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I see warnings on a variety of things, such as video game, stating that

A very small percentage of people may experience a seizure when exposed to certain visual images, including flashing lights ...

(Quote from: X-Box 360 Photosensitive Seizure Warning).

I have been riding home in the dark lately, and several bikes near me had flashing front or back lights. I don't have epilepsy, but it gave me a bit of a head-ache/eye strain after a while.

Which makes me wonder what it would do to someone who did have photosensitive epilepsy?

It it actually enough to cause a seizure, or would an epileptic person just feel the same eye strain as I do?

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Note: Part of this answer comes from personal experience. Judge as you wish.

I have a condition similar to epilepsy, that has caused me to experience a number of seizures over time (ranging from simple partial seizures to full convulsive tonic-clonic seizures, also called "grand mal" seizures). This means that over the past few years, I have taken numerous tests, including EEGs.

The basic principles of an electroencephalograph (EEG) test (also sometimes called an electroencephalogram) are well-known and can be looked up with ease. However, they do have specific parts designed to test specific potential triggers - you don't simply lie down in a darkened room for half an hour with wires strapped to your head.

One of these trigger tests involves a strobe light. The patient, lying in a darkened room, is instructed to close their eyes, if they have not already been doing so as per the instructions of the doctor, nurse, or technician in charge. A small light - sometimes jokingly referred to as a "bug light" - is then placed over the patient's eyes, at a distance of about one foot. It is a compound device, consisting of many small LEDs.

The person in charge then begins the test, which goes like so:

  • The light flashes on and off for about ten seconds fairly quickly.
  • There is a ten second pause.
  • The light flashes again for ten seconds, at a quicker speed.
  • There is a ten second pause.

This pattern repeats for some time, with the flashing getting quicker and quicker. It doesn't take long until it is impossible for the patient to tell whether the light is blinking or continuously shining, and even through closed eyelids, it is extremely bright. I've been told to not open my eyes during the test, as it could blind me. I don't know whether or not this is true, but I'm not keen to test it out any time soon.

In someone with photosensitive epilepsy, this light might trigger some sort of seizure - of any strength - or at least increased brain activity. While this has never happened to me, it does happen to people.

I wouldn't compare the experience to looking at bicycle lights like the ones you name, and for a few reasons:

  1. The bicycle lights are nowhere near as bright as the "bug light".
  2. The bicycle lights do not flash as quickly as the EEG light does. The Epilepsy Society notes that

    Between 3-30 hertz (flashes per second) are the common rates to trigger seizures but this varies from person to person. While some people are sensitive at frequencies up to 60 hertz, sensitivity under 3 hertz is not common.

    This Epilepsy Action pamphlet gives slightly different ranges.

  3. The EEG light takes up a much larger portion of a person's field of vision than do bicycle lights, because . . .
  4. . . . the bicycle lights are typically much farther away that one foot. If you're closer than one foot to a bicycle, you're probably too close. Again, the Epilepsy Society says that this could be a problem if the light completely dominates your field of vision:

    Flashing bicycle lights or other LED lights [could cause a seizure], if this creates a high enough flash rate against a dark background, and if the effect fills your vision.

    The Epilepsy Action pamphlet confirms this - if you're too close.

However, people with photosensitive epilepsy can have seizures when faced with lights less intense than the strobe lights I've been confronted with during EEGs (see the links). Those of us without photosensitive epilepsy should be fine; those with it might not.

The bottom line? Yes, it's possible - under certain conditions. There have been some outcries over this - see this article, for example - which has led to some changes. However, these cases occur when the lights are flashing pretty quickly - I haven't been able to find out average rates for bike lights, so any information there would be awesome - and when the person with epilepsy is relatively close to the lights. You don't necessarily have to be one foot away, but being a sensible distance from the bike would reduce the risk.

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