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Averager Artifact Rejection Limits

The real world is rarely as cooperative as we would like. If the desired response is produced "some of the time", interspersed with periods of spurious responses or "artifacts", we would like to average only the good stuff and skip the trash. A typical situation is where an evoked neural response is contaminated by occasional bursts of muscle activity when the subject fidgets.

CAUTION: Do not connect any electrical equipment to a living subject without proper signal isolation techniques. A lethal shock could result.

If the noise bursts are larger than the desired response, we can reject them based upon a simple test: We collect the data for each frame, but before adding it into the average it is scanned to see if any part of the data goes above a positive threshold or below a negative threshold. If so, that whole frame is thrown away and not counted toward the total number of frames requested. If the subject is very restless, this may greatly extend the averaging time (which will no doubt make the subject still more restless!), but there may be cases when this is the only way to get useful results.

Setting the threshold limits is a matter of judgement and experience, since there is a tradeoff between accepting too much artifact and rejecting too much signal. The best way to start is to simply observe the raw signal and note where the usual desired peaks reach in comparison to occasional bursts where there is obvious artifact. Then set the thresholds by eye to be just beyond the normal range.

Try a few averages at these settings, then at tighter and broader ranges. Compare not only the resulting waveforms, but also the time it takes to get them. If tighter thresholds give substantially better results but at a big increase in time, you will need to decide if this is acceptable or if another setting is more appropriate. It is rare to find that the same settings work best in all subjects, but you may at least find a good compromise as a starting point. Remember too that time spent tweaking the artifact limits is time that could be used to collect data at tighter settings.

See also Averager, Synchronous Waveform Averaging, Waveform Averager Artifact Rejection Limits, Spectrum Averager Artifact Rejection Limits, and Noise and Interference.


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