Televised Material and Photosensitive Epilepsy

Transcription

1 Epilepsia, 4O(Suppl , 1999 Lippincott Williams & Wilkins, Inc., Philadelphia International League Against Epilepsy Televised Material and Photosensitive Epilepsy G. F. A. Harding and P. F. Harding Clinical Neurophysiology Unit, Aston University, Birmingham, U. K. It has long been recognised that in Europe, the television (TV) monitor is the most common provocative stimulus for photosensitive seizures (1). More recent evidence confirms this view, whether the TV monitor is used for broadcast material, video playback, or electronic screen games. The monitor is an inherently provocative stimulus because its basic frequency causes flicker or changes in luminance. In Europe the frequency of this change is at 5 Hz (PAL), whereas for most of the world, the rate is 6 Hz (NTSC). At 5 Hz, 49% of patients are sensitive and will produce photoparoxysmal responses (PPRs) in the EEG, whereas at 6 Hz, only 15% of patients are sensitive (2). This marked difference in sensitivity may explain the relatively low incidence of TVinduced seizures in North America. The TV picture is made by a system called interlaced scanning. The picture is made up of a series of thin strips or lines. Normally 625 lines are used, and the scan begins at the top left corner of the picture and continues until the bottom right-hand corner is reached. When TV standards were established, it was realised that apparent flicker could be reduced if the 625 lines were divided into two line pictures, made up in one picture of even-numbered lines and in the other picture of oddnumbered lines. These two pictures are called fields. The alternating lines can be resolved only at relatively short viewing distances, typically around 1 m. At these short distances, spatial flicker is therefore present at 25 Hz for PAL and 3 Hz for NTSC TV systems. There are similar probabilities of inducing PPRs associated with these rates of flicker, 76% at 25 Hz and 72% at 3 Hz (3). In Europe, photosensitive epilepsy has an incidence of 1.1 : 1, per annum of the population, but between the ages of 7 and 19 years, it is 5.7 times as common (4). This incidence correlates well with the onset of seizures being maximal around puberty (2). Photosensitive epilepsy is more common in girls (2: 1) and has a marked genetic factor; 25% of children of photosensitive mothers show PPRs in the laboratory (5). Although the condition usually manifests itself around puberty, 9% of patients having their first seizure before Address correspondence and reprint requests to Dr. G.F.A. Harding at Clinical Neurophysiology Unit, Aston University, Aston Triangle, Birmingham B4 7ET, U.K. the age of years, only 25% of patients lose their photosensitivity, usually in their s and 3s; the others remain photosensitive throughout life (5). Many photosensitive patients also are pattern sensitive (6) and of those showing electronic screen game seizures, >% are not provoked by intermittent photic stimulation (IPS) but are sensitive to high-contrast bars, particularly if these alternate (7). With IPS in the laboratory, the relative sensitivity of the photosensitive population can be established (Fig. 1). It can be seen from this figure that most patients are sensitive around 16 flasheds, but a large majority are sensitive at 25 and 3 flasheds. These two frequencies are also the frequency of the frames or individual pictures that successively make up the moving picture in PAL and NTSC. In addition, many computer-generated cartoon sequences are produced at film rate of 24 frameds. There is therefore a risk that it is not just due to the construction of the picture inherent in cathode ray tube (CRT) displays but to the rate of change of material within successive frames of the televised material whether broadcast, video, or in computer games. The change may of course be temporal (intermittent flashes) or spatial (patterns in cycles per degree of vision). The presentation of flickering light pictures and Lop art backgrounds has long been recognised as provocative (1). More recent events have served to confirm this. In the 19s a U.S. television program called Captain Powers produced a seizure in a young male viewer (2). In the sequence there were frequent flashes from targets and from guns. When tested on photosensitive subjects, these flashes produced clear abnormalities in the EEG (Fig. 2). In 1993 an advertisement was shown on commercial TV in the United Kingdom (Golden Wonder, Pot Noodle). One of us (P.F.H.) immediately recognised the potential dangers of the stimulus, which on its first showing produced three proven complaints of seizures. This advertisement was made up of rapidly changing highcontrast film clips, often with picture reversal from frame to frame. Because of the unprecedented number of seizures, the Independent Television Commission (ITC), the U.K. statutory regulatory body for commercial television, requested that we produce draft guidelines to pre- 65

2 66 G. F. A. HARDING AND P. F. HARDINC loo 1 p 7.- c1 so E 5 P u) 4 3 s Flash rate (f/s) FIG. 1. The percentage of patients responding to flash rates between 1-65 flashes/s. 17 photosensitive patients took part in this analysis. Only photoparoxysmal responses were counted as significant. It should be noted that at 3 flashesis, only 3% of photosensitive patients are at risk. At 5 flashesk, 49% are at risk, and at 6 flashesk, only 15% are at risk. - I,,,,, I u u I I m I r I I I I I -nmi-lmmm 1 uv I 1 sec GUNFLASHES FIG. 2. The EEG response in one of our young photosensitive patients to a broadcast TV game called Captain Powers, which was screened in the USA. Each time a character in the picture fired his gun, the gun flashes produced discharges in the EEG. 1 UI * 4 6 t s Spatial Frequency FIG. 3. The relative sensitivity of patients to different spatial frequencies. It can be seen that peak sensitivity to a square wave bar pattern reversing at 1 Hz is maximum at 2-3 cycles per degree. Epilepsia, Vol. 4, Suppl. 4, 1999

3 TELEVISED MATERIAL AND PHOTOSENSITIVE EPILEPSY 67 vent future screening of provocative material (8). These guidelines were based on the scientific evidence available from our own studies and a survey of the literature. The most straightforward constraint was to restrict the rate of luminance change in both temporal and spatial terms. It is obviously not possible to produce a no-risk guideline because TV is inherently a provocative medium for any material, and luminance changes must occur. If, however, these luminance changes were restricted to a maximum of three per second, <3% of the photosensitive population would be at risk (Fig. 1). Because the prevalence of photosensitivity is one in 4, of the general population (2), this will produce a risk of one in 133,333 of the general untreated population. However, a significant percentage of this population will have been treated if previous photically induced seizures have taken place, thus reducing the number at risk. For new cases, the risk will be one in 3 million of the population, using known U.K. evidence figures of photosensitivity (4). In a recent incident in Japan, more than half of the seizures were in people with no previous history of photosensitivity (personal communication, Japanese Ministry of Post & Telecommunications). The most provocative image change is one in which bright and dark frames of the picture alternate, because this will produce the highest flash rate (12.5 fps PAL and 15 fps NTSC). However, from our previous studies of pattern sensitivity (6,7), changes that only occupy part of the screen also were considered provocative. Highcontrast bars that alternate were considered to be the most provocative condition (6). It was recognised that static patterns also are provocative in almost 6% of patients (6), but patterns that drift are not (9). Patients are most sensitive to these patterned stimuli presented on a TV monitor at a spatial frequency of 2-3 cpd (Fig. 3). However, because of the variation in size of TV monitors and the variation in viewing distance, it was recognised that only approximate guidelines could be produced. A minimal area of change was introduced to trigger the guideline. This figure of 1% was based on evidence that flickering stimuli that were not presented.centrally were not provocative (2). Because 9% of neurones in the visual cortex are known to subserve central vision (lo), the assumption was made that because a typical domestic TV monitor occupies the central area of vision, a stimulus that showed change in luminance in only 1% of the screen would not stimulate more than the presumed critical 1% of neurones. It is difficult to specify a critical level of contrast because the probability of PPRs is approximately linear (I 1). Patterns that are composed of isoluminant opponent colours also were thought to be nonprovocative (6), and indeed this finding was used to modify the Golden Wonder advertisement, which was subsequently rescreened for a period >6 months without further incident. FIG. 4. Figure 4 shows some of the stills in the Pocket Monster cartoon, which produced the seizures in viewers in Japan. The background red (stills 7 and 9) is of lower luminance than the background blue color (stills 8 and lo), and each is produced using either the redlblue gun of the cathode ray tube. (With acknowledgements to TV Tokyo.) Epilepsio, Vol. 4, Suppl. 4, 1999

4 68 G. F. A. HARDING AND P. F. HARDING FIG. 5. The percentage sensitivity of the cones of the human eye (shown as circles) and the relative output of the blue, green, and red guns of the television system (shown solid). The score along the abcissa is shown in nanometers between nm. It can be seen that although the blue (45 nm) and green (55 nm) guns approximate well to the sensitivity of blue and green cones, the red gun (64 and 71 nm) of the television only provides long wavelength red and does not allow normal colour opponency to take place (from Harding 1998) Since the introduction of the ITC Guidelines, there have been 13 further incidents of proven seizures (ITC, personal communication). These incidents were related to contravention of the Guidelines, and during this period, we evaluated and supervised the correction of >45 advertisements and programs. It has since become apparent that the ITC guidelines and similar ones produced by the BBC and based on Harding and Jeavons (2) are unique. Although the United Kingdom has maintained this control of broadcast material, both public and commercial, there has been no restriction in other areas of the world or on video- or computer-games material (3). In December 1997, an incident demonstrated the severe risk of provocative broadcast material. During a Japanese broadcast of a cartoon, Pocket Monsters, viewers, mainly children, began having seizures. Approximately 685 such incidents occurred, and viewers were detained in hospital overnight. Viewers and oouv sec EVENT BLACK AND WHITE COLOUR FIG. 6. The response in one patient to the critical sequence of the Pocket Monster cartoon shown in black and white and in colour. It can be seen that only the colour version of the cartoon produces a photoparoxysmal response, no abnormality being produced to monochromatic black and white form (From Harding 1998). Epilepsia, Vol. 4, Suppl. 4, 1999

5 TELEVISED MATERIAL AND PHOTOSENSITIVE EPILEPSY 69 parents complained that 15 min into the program, flashing lights and rapid colour changes provoked the seizures (Fig. 4). TV Tokyo, the broadcaster of this program, estimates the viewing population as 1 million, and it is watched by 55% of school children in Japan. The offending program was analysed frame by frame at the request of TV Tokyo and the Japanese government. If the ITC/BBC guidelines had applied in Japan, the program would have contained 18 contraventions. Some contraventions involved luminance changes at more than three per second. However, long sequences also occurred in which long-wavelength saturated-red stimuli of lower luminance alternated with higher luminance blue stimuli on a frame-by-frame basis at 12/s. The change in luminance was only from 45.6 c&m2 (red) to 7.2 c&m2 (blue). Our own studies in the United Kingdom have shown that colours are not more provocative than white stimuli (1,6). However, in Japan, Takahashi and his group (12,13) showed that red is a more provocative colour. Binnie et al. (9) neatly showed that this difference was due to the difference in wavelength of the red stimuli used. The U.K. studies were matched to red-cone function (5 nm), whereas the Japanese studies used longwave-length red at >6 nm, which would stimulate red cones in isolation and prevent natural colour opponency. Spectral analysis of the frames in the cartoon sequence showed that the red frames peaked sharply at 625 and 74 nm, correlating with the use of the red gun of the TV CRT in isolation. The blue frame had a single peak at 452 nm, correlating with the blue gun of the TV. Although the blue and green guns of the TV approximate well the absorption spectra of retinal cones, the red gun does not (Fig. 5). Thus whereas the blue colour will produce inhibitory responses from both green and red cones to compensate for blue-cone excitation, the longwave-length red will fail to evoke inhibitory responses from the blue and green cones. Because it was not clear whether the change in colour or the counterphased change in luminance was critical, the offending sequence was shown to eight photosensitive patients while their EEGs were recorded. -We minimised risk of seizures by using either patients who were not markedly photosensitive or those receiving valproic acid (VPA). The cartoon sequences were presented on a PAL system TV in monochromatic or colour mode in an ABBA design to avoid order effects. No patient showed abnormality to the monochromatic version, but seven of the eight patients showed abnormalities varying between posterior abnormalities and PPRs to the colour sequence (Fig. 6). It is therefore clear that colour was a critical factor in this incident, and ITC/BBC guidelines have been altered to outlaw changes using the isolated redgreen of the TV at rates above three per second. The unprecedented magnitude of the number of reported seizures is not so surprising. In countries using NTSC systems, only 15% of patients viewing at normal distance will be provoked by TV. In Europe, TV media precipitate a significant number of seizures each year. Countries using NTSC therefore have a latent unprovoked population waiting for the wrong stimulus. At the rate of change used in the Pocket Monster program (12-15 fps), % of the photosensitive population would be at risk to a luminance change. We do not know the comparable risk to a change in nonopponent chromicity, but both Takahashi et al. (13) and our results (3) indicated that the risk is not likely to be less. In addition, the program s 1 million viewers included 55% of schoolchildren in Japan. Both TV Tokyo and NHKTV, which had 16 reported seizures for a different cartoon, immediately instigated provisional guidelines based on those of the ITC and now have formal guidelines incorporating the colour restriction, which will be followed by the other Japanese stations. Other countries need similar guidelines, particularly those with NTSC systems, although these are inherently safer. Acknowledgment: We are grateful to the Japanese Epilepsy Association for financial support. Ian Fawcett and Paul Furlong provided technical assistance, and we are as usual most grateful to them. We are grateful for TV Tokyo for their cooperation, and to Nature (Medicine) for the use of Fig. 5 and 6. REFERENCES 1. Jeavons P, Harding GFA. Photosensitive epilepsy: clinics in developmental medicine, No. 56. London: Spastics International Medical Publications, Harding GFA, Jeavons P. Photosensitive epilepsy. (New Edition). London: Mac Keith Press, Harding GFA. TV can be bad for your health. Nat Med 1998;4: Fish DR, Quirk JA, Smith SJM. National survey ofphotosensirivify and seizures induced by electronic screen games: Interim findings. London: Department of Trade and Industry, Harding GFA, Edson A, Jeavons PM. Persistence of photosensitivity. Epilepsia 1997;38: Wilkins AJ, Stefansson SB, Jeavons PM, Harding GFA. Television epilepsy: the role of pattern. Electroencephalogr Clin Neurophysiol 1979;47: Harding GFA, Jeavons PM, Edson AS. Video material and epilepsy. Epilepsia. 1994;35: Independent Television Commission (ITC). Guidance note; use of flashing images or repetitive patterns. London: ITC, BinnieCD, Esterez ; Kasteleyn-Nolst Trenit6 DG, Peters A. Colour and photosensitive epilepsy. Electroencephalogr Clin Neurophysiol 1984;58: Drasdo N. The neural representation of visual space. Nature 1997; 266: Wilkins AJ. Neurophysiological aspects of pattern sensitive epilepsy. Brain 1979;12: Takahashi T, Tsukahara Y. Influence of colour on the photoconvulsive response. Electroencephalogr Clin Neurophysiol 1976;41: Takahashi T, Tsukahara Y, Kaneda S. Influence of pattern and red colour on the photoconvulsive response and photic driving. Tohoku J Exp Med 1981;133: Epilepsia, Vol. 4, Suppl. 4, 1999