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Summary: Television (TV) is the most provocative visual stimulus and evokes (first) seizures in susceptible children and adolescents, especially when flickering and patterned images are shown. This has led to the initiative to develop guidelines for broadcasters. The development of new types of TV screens will not remove the need for control of broadcast material. It could be argued that rather than protect the whole viewing audience by application of broadcasting guidelines, only those who are photosensitive should be protected. But maybe we should do both, because most known sensitive patients will benefit from greater safety and will not be dependent on fashionable ideas by commercial broadcasters that are not (yet) familiar with the guidelines. No such guidelines exist for video material, electronic screen games, and the Internet. It would be wise to adopt the guidelines for video material and electronic screen games.
Television (TV) is known to be a common precipitant of photosensitive seizures. In Europe it is by far the most common precipitant; >60% of patients with photosensitive epilepsy experience their first seizure while watching TV (1). Although the TV monitor, whether used for broadcast material, video playback, or electronic screen games, is inherently a flickering medium (2); specific broadcast material is known to be especially provocative. The TV picture is made up of a series of frames that occur at a rate of either 25/s (PAL) or 30/s (NTSC). Rapidly changing material from one frame to the next therefore has the potential to produce temporal flicker (flashes) or spatial oscillations or reversal (alternating patterns).
Because this type of material has been shown to provoke seizures (2), guidelines have been produced by the TV broadcasters in the U.K. to prevent transmission of this type of material. In 1997 the Pokémon incident in Japan (3,4) demonstrated the danger of broadcasting unregulated material. The episode, screened in December 1997, contained periods in which long-wavelength red frames were alternated with blue frames. Six hundred eighty-five persons were admitted to hospital, and further studies have shown that 560 had had definite seizures; of these, 76% had not experienced a previous seizure (5). This particular type of stimulation, which is related to the phosphors of the TV monitor, caused the Japanese TV industry immediately to adopt guidelines similar to those in the U.K. and specifically to warn of the dangers of long-wavelength red flicker. The U.K. guidelines also adopted this regulation.
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Subsequently guidelines were revised (6) and refined, but all regulate flicker, pattern, and the use of long-wavelength red. The adoption of guidelines is now under consideration in Italy, Russia, and has been followed by Disney and ABC in the United States.
In general, the principles of guidelines are as follows:
Flicker. Repetitive flashes of >3/s are not allowed.
High-contrast patterns that alternate, reverse, or oscillate are not allowed if more than five repetitions are present on the screen. High-contrast stationary patterns are similarly controlled but are allowed to occupy a larger screen area. However, patterns that move smoothly across, into or out of the screen, are allowed.
Flashes of long-wavelength red at >3/s are not to be used. In addition, clauses specify the amount of the screen that may be occupied by the flashes or patterns exceeding the guideline limits (typically <25% of screen area).
These guidelines have now received general acceptance by the TV industries in those countries that have adopted them, but two problems remain.
It could be argued that rather than protect the whole viewing audience, only those who are photosensitive should be protected. To achieve this, a device that modifies the TV picture at the receiver is required. However, only those with known photosensitivity realize that they have a need for such a device, and evidence in the U.K. and Japan indicates that many persons who have unrecognized photosensitivity will experience their first seizure while watching TV.
To prevent visually induced seizures due to flicker stimuli from display units such as TVs, Nomura (7) developed an adaptive temporal filter (ATF) able to reduce frame-to-frame flicker stimuli at ∼10–30 Hz, which are particularly risky for photosensitive individuals. A series of clinical investigations were undertaken to evaluate ATF efficacy.
With a problematic scene of the Pokémon incident containing 12-Hz red/blue flicker images lasting for 4 s (5), ATF efficacy was analyzed with a computer simulation in 11 photosensitive epilepsy patients (8). This image, shown with a 14-inch TV set, 2 m in front of subjects, promptly elicited generalized photoparoxysmal seizures (PPRs) in all cases; the filtered image did not induce any PPR.
Based on these results, a production ATF system (ATFS) was made for the purpose of realizing its clinical usefulness. With the same Pokémon scene, ATFS efficacies were studied in 12 photosensitive epilepsy patients (9). A nonfiltered Pokémon scene elicited generalized PPR in all patients, whereas a filtered Pokémon scene exhibited no PPR induction except in one case.
Because the problematic Pokémon images contain neither geometric pattern nor geometric pattern flicker stimuli, a 4-cycles/degree vertical stripe 15-Hz flicker video was produced to test ATFS efficacy on TV images with geometric pattern flicker. With this pattern flicker scene, ATFS function was studied in 13 photosensitive epilepsy patients (10). Pattern flicker stimulation promptly elicited generalized PPR in all patients, whereas a filtered image did not induce any PPR.
This series of clinical studies suggests that adding an ATFS device to display units may be useful as a preventive measure for photosensitive seizures triggered by flickering images, be they chromatic flicker or pattern flicker, from TV or other displays, in patients who have already suffered TV-induced seizures.
Automatic analyzer of broadcast material
An alternative method is to protect all viewers, as encapsulated in TV guidelines. The problem is that checking broadcast material for compliance with the TV guidelines is a laborious and time-consuming activity for the human observer. Typically the material must be viewed frame by frame. With PAL television, 25 frames exist in each second of material. With NTSC, there are 30 frames/s. One minute of broadcast material typically takes between 30 min and 1 h to evaluate by a human observer. What is therefore required is an analyzer that can assess broadcast material in real time (i.e., 1 min in 1 min). Such a computer-based automatic analyzer has now been developed (Cambridge Research Systems, Rochester, U.K.). The machine analyzes the input on a frame-by-frame basis in real time and assesses compliance independently in terms of flashes, pattern, or long-wavelength red. Failures are assessed as mild, moderate, or severe. The analyzer carried out computations of the screen area of the flash or pattern as well as the intensity contrast and temporal and spatial frequency. The exact frames at the start and end of the offending sequence are identified and are displayed as a film scroll that can be moved backward and forward in conjunction with the assessment output (Fig. 1). Because it is computer based, subsequent changes in guidelines can easily be incorporated, as well as compliance to specific guidelines in different countries. The machine does not fail to recognize and approve objects or patterns that show small movement across the screen or toward and away from the observer (2,11).
Figure 1. The CRS Flash Pattern Analyser. The film strip at the bottom of the illustration shows the frames preceding and following the point of analysis, which is marked with a red outline and shown as a red vertical line on the graph. The graph shows the categories of Pass or Failure. The failure in this case is a luminance flash failure in which the previous frames, in addition to the current frame, have exceeded the guidelines. The time code of the material being analyzed is indicated on the abcissa of the graph.
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In addition, the analyzer is programmed to warn of potential infringements when stimuli are presented just within the limits of the guidelines (say 3 flashes/s) for a prolonged period, as this is judged not to be risk free.
The availability of such machines to the industry has removed the final hurdle in compliance with these important guidelines. Material can be checked in real time, failures identified for correction, and the material rechecked. The amount of broadcast material is no longer critical, and it is even possible to monitor broadcasts online.
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With the increasing numbers of TV broadcast stations, independent monitoring becomes increasingly important. In the U.K., the guidelines are part of the program code covering terrestrial, cable, and satellite broadcasts. It should be emphasised, however, that no such guidelines exist for video material, electronic screen games, or anything that is not broadcast. Some interest exists in providing similar guidelines for the Internet, and it is to be hoped that video material and electronic screen games manufacturers also will consider the adoption of such guidelines.
The development of new TV monitors such as plasma screens, although removing some of the problems of inherent flicker of present TV monitors, will not of course remove the need for control of broadcast material, because the presentation of such material will remain the same with the same potential risks.