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:
- Level 1
- When content violates either the general flash threshold or the red flash threshold, users are warned in a way that they can avoid it
- Level 2
- Content does not violate the general flash threshold or red flash threshold
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.
Never thought I'd see a serious post on this topic Gez. I'm always joking about seizures caused by blink elements, and in particular animated GIF banner ads. Visit just about any porn site...
Posted by Douglas Clifton on
It's a serious post because there's nothing funny about having a seizure. Small flashing banners are unlikely to cause a seizure, but large flashing banners that flicker at dangerous frequencies, particularly those that make a transition from or to red, are highly likely to cause seizures.
Although small flashing banners are unlikely on their own to cause a seizure, there is the possibility they could be resized, in which case they too could cause seizures.
Posted by Gez on
I've been teaching accessibility to high school teachers in Texas, and in addition to the concern for people with photosensitive epilepsy, avoiding flicker benefits anyone with attention deficit disorder or similar cognitive disorder.
I'll never forget the speaker who came to my accessibility class one day. She was a researcher who had been in a car accident that had left her with cognitive disabilities. Flashing or moving content on the screen was so distracting to her she couldn't consume the other content.
I think of her every time I see unnecessary movement on a web page.
Posted by goodwitch on
I know all too well that epilepsy is nothing to joke about, I have a girlfriend who suffers from it. I didn't mean to make light of the issue at all, in fact I can't stand blinking, flashing banners (or anything on Web pages with too much movement or flicker). I can only imagine what it must be like for people who are sensitive to it. Hence the "joke" -- avoid adult sites if you have this problem.
There is something to be said for drawing your attention to certain places on a page, for instance hover events and such. But it should be done with great care and subtlety.
Posted by Douglas Clifton on
Sorry, Doug, I misunderstood what you meant. I thought it was out of character for you ... I should have realised. Sorry.
Posted by Gez on
Yes, very true, and an area that I think WCAG 2.0 is weak on. It was in WCAG 1.0 with the dreaded "until user agents..." preamble. User agents are now capable of disabling distracting images, so it's now considered a user-agent issue rather than a web content issue.
Like everyone else, I certainly don't think that accessibility is about pandering to people's whims, and passing responsibility to user-agents to empower users is the best solution for everyone. I just think it's misguided that guidelines that help people with cognitive problems are filed under "read the manual" - it's a contradiction in terms.
Posted by Gez on
My fault, the first post was poorly worded.
Posted by Douglas Clifton on
I used to work with adult epileptics with other compounding issues. One woman in particular was very photosensitive...even rapidly cycling through television channels with a remote was enough to provoke seizures occasionally, which she sometimes did on purpose.
I've seen many webpages and banners, not just animated gifs but also flash intros, that I have no doubt would cause her to seizure. It's a benchmark I use when I'm trying to decide if motion is adding to the presentation or just "bling."
Posted by J. Decker on
I have photosensitive epilepsy. Although flash content on the web has never triggered a grand mal seizure for me, I have experienced headaches and disruptions in cognitive functioning from such content--the same sort of headaches and disruptions that accompany the complex partial seizures I get in real (offline) life. Since even these baby seizures cause permanent brain scarring, I for one take them very seriously!
Posted by Emily Smit on
Thank you, J. Decker and Emily, for sharing real-life experiences. It adds weight to the argument that although there has never been a reported case of a seizure caused by web content, there could well be unreported cases - if not, then it's been more by luck than design.
That's a very good point, and one that's mostly overlooked. People usually consider the immediate effects, such as an injury sustained from falling. The long-term effects do tend to get overlooked.
Posted by Gez on
It's interesting to know it can result in permanent damage.
Posted by Robert Wellock on
I too have photosensitive epilepsy and while I also have not had any seizures specifically from webpages or banners, I do get petite mal or absent seizures from computer monitors that have the refresh rate turned lower than 85hZ. This is hard to avoid because I am learning to be a mechanical engineer and have to use a lot of different computers. Luckily most new monitors these days are flatscreen LCD and TFT which use different technology emitting stable light so they completely avoid the issue.
Posted by Anjrup on
Fine article and wonderful comments. I'm fortunate in not having this condition. Yet, I do leave the room when someone starts channel surfing the TV, and almost any flashing stuff on web sites drives me beyond my very low annoyance threshold.
There's one area where I think we could all use some help. Can anyone translate into plain language what those threshold formulas in the WCAG document really mean? I used to think I was good at math and visualization ... until I read those two descriptions.
Would it be so hard to have a description that says "Don't make a flashing thing any larger than this box (include an image)."
Posted by Bob Easton on
Thank you for your kind words.
I don't really understand it myself, but I'll attempt to provide my interpretation. As I don't understand it, I'll probably raise more questions than answers. If anyone does understand the descriptions, then please correct or extend on my attempt of an answer.
WCAG 2 Says:
I know that results of experiments have shown that both the frequency and the overall area of the flash are significant in provoking seizures, but I don't know how they arrived at these figures. Does it mean that seizures can only be induced on monitors with a resolution of exactly 1024 by 768? It's safe to assume that that isn't the case, but it could be interpreted that way. On different resolutions, does the effected rectangle change proportionally? I've no idea.
I suspect the reason is that the flashing areas don't have to be immediately next to each other; it could be that two smaller areas of the screen flash, but would occupy more than a quarter of the same 335 x 268 pixel rectangle and would be sufficient to provoke a seizure.
WCAG 2 Says:
That in itself is simple enough to understand, but the note that follows takes some understanding.
WCAG 2 Says:
As photosensitive epilepsy can be triggered by flashing (or flickering), the formula has to determine the change in brightness between the dullest and brightest point of each flash. The formula is to determine the brightness, which it does by converting RGB colour values into YIQ values, which yields the perceived brightness of a colour. The formula takes into account gamma correction, which is derived (including the constants for the RGB to YIQ conversion) from the Standard Default Colour Space for the Internet (sRGB): http://www.w3.org/Graphics/Color/sRGB
Even the simplest of concepts is difficult to follow in their explanation. It's a simple enough concept that the RGB system uses intensities of red, green, and blue to produce different colours. It's also simple to understand that using 8 bits, those intensities range from 0 to 255, where 0 has no intensity of that colour, and 255 is the full intensity of that colour (referred to as full scale in the explanation). The linearisation process is for gamma correction, and the constants are used to calculate the perceived brightness.
It's not much of an answer, but hopefully someone more knowledgeable will be able to explain it better.
Posted by Gez on
To see what a few different flicker rates look like (in case people don't think in Hertz), see the informational demo at:
Posted by AWK on
I am someone who has had photosensitive epilepsy all of my life. I was born with it and the degree of severity grew as I got older. Through out my childhood I was always told I would grow out of it, as its a child hood form, but as I am now 43 I can tell you definitely that it does not go away. The severity and intensity grows. To top it off I have grand mal seizures as well so the damage that can and has been done to me is intense. I have always wanted to be able to talk with other adults in the same situation who can help me cope at times. I have managed though, I have my medication, although my body does become immune after a certain period, which doesn't help me or my injuries!! I can't and won't drive, I have a very close family who are very protective and patient and understanding, my children are all blessed with an understanding of something that no child should grow up being aware of, but thank god none of them have it. I work and my work colleagues are a god send but if there is a miracle cure out there anywhere please let me know. I would love to live out the rest of my life without this over my head if possible, but as miracles never happen we do the best with what we've got. There are warnings on websites and video games, but trees are something that no one can avoid and that is my biggest threat, barring sand, which is more reflective than glass and the sea. I love it at night avoid it during the day!! Be aware protect yourself with non reflective sunglasses and visors and hats can be fashionable so don't be shy, it beats the alternative.
Take care one and all and know your not alone
Posted by Sonya on