Method and apparatus for estimating neural recruitment

1. A method of applying stimulus to a tissue, the method comprising:

delivering a therapeutic pulse having a first pulse width via one or more stimulus electrodes, wherein the therapeutic pulse evokes a neural response,

delivering at least one probe pulse having a second pulse width after the therapeutic pulse, wherein the second pulse width is less than the first pulse width, and

measuring a neural response subsequent to the therapeutic pulse via one or more sense electrodes:

determining a neural recruitment caused by the therapeutic pulse based on the neural response; and

determining a parameter for a subsequent therapeutic pulse based on the neural recruitment.

2. The method of claim 1 , wherein the first pulse width is in a range of 100-500 μs.

3. The method of claim 1 , wherein the determining the parameter comprises:

comparing the neural recruitment with a target neural recruitment;

deriving, using a controller, a command from the comparison; and

selecting, based on the command, the parameter for the subsequent therapeutic pulse.

4. The method of claim 3 , wherein the selecting the parameter comprises refining a parameter for the delivered therapeutic pulse.

5. The method of claim 3 , wherein the target neural recruitment is a predetermined neural recruitment.

6. The method of claim 1 , further comprising:

determining a parameter for a subsequent probe pulse based on the neural recruitment.

7. The method of claim 1 , wherein the probe pulse is delivered after a refractory period of fibres recruited by the therapeutic pulse.

8. The method of claim 1 , wherein the probe pulse is delivered at least 1 ms after the end of the therapeutic pulse.

9. The method of claim 1 , wherein measuring the neural response comprises:

settling measurement circuitry prior to the probe pulse by connecting the one or more sense electrodes to the measurement circuitry to allow the measurement circuitry to settle towards a bio-electrically defined steady state;

recovering charge on the one or more stimulus electrodes by short circuiting the one or more stimulus electrodes to each other;

keeping the one or more sense electrodes disconnected from the measurement circuitry while delivering the probe pulse;

imposing a delay during which the one or more stimulus electrodes are open circuited and the one or more sense electrodes are disconnected from the measurement circuitry and from the one or more stimulus electrodes; and

measuring, after the delay, the neural response by connecting the one or more sense electrodes to the measurement circuitry.

10. An implantable device for applying neural stimulus to neural tissue, the device comprising:

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

a stimulus source for delivering stimulus pulses via the one or more stimulus electrodes to neural tissue;

measurement circuitry for measuring a neural signal sensed at the one or more sense electrodes; and

a control unit configured to:

control the stimulus source to deliver a therapeutic pulse to neural tissue via the one or more stimulus electrodes, the therapeutic pulse having a first pulse width, wherein the therapeutic pulse evokes a neural response;

control the stimulus source to deliver at least one probe pulse to the neural tissue having a second pulse width after the therapeutic pulse, wherein the second pulse width is less than the first pulse width;

control the measurement circuitry to measure a neural response subsequent to the therapeutic pulse;

determine a neural recruitment caused by the therapeutic pulse based on the neural response; and

determine a parameter for a subsequent therapeutic pulse based on the neural recruitment.

11. The implantable device of claim 10 , wherein the first pulse width is in a range of 100-500 μs.

12. The implantable device of claim 10 , wherein the control unit is configured to determine the parameter for the subsequent therapeutic pulse by:

comparing the neural recruitment with a target neural recruitment;

deriving a command from the comparison; and

selecting, based on the command, the parameter for the

subsequent therapeutic pulse.

13. The implantable device of claim 12 , wherein the control unit is configured to select the parameter by refining a parameter for the delivered therapeutic pulse.

14. The implantable device of claim 12 , wherein the target neural recruitment is a predetermined neural recruitment.

15. The implantable device of claim 10 , wherein the control unit is configured to determine a parameter for a subsequent probe pulse based on the neural recruitment.

16. The implantable device of claim 10 , wherein the control unit is configured to control the stimulus source to deliver the probe pulse at least 1 ms after the end of the therapeutic pulse.

17. The implantable device of claim 10 , wherein the control unit is configured to control the measurement circuitry to measure the neural response by:

settling the measurement circuitry prior to the probe pulse by connecting the one or more sense electrodes to the measurement circuitry to allow the measurement circuitry to settle towards a bio-electrically defined steady state,

recovering charge on the one or more stimulus electrodes by short circuiting the one or more stimulus electrodes to each other;

keeping the one or more sense electrodes disconnected from the measurement circuitry while delivering the probe pulse,

imposing a delay during which the one or more stimulus electrodes are open circuited and the one or more sense electrodes are disconnected from the measurement circuitry and from the one or more stimulus electrodes; and

measuring, after the delay, the neural response by connecting the one or more sense electrodes to the measurement circuitry.

18. A method of applying stimulus to a tissue, the method comprising:

delivering a plurality of therapeutic pulses having a first pulse width, wherein the therapeutic pulses evoke neural responses;

delivering a plurality of probe pulses having a second pulse width interleaved with at least some therapeutic pulses of the plurality of therapeutic pulses, wherein the second pulse width is less than the first pulse width,

measuring, after a therapeutic pulse of the plurality of therapeutic pulses and prior to a subsequent probe pulse of the plurality of probe pulses, a neural response evoked by the therapeutic pulse;

determining a neural recruitment caused by the therapeutic pulse based on the neural response; and

determining a parameter for the subsequent therapeutic pulse based on the neural recruitment.

19. The method of claim 18 , wherein the first pulse width is in a range of 100-500 μs.

20. The method of claim 18 , wherein the determining the parameter comprises:

comparing the neural recruitment with a target neural recruitment;

deriving, using a controller, a command from the comparison; and

selecting, based on the command, the parameter for the subsequent therapeutic pulse.

21. The method of claim 20 , wherein the selecting the parameter comprises refining a parameter for the preceding delivered therapeutic pulse.

22. The method of claim 20 , wherein the target neural recruitment is a predetermined neural recruitment.

23. An implantable device for applying neural stimulus to neural tissue, the device comprising:

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

a stimulus source for delivering stimulus pulses via the one or more stimulus electrodes to neural tissue;

measurement circuitry for measuring a neural signal sensed at the one or more sense electrodes: and

a control unit configured to:

control the stimulus source to deliver a plurality of therapeutic pulses to neural tissue via the one or more stimulus electrodes, the therapeutic pulses having a first pulse width, wherein the therapeutic pulses evoke neural responses;

control the stimulus source to deliver a plurality of probe pulses to the neural tissue having a second pulse width interleaved with at least some therapeutic pulses of the plurality of therapeutic pulses, wherein the second pulse width is less than the first pulse width;

control the measurement circuitry to measure, after a therapeutic pulse of the plurality of therapeutic pulses and prior to a subsequent probe pulse of the plurality of probe pulses, the neural response evoked by the therapeutic pulse;

determine a neural recruitment caused by the therapeutic pulse based on the neural response; and

determining a parameter for the subsequent therapeutic pulse based on the neural recruitment.

24. The implantable device of claim 23 , wherein the first pulse width is in a range of 100-500 μs.

25. The implantable device of claim 23 , wherein the control unit is configured to determine the parameter for the subsequent therapeutic pulse by:

comparing the neural recruitment with a target neural recruitment;

deriving a command from the comparison; and

selecting, based on the command, the parameter for the subsequent therapeutic pulse.

26. The implantable device of claim 25 , wherein the control unit is configured to select the parameter by refining a parameter for the preceding delivered therapeutic pulse.

27. The implantable device of claim 25 , wherein the target neural recruitment is a predetermined neural recruitment.

28. The implantable device of claim 23 , wherein the control unit is configured to determine a parameter for a subsequent probe pulse based on the neural recruitment.

29. The implantable device of claim 23 , wherein the control unit is configured to control the stimulus source to deliver each probe pulse at least 1 ms after the end of the preceding therapeutic pulse.