3. The brain has response susceptibilities that mostly originate from its intrinsic (ie, spontaneous) rhythmic activity.15,38-41 A brain system responds to external or internal stimuli with those rhythms or frequency components that are among its intrinsic (natural) rhythms. Accordingly, if a given frequency range does not exist in its spontaneous activity, it will also be absent from its evoked activity. Conversely, if activity in a given frequency range does not exist in evoked activity, it will also be absent from spontaneous activity.
