Feedback control of neurostimulation
Controlling a neural stimulus comprises applying the neural stimulus to a neural pathway in order to give rise to an evoked action potential on the neural pathway, the stimulus being defined by at least one stimulus parameter. A neural compound action potential response evoked by the stimulus is measured, and from the measured evoked response a feedback variable is derived. Loop backoff behaviour is adjusted by applying a non-identity function to the feedback variable to produce a revised feedback variable, and completing a feedback loop by using the revised feedback variable to control the at least one stimulus parameter so as to maintain the feedback variable at a setpoint. Additionally, or alternatively, loop gain can be adjusted in response to a change in the loop setpoint, and/or a logarithm of the stimulus parameter can be controlled.
1 . An implantable device for controllably applying a neural stimulus, the device comprising:
a plurality of electrodes including one or more stimulus electrodes and one or more sense electrodes;
a stimulus source for providing a neural stimulus to be delivered from the one or more stimulus electrodes to a neural pathway in order to give rise to an evoked compound action potential on the neural pathway of a patient;
measurement circuitry for recording a neural compound action potential signal sensed at the one or more sense electrodes; and
a control unit configured to:
control application of the neural stimulus as defined by a stimulus current parameter;
measure via the measurement circuitry the compound action potential evoked by the neural stimulus;
determine from the measured evoked compound action potential a feedback variable; and
implement a feedback controller which completes a feedback loop, the feedback controller configured to use the feedback variable to control a logarithm of the stimulus current parameter so as to maintain the feedback variable at a setpoint.
2 . The implantable device of claim 1 , wherein the control unit is configured to control the logarithm of the stimulus current parameter by:
calculating an error between the feedback variable and the setpoint;
multiplying the error by a control gain to generate a multiplied error; and
adjusting the logarithm of the stimulus current parameter by the multiplied error.
3 . The implantable device of claim 1 , wherein the control unit is further configured to, in response to a change in the setpoint, adjust the control gain of the feedback controller.
4 . The implantable device of claim 3 , wherein the control unit is configured to adjust the control gain of the feedback controller by:
calculating a slope of a control transfer function between the stimulus current parameter and the feedback variable at the changed setpoint; and
adjusting the control gain in inverse proportion to the calculated slope.
5 . The implantable device of claim 4 , wherein the adjusting the control gain comprises dividing a predetermined open loop gain by the calculated slope.
6 . The implantable device of claim 1 , wherein the stimulus current parameter comprises a stimulus current amplitude.
7 . The implantable device of claim 1 , wherein the feedback variable is determined from a measure of an amplitude of the evoked compound action potential.
8 . The implantable device of claim 1 , wherein the feedback variable is determined from the stimulus current parameter.
9 . The implantable device of claim 8 , wherein the feedback variable comprises C=I k V, where I is the stimulus current parameter, V is a measure of an amplitude of the evoked compound action potential, and k is a ratio of an exponent m of a distance-dependent transfer function of stimulation to an exponent n of a distance-dependent transfer function of measurement.
10 . The implantable device of claim 8 , wherein the feedback variable comprises an amplitude of a measured evoked compound action potential that would be obtained from the neural stimulus if the patient were in a reference posture.
11 . An automated method of controlling a neural stimulus, the method comprising:
applying the neural stimulus to a neural pathway in order to give rise to an evoked compound action potential on the neural pathway, the neural stimulus being defined by a stimulus current parameter;
measuring a compound action potential evoked by the neural stimulus, and deriving from the measured evoked compound action potential a feedback variable; and
completing a feedback loop by using the feedback variable to control a logarithm of the stimulus current parameter so as to maintain the feedback variable at a setpoint.
12 . A non-transitory computer readable medium for controllably applying the neural stimulus, comprising instructions which when executed by one or more processors carry out the method of claim 11 .