Creating the Feedback Loop: Closed-Loop Neurostimulation

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Key points

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    Closed-loop stimulation may be superior to open loop therapy by reducing the impact of DBS on cognitive processes that depend on coordinated neuronal oscillations.

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    Understanding the relationship between the gross patient behavior (or severity of disease) and a neuronal signal that is under the influence of external stimulation is fundamental to using the signal in a control system.

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    A closed loop system extracts a particular feature of a biological signal that has a desired reference value

Rationale for closed-loop stimulation: Parkinson disease

Open-loop DBS is effective for treating the motor signs of Parkinson disease, but the side effects of this therapy and its inefficiencies may be diminished within a closed-loop system. Side effects of open-loop DBS experienced by some patients include impaired cognition, speech, gait, and balance.15 Open-loop DBS could potentially disrupt decision making, learning, and cognitive association through its effect on LFP oscillations of the brain. Open-loop DBS therapy was developed before an

Restoring the desired state: a role for closed-loop stimulation

A central challenge for closed-loop therapy is the definition of a therapeutic or optimal state that neurostimulation attempts to maintain or restore. Therefore, closed-loop systems incorporate a single or multiple set points, that is, reference values corresponding to the desired state. Returning to the example of optimizing behavioral goals, each of these set points could correspond to a different behavioral intention, such as walking, talking, or writing.

The goal of closed-loop DBS in

Available signals

The availability of reliably detectable biosignals capable of driving feedback is essential to a closed-loop neuromodulation system. In current open-loop systems, the patient’s clinical status and the provider’s assessment of the clinical status via physical examination provides the feedback to regulate neuromodulation. Although effective and the basis for newer neuromodulation models, this approach may be overly subjective/observer dependent, time intensive, overly consumptive of battery

Feature Extraction

The purpose of feature extraction is to transform the time series data for successive processing and/or improved computational efficiency. For example, time series data could be transformed from the time domain into the frequency domain, thus changing the meaning of the data stream from when an event occurs to how frequently an event occurs. Neuronal ensembles may use frequency coding to communicate, and therefore transformation of data into the frequency domain can be considered translation of

Closing the loop: DBS parameter interventions

Current DBS system neurostimulation consists of delivering a train of biphasic pulses with adjustable parameters (amplitude, pulse width, and frequency). This train is spatially applied across a cathode and an anode that are adjusted according to the size of the stimulation electrode array. The charge is deposited at the cathode, the negative pole, and the current flows from the cathode to the anode.132 Safety of the DBS is ensured because of a net zero current application across the biphasic

Alternate applications

Although this article has focused on movement disorders, the principles involved in system design, signal sources, feature extraction, signal classification, and effector control are applicable to virtually any neuropsychiatric disease neuromodulation system under development.

The most work on closed-loop neurostimulation has been done in the area of epilepsy and signal classification algorithm design for prediction of seizure onset.144 Such work has led to the first human implanted seizure

Closing remarks

Closed-loop neurostimulation is an interdisciplinary science incorporating disciplines of clinical neurosciences and electrical engineering. Given the importance of neuronal oscillations in the cooperative functioning of brain ensembles, the appeal for a neurostimulation system with a small electrical footprint is evident. Thus, closed-loop stimulation is preferable to open-loop stimulation for its less disruptive impact on cognitive processes that depend on coordinated neuronal oscillations.

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