When looking at all the promotional material for audio-visual stimulation products you could be forgiven for gaining the impression that brainwave entrainment is the best thing since sesame bagels. Unfortunately ‘entrainment’ has become a catchword that has come unstuck from its actual meaning.

When the brain receives a pulse of sound via the ears, there is an evoked potential visible by EEG in the region of the auditory cortex (temporal region). If this pulse is repeated at a consistent frequency the EEG will show a frequency following response (FFR) in the same region. Under certain circumstances, the conditions for which are unclear, other regions of the brain may fall into lockstep with the auditory cortex and a signal corresponding to the stimulus frequency may be seen in other areas of the cortex. When other areas start following the initial stimulus, then we have the phenomenon correctly referred to as entrainment.

Exactly the same sequence of occurs with visual stimulation in the form of flashes of light. First, there’s the evoked response in the visual cortex and secondary visual areas, then there’s entrainment if areas of the brain not generally associated with vision start following…

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