(Warning! Do not view if you have epilepsy.)
Steady State Visually Evoked Potential (SSVEP) are brain signals which occur in response to visual stimulation. These signals may be triggered by any repeatedly flashing light, such as your computer screen refreshing every 60Hz.
Viewing a flashing light at a particular frequency stimulates the visual pathway, causing this frequency to radiate throughout the brain. This stimulation produces electrical signals at both the base frequency and multiples thereof. For example: if you view an image flashing at 5hz, your brain should produce frequencies at 5Hz, 10Hz, 15Hz, …, etc.
Typically, SSVEP is only identifiable between 5Hz – 60Hz, with individual physiology determining particularly responsive frequencies. Inside the appropriate range, SSVEP is an excellent Brain-Computer Interface (BCI) tool because it has a very low signal to noise ratio, meaning even if an individual shifts and disrupts the EEG signal slightly, the SSVEP signal is still likely to come through.
These signals can be identified using Electroencephalography(EEG), which is a recording of electrical signals measured on the scalp.
Processing these signals makes it possible to identify various components of the EEG signal. For example, the EEG signal shown below is mostly made up of the frequencies 10Hz, 13Hz, 15Hz, 21Hz, 25Hz, 35Hz, and 38Hz.
The differences in the underlying signals when viewing and not viewing a flashing light allows for easy identification using SSVEP.
There are a number of uses for SSVEP technology, perhaps most notably in cases of paralysis victims who can use these systems to interact with their environment.
One such use case is a text-based spelling system, which uses flashing lights to identify various words. Individuals such as Stephen Hawking who suffer from Lou Gehrig’s disease or similar conditions could use this to communicate so long as brain control is intact.
Another interesting (though admittedly less useful) case of SSVEP being employed is a game of checkers using a robot!