Why do some images cause seizures while others do not?

A new study has investigated why it is that some images can cause seizures in people with photosensitive epilepsy while other images don’t.

We know that in people with photosensitive epilepsy, flashing lights can cause seizures. The impact of such provocative visuals can be quite staggering. In 1997, for example, a certain Pokémon episode triggered seizures in 685 people in Japan, and in 2012, the promotional video for the Olympics had to be taken down from the website because it caused seizures in multiple epileptic patients.

However, seizures can also be caused by static images, with no motion or flicker. New research set out to examine why that is. A team of international researchers reviewed the literature available in the field of neurophysiology to see if the neural responses in a healthy visual cortex can predict how people with photosensitive epilepsy might respond to static images.

The study, led by Dora Hermes, of the University Medical Center (UMC) Utrecht in the Netherlands, and published in the journal Current Biology, has looked into the role of gamma oscillations in the brain.

Their review focused on gamma oscillations induced by the spatial features of some static images, such as those depicting black and white bars.

The repetitive pattern of brain activity of gamma oscillations takes place when people are exposed to these images. In fact, the authors note that these images can cause headaches and migraines in photosensitive people as well as discomfort in perfectly healthy people.

“Our findings imply that in designing buildings, it may be important to avoid the types of visual patterns that can activate this circuit and cause discomfort, migraines, or seizures. Even perfectly healthy people may feel modest discomfort from the images that are most likely to trigger seizures in photosensitive epilepsy.” Said Dr Hermes.

Gamma brain oscillations can be measured with the help of a simple electroencephalogram, and they have been known to scientists since the 1980s. However, researchers have not yet agreed on the role that these oscillations play in perception, thought, or generally in neural processing.

Gamma oscillations occur in the brain only upon viewing certain images, however, which runs counter to the hypothesis that they may be key to neural processing. For instance, images of grating patterns cause strong gamma oscillations, but images of clouds or natural landscapes do not. Why this happens remains largely unknown.

The authors conclude that these grating patterns are most likely to induce seizures, and they suggest various ways in which the images can be adjusted so as to avoid producing gamma oscillations in the brain.

“The likelihood that a [photosensitive seizure] is induced by viewing a grating can be reduced by decreasing the size of the grating [pattern], by reducing the contrast, by superimposing a second grating [pattern] to create a plaid or checkerboard, or by superimposing noise. Both sine and square wave gratings are provocative whereas chromatic contrast alone […] is not.”

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