Photosensitive Epilepsy

Biology 202
1999 First Web Reports
On Serendip

Photosensitive Epilepsy

Carly Cenedella

Two 13-year-old girls are playing Super Mario Brothers. When the pace of the action picks up in the third straight hour of their play, one girl starts to shake and, for three minutes, has nonstop epileptic seizures (1) .

Six hundred eighty five Japanese people ranging in age from five to fifty-eight suffer spasms, convulsions, vertigo, and breathing difficulty while watching a colorful cartoon program. Doctors determine that most were suffering from epileptic seizures induced by a flashing white light sequence during the show (2) .

In the United States, there are close to one million people with epilepsy-- about 1 in 200 people around the world have epilepsy. For most of those people, video game playing and watching television are not a risky activities. The flashing patterns of certain games and television shows trigger epileptic seizures in only 5% of epileptics (1) . Photosensitive epileptics have with a peak age of onset of 10-14 years, are mostly woman, and experience a decline in the photosensitivity after 25 years of age (3) .

In normal brain function millions of tiny electrical charges pass from nerve cells in the brain to all parts of the body. In patients with epilepsy, this normal pattern is interrupted sometimes by sudden and unusually intense bursts of electrical energy, which may briefly affect a person's consciousness, bodily movements, or sensation (4) . During a seizure, nerve cells in the brain fire electrical impulses at a rate of up to four times higher than normal. This causes a sort of electrical storm in the brain (5) . A pattern of repeated seizures is referred to as epilepsy (4) .

Seizures cause different physical effects depending on which parts of the brain are involved and how far the signals fan out. Some people have violent seizures that knock them to the floor unconscious and twitching. Others experience less severe seizures that may only blank them out for a few seconds or more. Some mild seizures pass so quickly that it seems the person is just daydreaming. But, when these "absences" happen hundreds of times a day, they can be debilitating. Often, people sense that something is about to happen to them right before a seizure. This feeling, called an "aura," makes them restless, irritable, or just vaguely uncomfortable. After a seizure, people often don't remember having the seizure (1) .

The likelihood of a seizure, as well as its type, in photosensitive individuals depends on the intensity, the contrast of the visual stimulus, and the specific frequency of flashing. Extensive EEG studies have shown that a flicker stimuli between 10 and 30 flashes per second induces the generalized epileptiform discharges and the clinical features characteristic of an epileptic seizure particularly well. Television, computer, and video game screens produce a 50 Hz flicker and a vibrating pattern at half the alternating-current frequency, or 25 flashes per second within the 10 to 30 flashes per second range. The vibrating pattern is only visible when sitting close to the television. Therefore, most television-induced seizures occur at viewing distances between 1.5 and two meters. 100 Hertz televisions cause a vibrating pattern of 50 flashes per second and do not induce seizures (3) .

Video game seizures and television-induced seizures occur in people who have heightened sensitivity to pulsing light. It is not uncommon for this so-called photosensitivity to run in families. Some drugs, like valproate, are helpful in reducing photosensitivity and preventing these seizures. Some people simply outgrow epilepsy or are able to evade seizures by avoiding the stimuli that provoke them (1) . Individuals predisposed to seizures may have an increased risk for having a seizure following stress, sleep deprivation, fatigue, insufficient food intake, or failure to take prescribed medications (4) .

Many questions arise for me when studying photosensitive epilepsy. Which area or areas of the brain are affected by the visual stimulus? It seems like the affects can be quite dispersed since the outcome can affect so many functions of the body. The person's muscles are affected. They cannot stand and often twitch. Memory is affected. The person cannot remember having the fit. The description of absences seems to suggest that alertness or wakefulness seem to be affected.

Also, what does a particular frequencies have to do with the signaling in the brain? It seems that a specific frequency, 10 to 30 flashes per second, aptly induce photosensitive seizures. Higher or lower frequencies don't have the same effect. What is it about that rate of stimulus input that affects the brain of photosensitive epileptics? Understanding the frequency effect has vast implications for understanding brain function. The rate of stimuli that a neuron or group of neuron can take in and why could tell a lot about the function of neurons in general. It must have something to do with timing-- perhaps the delay in the voltage-sensitivity in sodium ion channels plays a role.

The model that I am currently assuming in my study of epilepsy is what I call the trigger area dispersion theory. The epilepsy trigger area in the brain of photosensitive individuals is connected to the visual processing area of the brain and is sensitive to a trigger stimulus, which is a light display flashing between 10 and 30 time per second. When the trigger stimulus is presented to the photosensitive individual, the trigger area begins firing abnormally. This abnormal firing spreads to other areas of the brain. The functions served by all affected areas are compromised as the abnormal firing, the seizure, proceeds.

WWW Sources

2)Health Authorities Look into Link between TV and Seizures. Medical Industry Today. December 19, 1997.

3) Trenite, D.G.A. Video Game Epilepsy. The Lancet. October 22, 1994.

4) The National Institute of Neurological Disorders and Stroke: What is Epilepsy?

5) The National Institute of Neurological Disorders and Stroke: Epilepsy