggered by flicker rates between 3Hz to 60Hz. The condition most commonly effects children, and is usually developed between the ages of 9 and 15 years, and most prevalent in females.
Author: Gez Lemon
- Photosensitive Epilepsy and the Web
- Causes of Photosensitive Epilepsy
- Accessibility Guidelines
- When it's Too Late
Photosensitive Epilepsy and the Web
As far as I'm aware, there has never been a reported case of photosensitive epilepsy caused by web content (if you know differently, then please post a comment to that effect), but there have been cases where photosensitive epilepsy has been caused through computer games. That's not to say that photosensitive epilepsy cannot be triggered by web content, and it stands to reason that designers/developers should take care never to create material that could potentially cause photosensitive epilepsy. There could well be unreported cases, and it follows that as computer games can induce photosensitive epilepsy, then it's obviously possible through web content.
Television programmes are thought to be the most common cause for triggering photosensitive epileptic seizures. The most famous incident of photosensitive epilepsy caused by a television programme is the Pokémon episode, Electronic Soldier Porygon, which was aired in Japan in 1997. Nearly 700 children were admitted to hospital through photosensitive epilepsy that was thought to have been induced by the episode.
Causes of Photosensitive Epilepsy
Flickering is the opposing changes in intensity of luminosity. This is usually caused by flashing, but can also be caused by spatial contrast patterns that oscillate at dangerous frequencies; the type of images that people create to deliberately stimulate a response in the recipient that makes them believe the image is moving or changing. For people with photosensitive epilepsy, flickering causes many of the nerve cells that process visual stimuli to all fire at once, resulting in a seizure.
Along with the frequency of the flickering, the size and luminous intensity of the stimuli is significant for people with photosensitive epilepsy. The greater the intensity and larger the size of the stimuli, the greater the danger of provoking seizures caused by flickering at dangerous frequencies. The colour red is particularly dangerous due to its longer wavelength that stimulates cones in the retina. There have been cases where photosensitive epileptic seizures have been triggered by cyclists while setting up the red flashing rear lights on their bicycle. Even when there is no perceived difference in the luminosity of the contrasting colours, red flickering is far more likely to cause seizures than other colours.
Checkpoint 7.1 of WCAG 1.0 is a priority 1 checkpoint that states:
Until user agents allow users to control flickering, avoid causing the screen to flicker, with the following advice:
Note. People with photosensitive epilepsy can have seizures triggered by flickering or flashing in the 4 to 59 flashes per second (Hertz) range with a peak sensitivity at 20 flashes per second as well as quick changes from dark to light (like strobe lights).
Guideline 2.3 of the November 2005 draft of WCAG 2.0 states:
Allow users to avoid content that could cause seizures due to photosensitivity. There are two success criteria associated with this guideline:
Allowing people to choose whether or not they receive the content is better than not providing a warning, but there are other factors to consider. The first is that as photosensitive epilepsy is most common in children, it could be that they don't understand or appreciate the significance of the warning. It isn't just children; people with reading difficulties or speakers of languages other than the language of the warning may also inadvertently be exposed to content that could induce seizures.
Although the size of the stimuli is significant, there is still a danger of material that is considered to be safe being changed by the visitor. For example, low vision users increasing the size of flickering material, or someone leaning in close to the screen.
The safest way to avoid causing photosensitive epilepsy is to completely avoid creating web content that flickers.
When it's Too Late
People with photosensitive epilepsy who suddenly find themselves exposed to material that could trigger a seizure should immediately cover one eye with the palm of their hand to reduce the number of brain cells that are stimulated by the flickering content, and either close the page or navigate away from the page.