The measurement and modulation of brain electrical activity have been investigated for close to a century. In general the lower-frequency endogenous potentials (e.g., 1-10 Hz) have been linked to inhibition and suppression, and are more prevalent during diminished arousal levels and sleep, while a shift to higher frequency activity, such as beta (e.g., 12-30 Hz) and gamma (e.g. 30-80 Hz) band activity, is associated with increased arousal and excitation. Neurostimulation which alters these endogenous frequencies in areas of the brain can be used to modulate the relative arousal level of various brain structures, in the treatment of disorders. Different brain disorders can be associated with deviations in the activity of a particular region of neural tissue compared to those of healthy brains, and stimulation may attempt to normalize or compensate for this activity. Neurostimulation may utilize a wide range of signals such as pulsatile or sinusoidal waveforms, which can be provided using low or high repetition/modulation rates. Recently, slower frequency neurostimulation has shown promise in the treatment of different disorders. Although therapy for a disorder may be obtained by neurostimulation, side-effects due to disruption of endogenous activity may also result. These can include alterations in processes related to learning, cognition, memory, and attention. Certain side-effects are more likely with neurostimulation signals which have a primary component which occurs at the same frequencies as endogenous signals, especially below 15 Hz. Adjusting characteristics of the stimulation signal, in relation to those of endogenous signals, for example, in order to match or avoid matching certain characteristics of the endogenous signals, may be increasingly important when providing therapy at these lower rates of stimulation. Another solution is to rove a parameter of the neurostimulation signals, such as the dominant frequency of pulse repetition rate, to such an extent that a particular type of stimulation does not continuously interfere with endogenous potentials. Unlike conventional neurostimulation protocols which set the stimulation parameters and then provide stimulation in a consistent manner, the current invention describes different methods of roving the stimulation parameters so that the stimulation signals alternate regularly over time.
Roving of stimulation parameters can be used to address a number of well-known factors which impede treatment. For example, since most neurological disorders comprise a cluster of symptoms roving can be designed so that the stimulation treatment is provided across time in a sequential manner in order to intermittently deter the emergence of different symptoms. Further, similar symptoms may be related to different disorders, and have different underlying biological causes, each of which can be addressed by roving the stimulation parameters to using parameters which have been empirically shown to decrease these symptoms in question. There may be different mechanisms behind different symptoms of a disorder which require relatively different treatment approaches. The treatment of epilepsy, provides an illustrative example, since therapy has been successfully provided using both slow (e.g. 1 Hz) and fast (e.g. 50 Hz) stimulation rates. Neurostimulation at lower and higher frequencies may work via several mechanisms such as depolarization blockade, synaptic inhibition/depression, and modulation, such as entrainment or suppression of endogenous activity and modulation of brain networks. The correct adjustment of neurostimulation parameters for the treatment of a wide array of disorders may depend upon multiple factors, and a consideration of the advantages of different stimulation strategies and signals is important. Strategies which alternate, rove between, or synchronously provide two or more stimulation signals may serve to modulate the brain in different manners and likely offer a number of advantages over chronic stimulation with a particular signal.