640 likes | 887 Vues
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
E N D
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)
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
Effect of monopolar reference (linked ears) (temporal lobe activity attenuated)
Artifact • Equipment-related • Physiological (non-cerebral signals) • Computational • Functional (unstable background/state transitions; transients, sleep!)
Computational Artifact:Undersampling • Heart beat of 60 sec • 60 samples/min = DC • 90 samples/min = 15 bpm
But they come with two artifacts of their own: 1. Smearing (spectral broadening), & 2. Sampling bias
Arbitrary segmentation (epoching) of signal can produce different spectral means