Method and system for controlling electrical conditions of tissue

1. A method for measuring a neural response to a stimulus, the method comprising:

providing a plurality of implantable electrodes including at least one nominal stimulus electrode, at least one nominal measurement electrode, and at least one nominal compensation electrode, the electrodes being implanted proximal to a neural tissue and being in electrical contact with the neural tissue;

applying an electrical stimulus to the neural tissue from the at least one nominal stimulus electrode;

connecting the nominal compensation electrode to a desired electrical value so as to drive the neural tissue to an altered electrical condition for neural response measurement; and

obtaining a measurement of a neural response of the neural tissue to the stimulus from the at least one measurement electrode during the altered electrical condition of the neural tissue.

2. The method of claim 1 , wherein connecting the nominal compensation electrode to the desired electrical value comprises connecting an output of a feedback amplifier to the nominal compensation electrode such that the feedback amplifier drives the neural tissue via the nominal compensation electrode to the desired electrical value in a feedback arrangement.

3. The method of claim 2 , wherein a negative terminal of the feedback amplifier is connected to a nominal feedback sense electrode, and a positive terminal of the feedback amplifier is connected to the desired electrical value.

4. The method of claim 2 , wherein a negative terminal of the feedback amplifier is connected directly to the output of the feedback amplifier, and a positive terminal of the feedback amplifier is connected to the desired electrical value.

5. The method of claim 1 , wherein connecting the nominal compensation electrode to the desired electrical value comprises connecting a capacitance between the nominal compensation electrode and the desired electrical value.

6. The method of claim 5 wherein the capacitance is a capacitor.

7. The method of claim 1 , wherein applying the electrical stimulus comprises delivering to the neural tissue via the nominal stimulus electrode a first current produced by a first current source.

8. The method of claim 7 , wherein applying the electrical stimulus further comprises extracting from the tissue via a nominal stimulus return electrode a second current drawn by a second current source, the second current source being matched with the first current source.

9. The method of claim 8 , wherein the desired electrical value is electrical ground, and wherein connecting the nominal compensation electrode to the desired electrical value comprises connecting the nominal compensation electrode directly to electrical ground.

10. The method of claim 8 , wherein the nominal compensation electrode is the nominal stimulus return electrode, wherein connecting the nominal compensation electrode to the desired electrical value comprises connecting an output of a feedback amplifier to the nominal compensation electrode such that the feedback amplifier drives the neural tissue via the nominal compensation electrode to the desired electrical value in a feedback arrangement.

11. The method of claim 10 , wherein the desired electrical value is half of a supply rail voltage for the first current source.

12. The method of claim 1 , wherein the desired electrical value is a fixed voltage.

13. The method of claim 1 , wherein the connecting occurs at least partly during the application of the stimulus.

14. An implantable device for controlling electrical conditions of tissue, the device comprising:

a plurality of implantable electrodes including at least one nominal compensation electrode, at least one nominal stimulus electrode, and at least one nominal measurement electrode; the electrodes being configured to be implanted proximal to neural tissue to make electrical contact with the tissue;

a first current source for providing an electrical stimulus to be delivered from the at least one stimulus electrode to neural tissue; and

measurement circuitry for measuring a neural response of the neural tissue to the electrical stimulus from a signal sensed at the at least one measurement electrode; wherein:

the nominal compensation electrode is connected to a desired electrical value so as to provide a conductive path between the neural tissue and the desired electrical value in order to improve an electrical condition of the neural tissue for neural response measurement; and

the measurement circuitry measures the neural response during the improved electrical condition of the neural tissue.

15. The implantable device of claim 14 , wherein the nominal compensation electrode is connected to an output of a feedback amplifier such that the feedback amplifier drives the neural tissue via the nominal compensation electrode to the desired electrical value in a feedback arrangement.

16. The implantable device of claim 15 , wherein a negative terminal of the feedback amplifier is connected to a nominal feedback sense electrode, and a positive terminal of the feedback amplifier is connected to the desired electrical value.

17. The implantable device of claim 15 , wherein a negative terminal of the feedback amplifier is connected directly to the output of the feedback amplifier, and a positive terminal of the feedback amplifier is connected to the desired electrical value.

18. The implantable device of claim 14 , wherein the nominal compensation electrode is connected to the desired electrical value via a capacitance.

19. The implantable device of claim 18 , wherein the capacitance is a capacitor.

20. The implantable device of claim 14 , further comprising:

a nominal stimulus return electrode, and

a second current source configured to extract a second current from the tissue via the nominal stimulus return electrode, the second current source being matched with the first current source.

21. The implantable device of claim 20 , wherein the desired electrical value is electrical ground, and wherein the nominal compensation electrode is connected directly to electrical ground.

22. The implantable device of claim 20 , wherein the nominal compensation electrode is the nominal stimulus return electrode, and is connected to an output of a feedback amplifier such that the feedback amplifier drives the neural tissue via the nominal compensation electrode to the desired electrical value in a feedback arrangement.

23. The implantable device of claim 22 , wherein the desired electrical value is half of a supply rail voltage of a power supply for the first current source.

24. The implantable device of claim 14 , wherein the desired electrical value is fixed in relation to a power supply for the first current source.