• 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

• 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)

Dis/Advantages

• 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)

98% of EEG energy is between 0.1 & 30 Hz

Artifact

• Equipment-related

• Physiological (non-cerebral signals)

• Computational

• Functional (unstable background/state transitions; transients, sleep!)

Impedence <5-10K Ohm

Impedence artifact?

Eye movement & blinks

Muscles: Heart, jaw, and neck

Non-biological artifacts60 Hz, electrode pops

Equipment or gross movement artifacts

Eye blinks in 19 channel NeuroNavigator

Muscle, forehead and jaw

Sleep “artifact”

The Problems with Artifact

Computational Artifact:Undersampling

• Heart beat of 60 sec– 60 samples/min = DC– 90 samples/min = 15

bpm

Spectral Leakage

See ShowDFT.xlsMAGN

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MAGN

MAGN

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Data Windows eliminate leakage significantly

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

Sampling bias makes analysis sensitive to epoch positions

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

Artifact Management

Seaming

Power vs Magnitude (the square root of power)

Ln Magn (or Ln Power)

• Ln = Natural log (base e, not base 10) • e = 1 + 1/1! + 1/2! + 1/3! + ... or ~= 2.718…

Skewed distribution of power

Greek Astronomer Hipparchus (190-120BC)

• 6 brightness classification for stars

• Each 2.5x as bright as next classification

• Logarithmic

relationship

Psychophysical Function

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ub

ject

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jnd

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Light energy (I)

Psychophysical Function Fechner’s Law: S = (1/k) log (I)

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ean

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(S)

Stimulus intensity (I)

Psychophysical Function Stevens’ Power Law: S = aIm

Electric shock (m > 1)

Brightness (m < 1)

Apparent length (m = 1)

Basic law of psychophysics (correspondence between physical energies

and mental experiences) appears linear

Untailored Dominant Frequency

• IAF – individual’s alpha frequency

State Transitions

State transitions

Sources of artifact by frequency