Neural measurement
Measuring a neural response to a stimulus comprises applying an electrical stimulus, then imposing a delay during which the stimulus electrodes are open circuited. During the delay, a neural response signal present at sense electrodes is measured with a measurement amplifier, while ensuring that an impedance between the sense electrodes is sufficiently large that a voltage arising on the sense electrode tissue interface in response to the stimulus is constrained to a level which permits assessment of the neural response voltage seen at the sense electrode. For example the input impedance to the measurement amplifier (ZIN) can be, where ZC is the sense electrode(s) constant phase element impedance, Vs1−Vs2 is the differential voltage arising on the sense electrode tissue interface, and VE is the neural response voltage seen at the sense electrode.
1. A method for measuring a neural response to a stimulus, the method comprising:
applying an electrical stimulus from stimulus electrodes to neural tissue of a patient in order to evoke a neural response, wherein the electrical stimulus causes a differential voltage to arise on a sense electrode-tissue interface;
amplifying a neural response signal sensed at two sense electrodes with a measurement amplifier to generate an amplified neural response signal;
providing a respective sense electrode capacitor in series between each of the two sense electrodes and each of two respective inputs of the measurement amplifier,
wherein the sense electrode capacitors are configured to have a capacitance that is greater than a threshold capacitance value, wherein the threshold capacitance value presents an impedance which ensures that a differential voltage arising across the sense electrode capacitors in response to the electrical stimulus is constrained to a level which permits differential measurement of the neural response signal sensed between the two sense electrodes; and
measuring the amplified neural response signal.
2. The method of claim 1 , wherein the measurement amplifier is configured to have an input impedance that is greater than a threshold impedance value, wherein the input impedance configuration of the measurement amplifier causes the differential voltage arising on the sense electrode-tissue interface to be constrained to a level which permits differential measurement of the neural response signal between the two sense electrodes, wherein the threshold impedance value is based on the differential voltage arising on the sense electrode-tissue interface in response to the electrical stimulus.
3. The method of claim 2 wherein the threshold impedance value is defined as:
A
×
Z
C
(
V
S
1
-
V
S
2
)
V
E
where
A is a scalar provided to give sufficient margin of V E over (V S1 −V S2 ),
Z C is a constant phase element impedance of each sense electrode,
V s1 −V s2 is the differential voltage arising on the sense electrode-tissue interface in response to the stimulus, and
V E is a neural response voltage seen at the sense electrodes.
4. The method of claim 3 wherein A=1.
5. The method of claim 3 wherein A is greater than 0.067.
6. The method of claim 5 wherein A is greater than 0.5.
7. The method of claim 5 wherein A is greater than 1.
8. The method of claim 5 wherein A is greater than 2.
9. The method of claim 1 further comprising providing a respective stimulus electrode capacitor in series with each stimulus electrode.
10. The method of claim 9 wherein the stimulus electrode capacitors each have a respective capacitance which is the same as a capacitance of each sense electrode capacitor.
11. The method of claim 10 further comprising electrically reconfiguring at least one stimulus electrode as a sense electrode.
12. The method of claim 10 further comprising electrically reconfiguring at least one sense electrode as a stimulus electrode.
13. The method of claim 9 further comprising closing switches connecting the stimulus electrodes together, to equilibrate the stimulus electrodes prior to each stimulus.
14. The method of claim 13 , wherein the switches are closed only in short bursts so that an equilibration current does not rise to a level which is perceivable by the patient.
15. The method of claim 1 wherein the sense electrode capacitors are chosen to have a respective capacitance of 5 pF.
16. The method of claim 1 further comprising obtaining neural measurements repeatedly over time and monitoring for changes in the neural response to a given stimulus.
17. The method of claim 16 further comprising providing feedback control of a therapy delivered to the patient.
18. An implantable device for measuring a neural response to a stimulus, the device comprising:
a plurality of electrodes including one or more stimulus electrodes and two sense electrodes;
a stimulus source for providing an electrical stimulus to be delivered from the one or more stimulus electrodes to neural tissue of a patient in order to evoke a neural response, wherein the electrical stimulus causes a differential voltage to arise on a sense electrode-tissue interface;
a measurement amplifier for amplifying a neural response signal sensed at the two sense electrodes;
a respective sense electrode capacitor in series between each of the two sense electrodes and each of two respective inputs of the measurement amplifier, the sense electrode capacitors each having a capacitance that is greater than a threshold capacitance value, wherein the threshold capacitance value presents an impedance which ensures that a differential voltage arising across the sense electrode capacitors in response to the electrical stimulus is constrained to a level which permits differential measurement of the neural response signal sensed between the two sense electrodes; and
a control unit configured to control application of an electrical stimulus to the neural tissue and measurement of an evoked neural response, the control unit configured to apply an electrical stimulus from the stimulus electrodes to neural tissue, and the control unit further configured to amplify a neural response signal sensed at the sense electrodes with the measurement amplifier.
19. The device of claim 18 , wherein the measurement amplifier is configured to have an input impedance that is greater than a threshold impedance value, wherein the input impedance configuration of the measurement amplifier causes the differential voltage arising on the sense electrode-tissue interface to be constrained to a level which permits differential measurement of the neural response signal between the two sense electrodes, wherein the threshold impedance value is based on the differential voltage arising on the sense electrode-tissue interface in response to the electrical stimulus.
20. The device of claim 19 wherein the threshold impedance value is defined as:
A
×
Z
C
(
V
S
1
-
V
S
2
)
V
R
where
A is a scalar provided to give sufficient margin of V E over (V S1 −V S2 ),
Z C is a constant phase element impedance of each sense electrode,
V s1 −V s2 is the differential voltage arising on the sense electrode-tissue interface in response to the stimulus, and
V E is a neural response voltage seen at the sense electrodes.
21. The device of claim 20 wherein A=1.
22. The device of claim 20 wherein A is greater than 0.067.
23. The device of claim 22 wherein A is greater than 0.5.
24. The device of claim 22 wherein A is greater than 1.
25. The device of claim 22 wherein A is greater than 2.
26. The device of claim 18 further comprising a respective stimulus electrode capacitor in series with each stimulus electrode.
27. The device of claim 26 wherein the stimulus electrode capacitors each have a respective capacitance which is the same as a capacitance of each sense electrode capacitor.
28. The device of claim 27 wherein the control unit is further configured to electrically reconfigure at least one stimulus electrode as a sense electrode.
29. The device of claim 27 wherein the control unit is further configured to electrically reconfigure at least one sense electrode as a stimulus electrode.
30. The device of claim 26 further comprising switches connecting the stimulus electrodes together, and wherein the control unit is further configured to close the switches to equilibrate the stimulus electrodes prior to each stimulus.
31. The device of claim 30 , wherein the control unit is further configured to close the switches only in short bursts so that an equilibration current does not rise to a level which is perceivable by the patient.
32. The device of claim 18 wherein the sense electrode capacitors are chosen to have a respective capacitance of 5 pF.
33. The device of claim 18 wherein the control unit is further configured to obtain neural measurements repeatedly over time and monitor for changes in the neural response to a given stimulus.
34. The device of claim 33 wherein the control unit is further configured to provide feedback control of a therapy delivered to the patient.