Photosensitive epilepsy is a form of epilepsy in which seizures are triggered by flickering light or other visual stimuli, such as bold or moving patterns. Of those who suffer from epileptic seizures, between 3% and 5% are known to be of the photosensitive type (approximately two people per 10,000 of the general population). Often they have no other history of epilepsy. Females are more commonly affected than males, and there is distinct genetic correlation. more...
In affected people, the symptoms usually first occur during childhood or adolescence and few people develop them after the age of 20. Sufferers generally learn to avoid the stimuli that trigger seizures and in many cases, the symptoms subside with time. There is no cure, although effective medication is available in appropriate cases.
Sensitivity is increased by alcohol consumption, sleep deprivation, and other forms of stress.
The response varies with the individual and can be any type of epileptic seizure, with characteristics ranging from a disconcerting loss of awareness to alarming fits. The seizure may be preceded by a period of disorientation sufficiently lengthy for the subject to take avoiding action, which may be simply to look away from the stimulus if possible, or to cover one eye so that fewer nerve cells are subjected to the stimulus.
Vulnerable people can be induced into seizure by any flickering light, such as from stroboscopic lamps in discotheques and faulty fluorescent lamps. The frequencies most likely to induce a seizure are between 15 Hz and 25 Hz (i.e. between 15 and 25 times per second), but some people are susceptible to frequencies as low as 3 Hz or as high as 50 Hz.
Travelling along tree-lined avenues with the sun flashing between the tree trunks can be a trigger, as can the flickering of sunlight among the leaves of trees as they move in the wind, or the reflection of light from the surface of rippling water.
Flashing light is not the only trigger and in some cases, looking at certain geometric patterns such as bold stripes or chequers can cause a seizure, or looking between railings while walking, or watching a rhythmically moving object such as a moving staircase.
Images displayed by some computer games can also trigger seizures, which is a particular hazard for affected children.
Apart from the nature of any image displayed on a television screen, the way in which the screen functions can serve as a trigger. In particular, PAL, one of the colour encoding systems used in broadcast television (the standard in the UK), refreshes at an interlaced frame rate of 25 Hz (half the mains frequency) and is a known cause of seizures. In most circumstances, television screens are viewed from a distance such that the refresh is indiscernible, but with the trend towards larger television screens, the problem becomes more evident.
The triggering effect of a flickering light is greatly increased with the contrast it produces, and it is more likely to induce a seizure in an otherwise dark room compared to one with bright ambient lighting. So, watching television from a reasonable distance and in a well-lit room greatly reduces the likelihood of seizure.
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