We often associate epilepsy and seizures with flashing lights. But despite this, photosensitive epilepsy (or PSE) is in fact quite rare, occurring in around 3% of all people with epilepsy (Epilepsy Society UK, 2016). For this relatively small proportion of people, seizures can be triggered by certain visual stimulation. The nature of the visual stimulation is specific to each individual, but typically involves particular rates of flashing lights (typically between 3-30 flashes per second) or anything that forms a regular (cyclic) visual pattern (such as a rapidly flickering image, or geometric shapes and patterns moving in a particular direction).
To test whether someone’s seizures can be triggered in this way, a procedure known as photic stimulation can be performed. This involves showing the person with epilepsy various kinds of flashing lights whilst they undergo an EEG (electroencephalogram - typically the first test someone with suspected epilepsy would have - see picture below!) in order to measure electrical responses in the brain caused by the visual stimulation. Photo-sensitivity can be confirmed if the recorded brain signals are abnormal.
An electroencephalogram, image from https://www.britannica.com/science/electroencephalography
Above is an EEG: a number of electrodes are placed on the surface of the scalp in various different positions (to cover as much of the head as possible). The electrodes, which are conductors, allow us to measure tiny electrical signals within the brain from, and between, the different positions at which the electrodes are placed. These signals (the squiggly lines on the left hand side of the picture) are very weak, and thus need to be amplified (using an amplifier) so we can see them better. When the EEG is performed at the same time as the visual stimulation, it is possible to measure the responses produced by the changes in light.
The reason why some people with epilepsy experience seizures as a result of flashing lights (or any cyclic visual pattern) is not fully understood. However, and based on what is known about epilepsy and what we know about the visual system and brain networks, we can try to build up a picture of what might be happening. But before delving into how a seizure might be triggered by a flashing light, we must first understand what an epileptic seizure actually is.
Normally, electrical activity inside the brain (resulting from the firing of billions of cells, known as neurons) is asynchronous. This means that neurons fire at different times to each other, and not in synchrony! In epilepsy, and due to an irritable area in the brain (due to damage, or because the part of the brain has not developed properly), this electrical activity becomes synchronous. This means that large groups of neurons begin to fire at the same time and produces a range of different symptoms over which the person having the seizure has no control. The area from where the seizures arise is known as the seizure focus, or epileptogenic zone (EZ), and the cells here are much more excitable than in other parts of the brain.
The symptoms of seizures depend on where in the brain the seizure begins, and where it spreads to, if at all. A seizure that starts in the part of the brain responsible for speaking, for example, might temporarily disrupt a person’s ability to speak properly. Or, if a seizure comes from the part of the brain involved in moving the right arm, the person having the seizure may experience a twitching or shaking in that arm. These are both examples of focal seizures - one that starts in one side (hemisphere), or specific part, of the brain. Sometimes a seizure might start in both hemispheres, or many parts of the brain: this is known as a generalised seizure. This is the typical kind of seizure that we associate with epilepsy, and involves loss of consciousness, collapsing to the floor and shaking of the limbs or whole body. Focal seizures can also lead to generalised seizures as a result of the abnormal electrical activity spreading to other brain areas.
Now that we know a little bit about electrical activity in the brain and how it is different in epilepsy, we can begin to look at how changes in light might affect the brain and trigger a seizure in this puzzling type of epilepsy. But we first need a very brief primer on how the brain deals with visual information…
The brain is a very complex lump of tissue indeed. Roughly speaking, billions of neurons are interconnected with one another to form a mass web of neural networks. The brain processes visual information by first converting light, received by the eye, into electro-chemical signals. After the eye, these signals then reach the brain where they pass through a series of waypoints before reaching the visual cortex (at the very back of the brain).
Visual pathway of the brain. Image from https://commons.wikimedia.org/wiki/File:Constudeyepath.png
From the visual cortex, these signals are then projected and processed across a range of further regions and networks throughout the brain. It is important to remember that the parts of the brain that deal with vision are very well connected to many other areas.
Connections to the visual cortex in the brain. Image from http://blog.myesr.org/mri-reveals-the-human-connectome/
Repetitive flashing lights, or patterns moving around very quickly in front of our eyes, would have a corresponding effect throughout the visual pathway by exciting neurons. If an area of cortical irritability (seizure focus) was to exist along this pathway (in the visual cortex for example), the neurons here could be activated by this visual stimulation over and over again. Given that the neurons at the site of the seizure focus are irritable - they are hyper-excitable as they possess a lower firing threshold than neurons in other parts of the brain - it is possible that repeated activation of the visual system could disturb the electro-chemical balance within these cells, leading to the passing of some threshold required for a seizure to occur. However, this would only really be applicable for people whose epilepsy originates in the visual cortex, but what if someone with PSE has seizures which are coming from another part of the brain (which can be the case!)? It is also possible that the area of the brain that is producing seizures is connected, maybe via a network of many neurons, to the visual areas in the brain. So instead of being directly excited by visual stimulation, the area where seizures are coming from could be continually activated and disturbed as a result of communication between the visual and other areas or networks in the brain.
A study detailed in Nature, (Porciatti et al., 2000), suggests that the brain mechanisms which help to account for rapid changes in light, and changes in contrast, are either lacking or not working as they should in patients with PSE – one of very few mechanical explanations for PSE.
In conclusion: the reason why flashing lights provoke seizures in people with photosensitive epilepsy is rather poorly understood. But it could be that the mechanisms in the brain which help us deal with very fast changes in light, or quickly moving things, are affected in people with PSE and are not working in their normal way. It is also possible that the place in the brain where seizures are coming from is within, or is in some way linked with, the visual circuits in the brain, and is therefore directly or indirectly disturbed by certain visual stimulation, thus causing seizures.
References and further reading
Epilepsy Society, UK:
Brain network dysfunction in FTD:
Nature paper - Lack of cortical contrast gain control in human photosensitive epilepsy: