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by G.F.A. Harding,
Professor of Clinical Neurophysiology,
Ashton University, Birmingham, U.K. |
Epilepsy is best described as a tendency to recurrent convulsions. Since evidence suggests that each individual has a convulsive threshold and since many people have a single seizure when very young under provocation (febrile or teething convulsions), the convulsive threshold must be a continuum between 'normal' (non-epileptic) individuals and those with severe epilepsy. Epilepsy is a common condition occurring at approximately 1:200 of the population.
Photosensitivity is sensitivity to flickering or intermittent light stimulation but includes sensitivity to visual patterns. Photosensitive epilepsy therefore can be defined as recurrent convulsions precipitated by visual stimuli, particularly flickering light (Harding & Jeavons, 1994).
It is in addition necessary to divide the photosensitive population into three groups:
- Persons who only ever have convulsions in the presence of a flickering light source or visual patterns;
- Persons who have convulsions both with a flickering light source (or pattern) and without any such stimuli being present;
- Persons who are sensitive to intermittent photic stimulation (IPS) in the EEG laboratory but have not had a convulsion with flickering light or patterns in the outside world. This group contains both classes 1 and 2 and also some patients with epilepsy who have no history of photosensitivity (1 in 20) and rarely 'normal' (non-epileptic) individuals or patients with conditions other than epilepsy.
Photosensitive epilepsy occurs at approximately 1:4000 of the population (incidence 1:100,000 per annum). In addition there is an unknown number of photosensitive persons who have as yet not had a convulsion. It is almost impossible to define this number. There are a few studies of photo-convulsive responses in normal volunteers; few have used adequate techniques of stimulation and adequate definitions of the photo-convulsive response.
Photosensitivity is tested in the EEG laboratory. Electrodes are attached to the scalp and a record made of the electrical activity of the brain. The patient is then shown bright flashes of light from a stroboscope at specific test frequencies; he is then shown black/white patterns presented on a television monitor. If the patient is photosensitive a photo-convulsive response occurs in the electrical activity of the brain.
The age distribution of photosensitive epilepsy has been described in terms of age of onset (Harding 1980). Seventy six percent of patients have their first convulsion between the ages of 8 and 20. Only 11 percent have their first photosensitive convulsion above the age of 20. Amongst the 7 - 19 year-olds the incidence rises to 5.7 per 100,000 per annum. The sex distribution is unusual in that the condition is more prevalent amongst girls, 1.69:1 (Jeavons & Harding 1975).
There is no doubt that photosensitivity is probably a genetically determined predisposition. Its inheritance is however complex, under ten per cent of patients giving a family history of photosensitivity. However, sensitivity to intermittent photic stimulation can be demonstrated in approximately 40 percent of siblings of clinically photosensitive patients (Doose & Jerkin 1973).
Television is a Flickering Medium
Of the precipitation stimuli, television is the most common precipitant (60 percent), followed by natural and artificial lighting. With television viewing, nearness to the set (60 cm or less) is a common factor in over 70 percent of patients (Jeavons & Harding 1975). There is no doubt that television always contains two precipitation factors.
The 50 Hz flicker occurring at the main frequency in Europe can be perceived at all distances. Forty-nine percent of patients are sensitive to this frequency of intermittent photic stimulation (IPS). However at 60 Hz, the frequency of television in North America and Japan, only 15% are sensitive. The vertical refresh rate due to two horizontal lines produces flicker at half these rates, i.e. 25 Hz (Europe) or 30 Hz (North America). This can only be seen when the patient is close enough to resolve the linear pattern, typically less than one metre. At these frequencies 72-75 percent of patients are sensitive. However, at this distance the presence of the lines may well be a factor related to pattern sensitivity.
Television Content Causes Seizures
The further provocative factor is the broadcast material itself. Obviously all flashing lights or temporal changes from light to dark with high contrast represent intermittent light stimulation. At three flashes per second only three per cent of the photosensitive population are sensitive. At 65 flashes per second only four per cent are sensitive. Maximum sensitivity is between fifteen and twenty flashes per second. The precise distribution is given in Harding and Jeavons 1994 (see graph).
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96% of people with photosensitive epilepsy are sensitive to lights flashing at between 15 and 20 flashes per second |
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Patterns can also cause seizures
Besides temporal contrast (flickering light) there is also spatial contrast (pattern). The pattern needs to be between one and four cycles per degree. This means that there has to be between one and four black and white bars in each degree of the person's vision. Thus the spacing of the bars on the television screen (and its size) and the distance between the person's eyes and the television change this ratio.
Examples of photosensitive patterns
There is no significant difference in the orientation of the bars but bars are more provocative than plaids or checks. The contrast is critical, there being a linear positive relationship between contrast and abnormal brain responses. Isoluminance (normal red-green) bars are non-provocative. However, long wavelength red is the most provocative colour. Patterns, bars or grating that oscillate (white to black and vice versa) are more provocative than static patterns. The least provocative are drifting patterns (i.e. moving in one direction). If the patterns oscillate, the "dangerous" IPS frequency of 15 - 20 Hz is the most provocative (Wilkins, Darby, Binnie, Stefanson, Jeavons & Harding 1979; Wilkins, Binnie & Darby 1980).
For all stimuli, whether flashing light or pattern, it is essential that stimulation is in the central area of vision (ten degrees) and must be binocular [viewed through both eyes] (Harding & Jeavons 1994).
From the above preamble it is possible to produce a definition of provocative visual stimuli which are most likely to precipitate convulsions in the photosensitive population and therefore should be avoided in the production of video material for television transmission.
Flashing lights or temporal change in light-dark contrast, that is, between successive frames or sequences:
No light flashes or image changes in light to dark or long wavelength red to blue should exceed three per second, i.e. each image luminance level should be present for at least eight frames. For North American TV, a light frame should be followed by ten dark frames. This is particularly true if the contrast between light and dark is high. When changes from dark to light or flashes of light occur they should not exceed one tenth of the projected screen, and should not be in the centre of the screen.
If stimuli change in luminance at faster rates the change should not exceed twenty per cent. Higher changes should be "ramped" i.e., increased or reduced frame by frame. Stimuli occurring in the centre of the screen are more provocative.
Changes can occur in chromicity without changes in contrast, i.e. red flashes against an isoluminant green background or equivalent.
There are specific patterns which are provocative. In general high contrast bars of light-dark are provocative, particularly if their spatial frequency is between one and four cycles per degree of visual angle. Determining the size of bars on the screen is almost impossible but if we assume an average viewing distance of two metres each bar of either black or white must be less than 0.4 cm or more than 2 cm. Again, stimuli in the centre of the visual field are more provocative and "dartboard" or sectored or "whirling wheel" type patterns maximally stimulate the visual cortex due to the cortical magnification of the centre of the visual field and are therefore more provocative and should be avoided.
The rate of change of high contrast pattern is also critical and should be less than three image changes per second. In particular oscillating bars or patterns oscillating at between 15 - 20 changes per second must be avoided.
Black and white bars at 70 per cent contrast affect more than 50 per cent of the photosensitive population. Bars or stripes or patterns which are isoluminant (normal red-green, etc.) are not provocative.
Bars or stripes which drift from left to right across the screen, or optical flow, that is creating an impression of forward movement, are not provocative.
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References
Binnie D & Jeavons P M 1992. Photosensitive Epilepsies. In J Roger, M Bureau, C H Dranet, F E Dreifuss, A Perret & P Wolf. Epileptic syndromes in infancy childhood and adolescence. (2nd edition, J Liffy & Co Ltd).
Doose H & Gordon H, 1973. On the genetics of EEG - anomalies in childhood IV Photoconvulsive reaction. Neuropaediatric 4: 162 - 171.
Harding G F A, 1980. Photosensitive Epilepsy. In D J Oborne, M M Gruneberg & J R Eiser (Eds). Research in Psychology & Medicine, Academic Press, London.
Harding G F A, 1986. Photosensitive Epilepsy & Employment. In F Edwards, M Esprin & J Oxley (Eds) Epilepsy & Employment, Royal Society of Medicine International Congress Proceedings, 1986.
Harding G F A & Dimitrakoudi M,1977. The visual evoked potential in photosensitive epilepsy. In J Desmedt (Ed) Visual Evoked Potential in Man: New Developments, 509 - 513. Clarendon Press, Oxford.
Harding G F A & Jeavons P M, 1994. Photosensitive Epilepsy. MacKeith Press, London.
Harding G F A, Herrick C E, & Jeavons P M, 1978. A controlled study of the effect of sodium valproate on photosensitive epilepsy and its prognosis. Epilepsia 19: 555-565.
Jeavons P M & Harding G F A, 1975. Photosensitive Epilepsy. William Heinemann, London (out of print).
Jeavons P M, Bishop A, & Harding G F A, 1986. The prognosis of photosensitivity. Epilepsia 27: 569 - 575.
Wilkins A, Darby C E, Binnie C, Stafanson S B, Jeavons P M, & Harding G F A, 1979. Television epilepsy: the role of pattern. Electroenceph. Clin. Neurophysiol. 47: 163 - 171.
Wilkins A, Binnie C, & Darby C E, 1980. Visual induced seizures. Prog. Neurobiol. 15: 85 -117.
Related Story
On December 16, 1997, 685 Japanese children and adults made headlines after suffering apparent epileptic seizures while watching the popular animated TV program Pocket Monsters. Twenty minutes into the program hospitals all over Japan received calls requesting admission, and about 200 viewers, mainly children, were hospitalized overnight. They reported that flashing lights and rapid colour changes in the cartoon provoked the seizures.
This is a dramatic example of a well-known disorder: photosensitive epilepsy, one of the reflex epilepsies. In order to prevent a recurrence, TV Tokyo and NHK TV have voluntarily adopted a code based on the British television/advertising guidelines outlined in Dr. G.F.A. Harding's paper, reprinted here with permission.
Common Questions about Photosensitive Epilepsy
 What is photosensitive epilepsy?
Some people with epilepsy can be triggered into seizures by flashing or flickering lights, rapidly changing images or visual patterns. This is called photosensitive epilepsy. When given an EEG test, the majority of people with photosensitive epilepsy will show epileptic discharges in the brain when exposed to flashing lights.
Who has photosensitive epilepsy?
It is estimated that fewer than 5% of people with epilepsy are photosensitive. In addition, there are an unknown number of photosensitive persons who have as yet not had a seizure. Photosensitive epilepsy is largely genetically determined, although its inheritance is complex. It most commonly affects children, and usually appears between the ages of 8 and 20 years. The incidence is highest around ages 12 and 13, suggesting a link with early puberty, and girls are affected more often than boys. One quarter of patients lose their photosensitivity around 25 years of age. The rest remain photosensitive for life.
What kinds of flashing lights can trigger seizures?
Not all flashing lights or visual patterns will trigger a seizure, even in individuals who are photosensitive. The rate of the flashing light, the duration of the flashing, and the intensity of the light all play a part. A frequency of between 15 and 20 flashes per second is most likely to cause a seizure, whereas very few people are sensitive to a rate of 3 flashes per second. Recent advances in technology and special effects expose many people to a wide range of photosensitive stimuli. But natural sources, like the effect of sunlight through trees or dazzling reflections off water, are also known seizure triggers.
Can other visual patterns trigger seizures?
Besides flickering light, there are also visual patterns that can be seizure provoking. Stripes, bars, "dartboard", spiral and "whirling wheel" type patterns maximally stimulate the visual cortex and are therefore provocative, especially when they fill the field of vision. High light-dark contrast is critical to abnormal brain response; colours of equal brightness are less provocative. Patterns that move or reverse (white to black and vice versa) are more provocative than stationary patterns, especially if they oscillate in the "dangerous" frequency of 15 - 20 Hz. Long wavelength red is the most provocative colour.
Why do flashing images cause seizures in some people?
Lights flashing at roughly the same frequency as brain waves seem to entrain brain rhythms, causing nerve cells in the brain to misfire and produce a seizure. According to neurologist Jerome Engel, "Instead of the nerve cells firing individually, like fingers playing notes on the piano, many fire at once-like the banging of dissonant chords". This only happens when the flashing light or pattern is in the central area of vision and is seen by both eyes.
What do photosensitive seizures look like?
The seizures that are produced are of various types, such as tonic clonic (grand mal), absence, myoclonic, or, less often, simple or complex partial seizures. This means that they may consist of convulsions, or they may be brief episodes of blank staring, rapid blinking or twitching of the mouth or face, jerking movements in other parts of the body, loss of attention, the inability to talk or respond, or sensory hallucinations.
Can television cause seizures?
Television is by nature a flickering medium, and can pose a risk to viewers with photosensitive epilepsy. The closer the viewer to the set, the more the screen fills the entire field of vision and the greater the effect of the flicker frequency of the picture. Tiredness may be an associated factor. The further provocative factor is the material itself. Music videos and high-tech promotional materials are known for their quick-cut editing, rapidly changing images and high-contrast lighting patterns that often prove problematic for people with photosensitive epilepsy. Highly detailed computer-generated images can also be provocative.
Can discotheques or nightclubs cause seizures?
The strobe or flashing lights, which may be part of the entertainment in nightclubs or at theatre events, can trigger seizures in people who are photosensitive.
Can video games cause seizures?
Current medical opinion suggests that video games featuring patterns of flashing lights, rapidly changing images or patterns do not create a tendency to have seizures in persons where that tendency is not already there, but may trigger seizures in people with photosensitivity. Today, the manufacturers of video games often put a warning to that effect right on their products.
Can computer screens cause seizures?
Seizures may be triggered in individuals with photosensitive epilepsy who are sensitive to the flickering of the computer screen, although the flicker rate is not thought to be very provocative.
Is sunlight a seizure trigger?
Natural light sources are just as likely to trigger seizures in a photosensitive individual as artificial ones. For example, sunlight flickering off water or through a line of trees can trigger seizures.
How can photosensitive seizures be avoided?
If you have photosensitive epilepsy, taking a few simple steps can be helpful:
- Sitting at least 2.5 metres away from the television set and 30 cm. back from a computer monitor.
- Not increasing the contrast and brightness of a screen. Using a high-quality computer monitor, with a refresh rate of at least 60 Hz (VGA quality or better).
- Viewing in a well-lit room to help counteract the brightness of the screen. Eliminating reflected ambient light on the screen. Minimizing exposure to fluorescent lighting.
- Placing a hand over one eye or wearing an eyepatch when approaching the TV picture will also help, since images have to be viewed through both eyes in order to provoke photosensitive seizures.
- Wearing a good pair of polarized blue sunglasses (preferably with side shades) of tinted lenses when exposed to flickering light both indoors and outside.
- Calling ahead when planning on attending a movie, theatrical or other event which may contain strobing or provocative stimuli can reduce surprises.
- Sodium valproate (valproic acid) is the drug of choice for controlling photosensitive seizures.
- If something on television or elsewhere causes you to have a photosensitive seizure, bring it to the attention of the station or company responsible and/or to Epilepsy Toronto.
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