It is well known that flashing images can trigger a seizure in people with photosensitive epilepsy. However, some still images can have the same effect and the reason why is still poorly understood.
Dr Dora Hermes from the Netherlands was one of the researchers working on this study. She and her colleagues looked at previous studies into the brainwave patterns caused by different images in people with and without epilepsy. They wanted to try to get clues about the mechanism by which some images may trigger seizures in people with photosensitive epilepsy.
They found that some still images with particular patterns can cause a particular type of brain activity in people without epilepsy. This brain activity is called ‘gamma oscillations’. The researchers said it is caused by patterns like black and white bars or stripes, but not by other, more natural images. They added that these types of images may also cause migraine headaches and discomfort in people even when they don’t have epilepsy.
The researchers found that these types of stripe-patterned images are also more likely to trigger a seizure in people with photosensitive epilepsy. They believe the same brain network is involved in the response to the images in people with and without photosensitive epilepsy.
Scientists are divided over what the purpose of the ‘gamma oscillation’ brain pattern is and they are not sure why striped images cause it to happen.
The researchers explained that not all stripes and striped images are a problem. They said that vertical stripes are more likely to affect people than horizontal ones. Looking at some everyday things such as escalators, radiator grills or striped clothes may sometimes cause a reaction. However, they add that wider stripes or less sharp contrast between them dampens the effect.
There is more information on photosensitive epilepsy on the Epilepsy Action website.