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Artifact can make everything upside down and meaningless

Artifact can make everything upside down and meaningless. Remove artifact to find reliable answers. VHS Zimbardo #3, ~15 min, E Roy John’s work. Recording Montages. Montages. We measure different electrical potential between sensors Bipolar montage = two active channels

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Artifact can make everything upside down and meaningless

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  1. Artifact can make everything upside down and meaningless

  2. Remove artifact to find reliable answers

  3. VHS Zimbardo #3, ~15 min, E Roy John’s work

  4. Recording Montages

  5. Montages We measure different electrical potential between sensors • Bipolar montage = two active channels • Monopolar or referential montage = one active, one “inactive” such as ear • Linked Ears • Linked Mastoids • Nose • Average reference montage Mathematical sharpening techniques (e.g., Laplacian)

  6. Dis/Advantages

  7. Disadvantage with Monopolar • No such thing as inactive reference (including ear, neck, nose – cortical signal bleeds through – see scallop shaped topometric) • Disadvantage with Bipolar • Source of signal not localizable directly, but only through inference and comparison with other channels

  8. Effect of monopolar reference (linked ears) (temporal lobe activity attenuated)

  9. 98% of EEG energy is between 0.1 & 30 Hz

  10. Artifact • Equipment-related • Physiological (non-cerebral signals) • Computational • Functional (unstable background/state transitions; transients, sleep!)

  11. Impedence <5-10K Ohm

  12. Impedence artifact?

  13. Eye movement & blinks

  14. Muscles: Heart, jaw, and neck

  15. Non-biological artifacts60 Hz, electrode pops

  16. Equipment or gross movement artifacts

  17. Eye blinks in 19 channel NeuroNavigator

  18. Muscle, forehead and jaw

  19. Sleep “artifact”

  20. The Problems with Artifact

  21. Computational Artifact:Undersampling • Heart beat of 60 sec • 60 samples/min = DC • 90 samples/min = 15 bpm

  22. Spectral Leakage

  23. See ShowDFT.xls

  24. Data Windows eliminate leakage significantly

  25. But they come with two artifacts of their own: 1. Smearing (spectral broadening), & 2. Sampling bias

  26. Sampling bias makes analysis sensitive to epoch positions

  27. Arbitrary segmentation (epoching) of signal can produce different spectral means

  28. Artifact Management

  29. Seaming

  30. Power vs Magnitude (the square root of power)

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