Professor G.F.A. Harding Photic Hazards at Work and at Leisure - 8 June 1998

Photosensitive Epilepsy occurs in approximately 1 in 4000 of the population (Harding & Jeavons

1994). The mean age of onset is around puberty, 90% of patients experiencing their first seizure by

the age of 20 years. It is more common in girls than boys (1.7- 1) and it is now clear that 75% of

patients remain photosensitive for life, the other 25% losing their photosensitivity in their 20s

(Harding et al 1997). The condition has a marked genetic factor, 25 '3% of children of photosensitive

mothers showing evidence of photosensitivity in the laboratory (Harding et al 1997).

Relatively few seizures are precipitated by natural phenomena such as sunlight through trees, or

reflected from surfaces such as water or snow. It has long been recognised that in Europe the

television monitor is the most common precipitant of photosensitive seizures (Harding & Jeavons

1994) whether the monitor is used for broadcast, video or computer game material (Hading et al

1994). Other sources of provocative stimuli in the urban environment are discotheques, cinema

("Titanic") light off shiny surfaces in supermarkets, sunlight through tress, chimneys and railings

whdst travelling and patterns such as bars on the stairs of escalators. Recently the importance of

pattem sensitivity has been recognised, less commonly in the environment but particularly when

combined with TV presentation (Harding et al 1994).

Provocative stimuli in the working environment are relatively few. There is little evidence of seizures

produced by photic stimuli whilst driving, seizures usually occuring in passengers who are free to

look towards peripheral flickering sunlight occuring at the side (Harding & Jeavons 1994). However

the normal driving license restrictions apply. Even more restrictions are found in employment as

merchant seamen or train drivers. The situation with commercial pilots is complex, exclusions apply

to any medical history or clinical diagnosis of epilepsy or any evidence of spontaneous or induced

paroxysmal discharges in the EEG.

Computer monitors present relatively few risks since the industry standard refresh rate is 70- and

the tubes have long persistence phosphors. However, high contrast patterns presented on this media

can still be prevocative to these who are pattem sensitive, and this includes the majority of patients

with photosensitive epilepsy.

1 0 1998 The Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WC2R OBL, UK. 4/1

In the U.K. therefore the most common precipitant is the domestic (or commencal) T.V. The monitor

is an inherently provocative stimulus since its basic frequency causes flicker or changes in luminance.

In Europe the frequency of this change is at 50Hz (PAL). Whereas for most of the world the rate is

60Hz (NTSC). At 50Hz 49% of patients are sensitive and will produce photoparoxysmal responses

(PPR) in the EEG, whereas at 60Hz only 15% of patients are sensitive (Harding & Jeavons 1994).

This marked difference in sensitivity may explain the relatively low incidence of the condition in

North America. Since the TV picture is made up of a raster of 2 alternating lines there is also flicker

present at half the basic frequency i.e. 25Hz for PAL and 30Hz for NTSC. There are similar risks

for the photosensitive population at these two frequencies, 76% at 25% and 72% at 30Hz (Harding

1998). The alternating lines of the picture can only be resolved at relatively short viewing distances,

typically around one metre.

With IPS in the laboratory the relative sensitivity of the photosensitive population can be established.

Most patients are sensitive around 16 flashes per second, but a large majority are sensitive at 25 and

30 flashes per second. These two frequencies are also the frequency of the "frames" or individual

pictures which successively make up the moving picture in PAL and NTSC. In addition many

computer generated cartoon sequences are produced at 'film rate' of 24 frames per second. Thus it is

apparent that the risk from all monitors is not just the construction of the picture inherent in CRT

displays but the rate of change of material within successive frames. The change may of course be

temporal (intermittent flashes) or spatial (patterns in cycles per degree of vision).

In 1993 an advert was shown on commercial TV in the UK (Golden Wonder, Pot Noodle), which

produced 3 proven complaints of seizures. This advert was made up of rapidly changing high

contrast film clips often with picture reversal from frame to frame. Due to the unprecedented number

of seizures the Independent Television Commission (ITC), the UK statutory regulatory body for

commercial television, requested that we produce draft guidelines to prevent future screening of

provocative material (ITC 1994). The rate of luminance change both in temporal and spatial terms

was constrained. It is obviously not possible to produce a no-risk guideline since TV is inherently a

provocative media for any material and luminance changes must occur. If however these luminance

changes are restricted to a maximum of 3 per second less than 3% of the photosensitive population are

at risk. Using known UK incidence figures (Fish et al 1993) this would produce a risk of 1:3 million

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of the population which was considered. The most provocative image change is one in which frames

of the picture altemate since this will produce the highest flash rate (12.5 f p s PAL & 15 f p s NTSC).

However from our previous studies of pattern sensitivity (Wilkins et al 1979; Harding et al 1994)

changes which only occupy part of the screen were also considered provocative. High contrast bars

which alternate were considered to be the most provocative condition (Wilkins et al 1979) but patterns

which drift are not (Binnie et al 1984).

It has since become apparent that the ITC guidelines and similar onces produced by the BBC and

based on Harding & Jeavons (1994), are unique. Although the UK 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 (Harding 1998).

In December 1997 an incident occured which demonstrated the severe risk of provocative broadcast

material. During a TV Tokyo broadcast of a cartoon "Pocket Monsters" viewers, mainly children,

began having seizures. Approximately 685 such incidents occurred and about 200 viewers were

detained in hospital overnight. The viewing population was estimated at 10 million and it is watched

by 55% of school children in Japan. The program would have contained 18 contraventions of ITC

Guideline. Some contraventions involved luminance changes at more than 3 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 per second. The change

in luminance was only from 45.6 cdm (red) to 70.2 c d m (blue).

Studies in the UK have shown that colours are not more provocative than white stimuli (Jeavons &

Harding 1975, Wilkins et a1 1979). However in Japan Takahaski and his group (1976, 1981) have

shown that red is a more provocative colour. Binnie et al(1984) neatly showed that this difference

was due to the difference in wavelength of the red stimuli used, the Japanese studies using long

wave-length red at more than 600nm 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

625nm and 704nm correlating with the use of the red gun of the TV CRT in isolation. The blue frame

had a single-peak at 452nm correlating with the blue gun of the TV. Although the blue and green

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guns of the TV approximate well to the absorption spectra of retinal cones, the red gun does not

(Harding, 1998).

Since it was not clear whether the change in colour or the counterphased change in luminance was

critical the offending sequence was shown to 8 photosensitive patients whilst their EEGs were

recorded. 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 7 out of the 8 patients showed abnormalities in their EEG to the colour

sequence (Harding, 1998).

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 we precipitate with TV media 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 (12 - 15 fps) approximately 80% of the photosensitive population would be at risk. In

addition the program's ten million viewers included 55% of school children in Japan. Japanese TV

stations 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.

References

HARDING, G.F.A., EDSON, A., JEAVONS, P.M. (1997) Persistence of Photosensitivity. Epilepsia 38 (6) : 6 63 - 6 6 9

HARDING, G.F.A., & JEAVONS, P. (1994), Photosensitive Epilepsy, New Edition London: Mac Keith Press

HARDING, G.F.A., JEAVONS, P.M., EDSON, A. (1994) Video Material & Epilepsy, Epilepsia, 35(6): 1208- 121 6

HARDING (1998), TV Can Be Bad For Your Health, Nature Medicine, 4, 265-267

FISH,D.R., QUIRK, J.A., SMITEI, S.J.M., SANDER, J.W.A.S., SHORVAN, S.D.,ALLEN,P.J., (1993/5??), Incidence of photosensitive epilepsy: A prospective national study. Electroencephalogr Clin Neurophysiol 95:4260-7

WILKINS, A.J., DARBY, C.E., BINNIE, C.D., STEFANSSON, S.B., JEAVONS, P.M., HARDING, G.F.A. (1979) Television Epilepsy: The role of the pattern. Electroencephalogr Clin Neurophysiol 47: 163-7 1

B I " , C.D., ESTEREZ, O., KASTELEYN-NOLST TRENITE, D.G., PETERS, A. (1985) Colour and photosensitive epilepsy. Electroencephalography and Clinical Neurophysiology 58, 387-39 1

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