Method and device for detecting a neural response in a neural measurement

1. A method for processing a neural measurement obtained in a presence of artifact, in order to detect whether a neural response is present in the neural measurement, the method comprising:

obtaining a neural measurement from one or more sense electrodes;

at a first time offset, correlating the neural measurement against a first filter template to produce a first measure m1, the first filter template comprising an alternating waveform of a first phase;

at the first time offset, correlating the neural measurement against a second filter template to produce a second measure m2, the second filter template comprising an alternating waveform of a second phase 90 degrees offset to the first phase;

at a second time offset, being at a non-integer multiple of 180 degrees offset from the first time offset, correlating the neural measurement against the first filter template to produce a third measure m3;

at the second time offset, correlating the neural measurement against the second filter template to produce a fourth measure m4; and

processing m1 to m4 to detect whether a neural response exists in the neural measurement.

2. The method of claim 1 wherein the first filter template is anti-symmetric so as to create an imaginary DFT output, while the second filter template is symmetric so as to create a real DFT output.

3. The method of claim 1 wherein the second time offset is offset by 90 degrees or 270 degrees out of phase from the first time offset.

4. The method of claim 1 wherein the first filter template comprises four half cycles of a first alternating waveform and the second filter template comprises four half cycles of a second alternating waveform 90 degrees out of phase with the first alternating waveform.

5. The method of claim 1 wherein the first filter template and second filter template are amplitude modulated by a triangular window.

6. The method of claim 1 wherein only a single point of a correlation is calculated.

7. The method of claim 6 wherein the single point of the correlation is calculated at a predefined optimal time delay.

8. The method of claim 7 , further comprising determining the predefined optimal time delay when a signal to artifact ratio is greater than one, at which a first point or a single point of a cross-correlation between the neural measurement and a filter template should be produced, by:

at an approximate time delay between the neural response and the filter template, computing real and imaginary parts of a fundamental frequency of a DFT of the neural measurement;

calculating a phase defined by the real and imaginary parts;

relative to a fundamental frequency of the filter template, calculating a time adjustment needed to change a calculated phase to pi/2; and

defining the predefined optimal time delay as being the sum of an approximate time delay and the time adjustment.

9. The method of claim 7 further comprising determining the predefined optimal time delay by:

at the first time offset, correlating the neural measurement against a third filter template to produce a fifth measure m5, the third filter template comprising an alternating waveform at double the frequency of the first filter template and of a third phase;

at the second time offset, correlating the neural measurement against the third filter template to produce a sixth measure m6; and

determining from m5 and m6 a decay in artifact between the first time offset and the second time offset.

10. The method of claim 7 wherein the predefined optimal time delay is recalculated prior to every attempted detection of a neural response.

11. The method of claim 7 wherein the predefined optimal time delay is recalculated in response to a detected change in a user's posture.

12. A device for processing a neural measurement obtained in a presence of artifact, in order to detect whether a neural response is present in the neural measurement, the device comprising:

measurement circuitry for obtaining a neural measurement from one or more sense electrodes; and

a processor configured to:

at a first time offset, correlate the neural measurement against a first filter template to produce a first measure m1, the first filter template comprising an alternating waveform of a first phase;

at the first time offset, correlate the neural measurement against a second filter template to produce a second measure m2, the second filter template comprising an alternating waveform of a second phase 90 degrees offset to the first phase;

at a second time offset, being at a non-integer multiple of 180 degrees offset from the first time offset, correlate the neural measurement against the first filter template to produce a third measure m3;

at the second time offset, correlate the neural measurement against the second filter template to produce a fourth measure m4; and

process m1 to m4 to detect whether a neural response exists in the neural measurement.

13. The device of claim 12 wherein the first filter template is anti-symmetric so as to create an imaginary DFT output, while the second filter template is symmetric so as to create a real DFT output.

14. The device of claim 12 wherein the second time offset is offset by 90 degrees or 270 degrees out of phase from the first time offset.

15. The device of claim 12 wherein the first filter template comprises four half cycles of a first alternating waveform and the second filter template comprises four half cycles of a second alternating waveform 90 degrees out of phase with the first alternating waveform.

16. The device of claim 12 wherein the first filter template and second filter template are amplitude modified by a triangular window.

17. The device of claim 12 wherein the processor is further configured to calculate only a single point of a correlation.

18. The device of claim 17 wherein the processor is further configured to calculate the single point of the correlation at a predefined optimal time delay.

19. The device of claim 18 , wherein the processor is further configured to determine the predefined optimal time delay when a signal to artifact ratio is greater than one, at which a first point or a single point of a cross-correlation between the neural measurement and a filter template should be produced, by:

at an approximate time delay between the neural response and the filter template, computing real and imaginary parts of a fundamental frequency of a DFT of the neural measurement;

calculating a phase defined by the real and imaginary parts;

relative to a fundamental frequency of the filter template, calculating a time adjustment needed to change a calculated phase to pi/2; and

defining the predefined optimal time delay as being the sum of an approximate time delay and the time adjustment.

20. The device of claim 18 wherein the processor is further configured to determine the predefined optimal time delay by:

at the first time offset, correlating the neural measurement against a third filter template to produce a fifth measure m5, the third filter template comprising an alternating waveform at double the frequency of the first filter template and of a third phase;

at the second time offset, correlating the neural measurement against the third filter template to produce a sixth measure m6; and

determining from m5 and m6 a decay in artifact between the first time offset and the second time offset.

21. The device of claim 18 wherein the processor is further configured to recalculate the predefined optimal time delay prior to every attempted detection of a neural response.

22. The device of claim 18 wherein the processor is further configured to recalculate the predefined optimal time delay in response to a detected change in a user's posture.