Effects of strobe lighting

[aka pixi]

not sure if this is right category but here goes anyway.

Last night me and my teenage son were watching a film. The opening credits had a few seconds of really mental fast strobing. The effect it had on us was crazy. Both of us felt like our eyes and our brains were shaking/vibrating back and forth really fast. Neither of us has epilepsy and neither of us have had any reactions to strobe or flash photography etc before. It was a horrible feeling. How on earth does it cause that to happen?

[fo3nix]

Which film?

These effects are usually down to the fact that your brain is trying to understand what's going on, and is being (deliberately or not) confused.

[Clanad]

A similar effect is caused by driving through forests with the light flashing through the trees. Additionally, the "strobe" effect can occur during helicopter operations and, in it's worst out come can cause vertigo. Called Flicker Effect, it can trigger epileptic seizures known as Photosensitive epilepsy or PSE.
Wikipedia adds "...The exact spacing of a pattern in time or space is important and varies from one individual to another: a patient may readily experience seizures when exposed to lights that flash seven times per second, but may be unaffected by lights that flash twice per second or twenty times per second. Stimuli that fill the entire visual field are more likely to cause seizures than those that appear in only a portion of the visual field. Stimuli perceived with both eyes are usually much more likely to cause seizures than stimuli seen with one eye only (which is why covering one eye may allow patients to avoid seizures when presented with visual challenges). Some patients are more sensitive with their eyes closed; others are more sensitive with their eyes open.

Sensitivity is increased by alcohol consumption, sleep deprivation, and other forms of stress."

[heathfield]

For the reasons Clanad explains, it's also why TV news presenters warn viewers that 'The following item contains flash photography'.
At one time, it was also a problem caused by the rate at which flashing direction indicators worked on some cars. It was found that these could bring on epilepticseizures in some people, and so regulations were introduced putting limits on their timing.

[aka pixi]

Thanks everyone. Although I knew a bit about strobe lights etc/epilepsy there was tons there that I didnt know. One thing I didnt know about was the fact that some people can avoid it by covering one eye, thats a brilliant information that can be passed on.

fo3nix, the film was a hillbilly slasher movie I recorded off tv the other night, I think it was called Backwoods though Im not 100% certain.