Certain images can cause several neurons to fire at once, as many as 500 times per second, which can cause seizures in some epileptic patients.
The National Institute of Neurological Disorders and Stroke (NINDS) define epilepsy as a group of neurological disorders in which bundles of neurons signal abnormally, triggering a seizure.
During a seizure, several brain cells may signal simultaneously as many as 500 times per second.
The NINDS also estimate that currently, around 2.3 million adults in the United States, as well as more than 450,000 children and teenagers, are living with epilepsy.
It is known 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 first author of the study is Dora Hermes, of the University Medical Center (UMC) Utrecht in the Netherlands, and it is published in the journal Current Biology .
The role of gamma oscillations in the brain
Hermes and colleagues followed up on previous studies, which have suggested that gamma oscillations in the brain - with a frequency of between 30 and 80 Hertz - may be responsible for generating seizures.
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."
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.
The study's first author explains further, saying, "Some scientists argue that these oscillations are hugely important and essential for awareness, attention, and neuronal communication, while others say that they are more likely a byproduct of normal neuronal processing, like the exhaust coming out of a car - a potentially useful diagnostic signal, but not one that makes the neuronal machinery work."
How images of grating patterns can cause seizures
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."
Study co-author Jonathan Winawer, of New York University, further explains the findings:
"What we distinguish in this proposal is that the link between images that trigger photosensitive epilepsy and normal brain activity is particular to gamma oscillations, and not to other forms or neuronal responses like the overall rate of action potentials," Winawer says.
Hermes and colleagues are currently working on studies that will further investigate these brain patterns in people with photosensitive epilepsy and people without. Additionally, they are creating a model that will hopefully predict which natural scenes are more likely to induce gamma oscillations and seizures.