Photosensitive epilepsy affects approximately one person in six thousand in the adult population, and five times that many between the ages of seven and twenty. Many who are susceptible to it are unaware of their vulnerability.

Photosensitive seizures can be triggered by various things: Dappled or flickering light, including fluorescent lighting (especially when it starts to fail) and strobe lights are one of the main dangers; repeating patterns, such as stripes and chequers, especially when they are moving ones, are the other main thing to look out for. The brighter and bigger the pattern or flashes, and the higher the contrast, the more danger of a seizure there is. Different people respond to different temporal and spatial frequencies of patterns; flashing at around 20Hz (ie. twenty flashes a second) sets off the highest proportion of sufferers, but for any individual the worst frequency could be anywhere between about 2Hz and 60Hz. The worst spatial frequency for striped patterns varies between about one and ten cycles per degree depending on the person. Stripes are around twice as likely as chequers to set off seizures.

The same sort of triggers which cause photosensitive seizures can also set off migraines and other headaches in susceptible people, and cause eye strain and perceptual distortions in those with Meares-Irlen Syndrome - a condition which is quite common in children with difficulty reading, which causes the words and letters sort of swim about on the page. Arnold Wilkins at the University of Essex has characterised the sort of stimuli which contribute to these in a general theory of visual stress. People who suffer from at least one of these visual-stress-related problems are statistically a good deal more likely to suffer from the others.

Increased awareness of photosensitive epilepsy (not to mention migraines) - largely due to a few particularly garish adverts and animations which set of a significant proportion of their audience - has led to a clampdown on excessive flashing and stripes on television, with warnings being given beforehand if a program contains strobe lighting and adverts being screened to make sure they won't cause dangerously high levels of visual stress. However, watching television remains a risky business for those who are most sensitive to these triggers.

Coloured filters have been found to help reduce visual stress in many cases; different people turn out to suffer from visual stress to a greater or lesser extent depending on the colours they're exposed to, with many reporting reduced or eliminated perceptual anomalies when seeing things through the appropriate filter or under light of that colour. Some children make rapid progress in reading and so on once they are provided with appropriate coloured filters, and trials are ongoing to determine how much coloured glasses might help with reducing seizures and migraines. Helen Irlen was one of the first to propose the use of coloured filters in this way; Arnold Wilkins and others have carried the work forward.

One thing to be aware of with photosensitive seizures is that many sufferers actively seek out the stimuli that set them off - presumably because they enjoy the early stages, marked as they are by interesting perceptual distortions and a feeling of distance from reality. It is not uncommon to find kids putting their face right up to a flickering television screen, too close to appreciate any pictures (if the TV is even showing anything but static) - close enough to appreciate the fast-pulsing light taking up their entire field of vision, and feel the effect this has on their brains. When looking after kids who get photosensitive seizures, it is important to keep an eye out for them spacing out on patterns or flickering screens, and snap them back to reality by distracting them and removing the stimulus, if possible. In severe cases it is probably wise to avoid clothing with a coarse weave, let alone any patterning, and to put stickers over bar codes on books and so on.

Based mainly on the course notes and web page of Arnold Wilkins: