Transcript M/EEG Data: What are we measuring? Jason Taylor MRC Cognition and Brain Sciences Unit (CBU) Cambridge Centre for Ageing and Neuroscience (CamCAN) 19 January 2011

M/EEG Data:
What are we measuring?
Jason Taylor
MRC Cognition and Brain Sciences Unit (CBU)
Cambridge Centre for Ageing and Neuroscience (CamCAN)
19 January 2011 | SPM M/EEG Course | Brussels
[ Much stolen from James Kilner, Jérémie Mattout, Olaf Hauk ]
M/EEG: What are we measuring?
• What do we want to measure?
• How do we measure it?
• What do we really measure?
• What do we not measure?
• Why don’t we use a different measure?
What do we want to measure?
+
Current flow in apical
denrites of cortical
pyramidal cells (not
action potentials)
= Electric dipole, with
magnetic field produced
according to the righthand rule
What do we want to measure?
~1 million synapses
must be simultaneously
active to be detected
+
Luckily, there are ~10
million cells per mm2
with 1000s of synapses
each.
Along with the
primary current,
volume currents
are induced in the
surrounding tissues
How do we measure it?
1771: Luigi Galvani (<- Bologna, Italy) experiments with
‘animal electricity’.
1840s, ’50s: Du Bois-Raymond
(Berlin) and Helmholtz (Berlin) describe
and quantify action potential.
1875: Richard Caton (Liverpool, UK ->) measures currents
between the cortical surface and the skull (ECoG) in dogs, monkeys
1924: Hans Berger (<- Jena, Germany) records first EEG
in humans, describes alpha and beta waves
-- early days of MEG (and MCG)
MCG: Cohen,
Science (1967)
(first: Baule &
McFee in 1963,
Syracuse, NY)
1960s: David Cohen
(<- MIT) MCG and
MEG, pre-squid
MEG: Cohen,
Science (1968)
MCG: Cohen,
Science (1967)
(first: Baule &
McFee in 1963,
Syracuse, NY)
1979: Joy Division (Manchester, UK)
release ‘Unknown Pleasures’,
featuring recordings of a pulsar on
the album cover (obviously inspired
by Cohen’s MEG waveforms ;-).
MEG: Cohen,
Science (1968)
-- and now, the SQUID!
1970: James Zimmerman (Ford Co.,
USA) invents the Superconducting
Quantum Interference Device
(SQUID), an ultrasensitive detector of
magnetic flux
Superconductivity is zero-resistance
electrical conduction that (typically)
occurs at extremely cold temperatures,
near absolute zero.
1973: Brian Josephson (Cambridge,
UK) awarded the Nobel prize for
prediction (in 1962) of ‘tunnel effect’
between two superconducting materials
separated by a thin insulating layer
(‘Josephson Junction’)
Brian Josephson
-- and now, the SQUID!
David Cohen knew something about
extreme cold temperatures, having
grown up in Winnipeg, Canada
(incidentally also my home town),
where it’s often -30°C for weeks at a
time in the winter.
Cohen (Science, 1972) describes the
first SQUID-based 1-sensor MEG
recording
This ->
is not it
David Cohen
http://www.etsy.com
‘squid hat’
-- and now, the SQUID!
MCG
MEG
MEG sensor types
http://meg.aalip.jp/scilab/CoilType.html
Elekta Neuromag Vector View System
Sensor Array
102 magnetometers
204 planar gradiometers
EEG Cap
(70-channel
montage shown)
What do we really measure?
-- methods for removing/avoiding noise
Magnetically
shielded
room (MSR)
Independent Component
Analysis (ICA)
IC1
VEOG
Signal Space
Separation (SSS)
- Neuromag
MEG1
MEG2
MEG3
MEG4
Plus:
Filtering, averaging,
robust averaging,…
Before / After
IC1 removed
What do we not measure?
-- dipole orientation
MEG vs. EEG
- MEG is insensitive to (purely) radially oriented dipoles
-- source depth
MEG vs. EEG
- MEG is insensitive to (purely) radially oriented dipoles
- MEG is less sensitive to deep sources
From Olaf Hauk:
http://http://www.mrc-cbu.cam.ac.uk/research/eeg/eeg_intro.html
-- cancelling sources
MEG vs. EEG
- MEG is insensitive to radially oriented dipoles
- MEG is less sensitive to deep sources
- NEITHER can detect closed (cancelling) sources
Ahlfors et al., HBM 2010
Why don’t we use a different measure?
- The End • Thanks!