Feedback control of neuromodulation

1. 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 action potential on the neural pathway, the stimulus being defined by at least one stimulus parameter;

measuring a neural compound action potential response evoked by the stimulus, and deriving from the measured evoked response (ECAP) a feedback variable;

completing a feedback loop by using the feedback variable to control the at least one stimulus parameter value; and

adaptively compensating for changes in a gain of the feedback loop caused by electrode movement relative to the neural pathway, by applying a compensating transfer function to the feedback variable, the compensating transfer function including the at least one stimulus parameter as a variable and being configured to compensate for both (i) a distance-dependent transfer function of stimulation, and (ii) a distance dependent transfer function of measurement which is distinct from the distance-dependent transfer function of stimulation;

wherein applying the compensating transfer function comprises multiplying the feedback variable with an exponentiated version (I k ) of the stimulus parameter (I) used to generate that ECAP.

2. The method of claim 1 wherein constant recruitment is effected by use of single measurement pairs of stimulus parameter (I) and observed ECAP amplitude (V).

3. The method of claim 1 wherein the parameter k reflects both a recruitment parameter n and a measurement parameter m, m≠n.

4. The method of claim 3 wherein the total number of fibres recruited N varies as N∝Ix −n −T 0 , where x is the electrode to cord distance, the measured ECAP amplitude V is approximated as ∝Nx −m , and k=m/n.

5. The method of claim 4 wherein k is selected to take a value which is based upon the stimulation and recording configurations in use.

6. The method of claim 5 wherein tripolar stimulation is delivered using first to third electrodes of a SCS lead, and recordings are taken using a sixth electrode of the same lead, and wherein k is selected to be in the range 0.1 to 0.7.

7. The method of claim 6 wherein k is selected to be in the range 0.22 to 0.53.

8. The method of claim 7 wherein k is selected to be about 0.37.

9. The method of claim 3 wherein k is determined clinically using a recruitment datum.

10. The method of claim 9 wherein the recruitment datum comprises one or more of: the patient's perceptual threshold, discomfort threshold, coverage of a certain area or body part, a qualitative characteristic of the patient's perception of a stimulation, patient's perception of optimal comfort, an electrophysiological measure, the onset of muscle response/twitching, and a measure of neural activity.

11. The method of claim 9 further comprising the patient assuming a series of postures; in each posture adjusting the stimulus parameter until the required recruitment datum is achieved; and estimating k from constant recruitment data in differing postures.

12. The method of claim 3 wherein k is partly or wholly determined clinically by using the recording electrode to measure neural responses to peripheral stimulation in a number of postures to obtain Vi data in each posture; using the stimulus electrode to deliver stimulation in each posture adjusted to a current level Ii which yields the respective Vi, and using the set of (I i , V i ) pairs of constant recruitment to derive k.

13. The method of claim 3 wherein k is partly or wholly determined clinically by placing the patient in a range of postures i, in each posture sweeping the stimulus parameter and recording a growth curve, linearly fitting the growth curve for each respective posture to determine the respective threshold T i and growth slope M i , and comparing log T i against log T i M i ; to derive k.

14. The method of claim 1 wherein an error signal is generated from the compensated feedback variable relative to a setpoint and fed into a controller which determines the next stimulus parameter value.

15. The implantable device of claim 1 , wherein the feedback variable is an amplitude of the measured evoked response.

16. An implantable device for controllably applying a neural stimulus, the device comprising:

a plurality of electrodes including one or more nominal stimulus electrodes and one or more nominal sense electrodes;

a stimulus source for providing a 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 (ECAP) on the neural pathway;

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 a neural stimulus as defined by at least one stimulus parameter;

measure via the measurement circuitry a neural compound action potential response evoked by the stimulus;

determine from the measured evoked response a feedback variable;

complete a feedback loop by using the feedback variable to control the at least one stimulus parameter value; and

adaptively compensate for changes in a gain of the feedback loop caused by electrode movement relative to the neural pathway, by applying a compensating transfer function to the feedback variable, the compensating transfer function include the stimulus parameter as a variable and being configured to compensate for both (i) a distance-dependent transfer function of stimulation and (ii) a distance dependent transfer function of measurement which is distinct from the distance-dependent transfer function of stimulation;

wherein applying the compensating transfer function comprises multiplying the feedback variable with an exponentiated version (I k ) of the stimulus parameter (I) used to generate the ECAP.

17. The implantable device of claim 16 , wherein the control unit is further configured to generate an error signal relative to a setpoint and feed the error signal into a controller which determines a future stimulus intensity.

18. A non-transitory computer readable medium for controllably applying a neural stimulus, comprising the following instructions for execution by one or more processors:

computer program code means for applying the neural stimulus to a neural pathway in order to give rise to an evoked compound action potential (ECAP) on the neural pathway, the stimulus being applied as defined by at least one stimulus parameter;

computer program code means for measuring a neural compound action potential response evoked by the stimulus and deriving from the measured evoked response a feedback variable;

computer program code means for completing a feedback loop by using the feedback variable to control the at least one stimulus parameter value; and

computer program code means for adaptively compensating for changes in a gain of the feedback loop caused by electrode movement relative to the neural pathway, by applying a compensating transfer function to the feedback variable, the compensating transfer function including the stimulus parameter as a variable and being configured to compensate for both (i) a distance-dependent transfer function of stimulation and (ii) a distance dependent transfer function of measurement which is distinct from the distance-dependent transfer function of stimulation;

wherein applying the compensating transfer function comprises multiplying the feedback variable with an exponentiated version (Ik) of the stimulus parameter (I) used to generate that ECAP.