When the brain receives an external stimulus, through ears, eyes or any other senses, it emits a response which is called a 'cortical evoked response'. These electrical responses travel throughout the brain and they can be measured using sensitive devices attached to the scalp.
When the rhythmic stimulus presented to the brain is, for example, a drum beat, the rhythm is reproduced in the brain in the form of a 'cortical evoked respose'. If the rhythm becomes fast and consistent enough, it can start to resemble the natural internal rhythms of the brain, called 'brainwaves'. When this happens, the brain starts responding by synchronizing its own electric cycles to the same rhythm as the drum beat. This is usually called a 'Frequency Following Response' or FFR.
