Methods and apparatus for removing artifacts from an electroretinogram
A method for removing artifacts from an electroretinogram, the method comprising: visually stimulating at least one eye of a test subject, and using at least two electrodes to measure electrical responses from the at least one eye of the test subject; recording the electrical responses during, and for a predetermined period of time after, the time at which a visual stimulation is applied, whereby to generate a data set, wherein each electrical response is represented as a sweep comprising a plurality of voltage amplitude values and a plurality of corresponding time values; applying an elevated high pass filter to each sweep, wherein the elevated high pass filter is configured to remove any signal components with a frequency value of <1 hz; and plotting the resulting data set as an electroretinogram, in which the plurality of voltage amplitude values of the remaining signal is plotted on a first axis of an electroretinogram, and the plurality of corresponding time values of the remaining signal is plotted on a second axis of the electroretinogram.
1. A method for removing artifacts from an electroretinogram, the method comprising:
visually stimulating at least one eye of a test subject, and using at least two electrodes to measure electrical responses from the at least one eye of the test subject;
recording the electrical responses during, and for a predetermined period of time after, the time at which one or more visual stimuli are applied, whereby to generate a data set, wherein each electrical response is represented as a sweep comprising a plurality of voltage amplitude values and a plurality of corresponding time values;
plotting the voltage amplitude values of the data set on a first axis of an electroretinogram, and plotting the time values of the data set on a second axis of the electroretinogram, so as to produce a electroretinogram comprising a plurality of sweeps, wherein each sweep is defined to start at the time of a stimuli and generally represents a combination of signal components from (i) the electrical response of the retina of the at least one eye of the test subject to the visual stimulation, and (ii) an artifact;
identifying a zone of the electroretinogram plot defined by a predetermined upper voltage amplitude limit, a predetermined lower voltage amplitude limit, a predetermined start time limit, and a predetermined end time limit, with at least one of the voltage amplitude limits being smaller than the largest voltage amplitude value representing a recorded electrical response of the retina;
removing sweeps having voltage amplitude values that do not fall within the zone, whereby to produce a filtered electroretinogram depicting the sweeps not removed.
2. A method according to claim 1 further comprising:
applying an elevated high pass filter to each sweep, wherein the elevated high pass filter is configured to remove any signal components with a frequency value of <1 hz; and
plotting the resulting data set as an electroretinogram, in which the plurality of voltage amplitude values of the remaining signal is plotted on a first axis of an electroretinogram, and the plurality of corresponding time values of the remaining signal is plotted on a second axis of the electroretinogram.
3. A method according to claim 2 wherein the elevated high pass filter is applied to a group of averaged sweeps.
4. A method according to claim 2 wherein the elevated high pass filter is configured to remove any signal components with a frequency value of <5 hz.
5. A method according to claim 2 wherein the elevated high pass filter is configured to remove any signal components with a frequency value of <7 hz.
6. A method according to claim 2 wherein the elevated high pass filter comprises an analog filter.
7. A method according to claim 6 wherein the analog filter comprises one selected from the group consisting of a Bessel filter, a Butterworth filter, and a Chebyshev filter.
8. A method according to claim 2 wherein the elevated high pass filter comprises a Fourier filter.
9. A method according to claim 1 wherein the time over which the electroretinography is recorded is 150 milliseconds after each visual stimulus.
10. A method according to claim 1 wherein the electroretinogram comprises at least 50 sweeps.
11. A method according to claim 1 wherein at least one of the predetermined upper voltage amplitude limit of the zone, and the predetermined lower voltage amplitude limit of the zone varies as a function of time.
12. A method according to claim 11 wherein plotting the predetermined upper voltage amplitude limit of the zone and the predetermined lower voltage amplitude limit of the zone as a function of time results in the zone having an oval shape.
13. A method according to claim 1 wherein, before the step of identifying a zone of the electroretinogram defined by a predetermined upper voltage amplitude limit, a predetermined lower voltage amplitude limit, a predetermined start time limit, and a predetermined end time limit, a blink artifact rejection filter is applied to the electroretinogram in order to remove (i) any sweeps that have a voltage amplitude larger than +250 μV, and (ii) any sweeps that have a voltage amplitude smaller than −250 μV.
14. A method according to claim 1 wherein the predetermined upper voltage amplitude limit of the zone is +10 μV, the predetermined lower voltage amplitude limit of the zone is −50 μV, the predetermined start time limit of the zone is 70 milliseconds, and the predetermined stop time limit of the zone is 150 milliseconds.
15. A method according to claim 1 wherein at least one of the predetermined upper voltage amplitude limit of the zone, and the predetermined lower voltage amplitude limit of the zone varies linearly as a function of time.
16. A method according to claim 15 wherein plotting the predetermined upper voltage amplitude limit and the predetermined lower voltage amplitude limit as a function of time results in a zone having the shape of a parallelogram.
17. A method according to claim 1 wherein at least one of the predetermined start time limit of the zone, and the predetermined end time limit of the zone varies linearly as a function of voltage amplitude.
18. A method according to claim 17 wherein plotting the predetermined start time limit and the predetermined end time limit as a function of voltage amplitude results in a zone having the shape of a parallelogram.
19. A method according to claim 1 further comprising:
calculating an average voltage amplitude value for the plurality of sweeps for each time value;
calculating the root mean square (RMS) difference between the voltage amplitude value of each sweep at each time value and the average voltage amplitude value for all of the plurality of sweeps for that time value;
ranking the root mean square (RMS) difference of each sweep based on the difference between the calculated root mean square (RMS) difference of each sweep at each time value and the average voltage amplitude value for the plurality of the sweeps for that time value; and
discarding those ranked sweeps that are ranked below a predetermined value relative to the remaining sweeps, whereby to produce an electroretinogram depicting the sweeps not discarded.
20. A system for removing artifacts from an electroretinogram, the system comprising:
apparatus configured to visually stimulate at least one eye of a test subject, and using at least two electrodes to measure electrical responses from the at least one eye of the test subject;
a processing unit configured to:
record the electrical responses during, and for a predetermined period of time after, the time at which one or more visual stimuli are applied, whereby to generate a data set, wherein each electrical response is represented as a sweep comprising a plurality of voltage amplitude values and a plurality of corresponding time values;
plot the voltage amplitude values of the data set on a first axis of an electroretinogram, and plot the time values of the data set on a second axis of the electroretinogram, so as to produce a electroretinogram comprising a plurality of sweeps, wherein each sweep is defined to start at the time of a stimuli and generally represents a combination of signal components from (i) the electrical response of the retina of the at least one eye of the test subject to the visual stimulation, and (ii) an artifact;
identify a zone of the electroretinogram plot defined by a predetermined upper voltage amplitude limit, a predetermined lower voltage amplitude limit, a predetermined start time limit, and a predetermined end time limit, with at least one of the voltage amplitude limits being smaller than the largest voltage amplitude value representing a recorded electrical response of the retina;
remove sweeps having voltage amplitude values that do not fall within the zone, whereby to produce a filtered electroretinogram depicting the sweeps not removed.
21. A system according to claim 20 wherein the processing unit is further configured to:
apply an elevated high pass filter to each sweep, wherein the elevated high pass filter is configured to remove any signal components with a frequency value of <1 hz; and
plot the resulting data set as an electroretinogram, in which the plurality of voltage amplitude values of the remaining signal is plotted on a first axis of a electroretinogram, and the plurality of corresponding time values of the remaining signal is plotted on a second axis of the electroretinogram.
22. A system according to claim 21 wherein the elevated high pass filter is applied to a group of averaged sweeps.