TABLE 3.

Control Systems Description

Control mode	Feedback type	Description of feedback	Time constant of activation
Continuous	Clinician observation	Clinician manual adjustment	Monthly or frequency of clinic visits
Scheduled Intermittent	None	Preset stimulation amplitude turned on or off at preset timing	Preset timing determined by system physiology or empirically
Responsive	Triggered by threshold event	Preset stimulation amplitude turned on or off by trigger (with defined lockouts)	0.5-5 s, can be repeated
Adaptive	Single biomarker input, continuous monitoring	Stimulation output can be turned on or off, or scaled, for continuous adjustment	Tremor ∼ 10 s Rigidity, Gait ∼ 60-90 s
Closed-Loop	Multiple channels of input biomarkers for continuous analysis	Continuous prediction of brain intent for action	20-50 ms for information update

The use of biomarkers can be described in various approaches, including continuous and intermittent. Responsive and adaptive show progressively more flexibility in when to perform stimulation (ie, triggered by an event or threshold) and adaptive has inherently further flexibility in prolonged stimulation and levels of stimulation when on. Closed loop can apply to any scheme where a feedback signal is used to alter stimulation, but commonly is used in a brain-machine context. The chart gives the type of feedback which can be used, the nature of the feedback and time constants to be considered in delivering the feedback.