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
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
See ShowDFT.xlsMAGN
0.00
0.10
0.20
0.30
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0.50
0.60
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33
MAGN
MAGN
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0.80
1.00
1.20
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MAGN
But they come with two artifacts of their own: 1. Smearing (spectral broadening), & 2. Sampling bias
Ln Magn (or Ln Power)
• Ln = Natural log (base e, not base 10) • e = 1 + 1/1! + 1/2! + 1/3! + ... or ~= 2.718…
Greek Astronomer Hipparchus (190-120BC)
• 6 brightness classification for stars
• Each 2.5x as bright as next classification
• Logarithmic
relationship
Psychophysical Function
0
1
2
3
4
5
0 10 20 30 40 50 60
S
ub
ject
ive
Bri
gh
tnes
s (
S)
jnd
un
its
Light energy (I)
Psychophysical Function Fechner’s Law: S = (1/k) log (I)
01020304050607080
0 10 20 30 40 50 60 70 80
M
ean
mag
nit
ud
e es
tim
atio
ns
(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
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