1. MEG introduction
Brain Signals
MEG seminar
Oct 06 2011
Bernhard Ross
Rotman Research Institute
Department of Medical Biophysics 400 fT
University of Toronto
1.0 s

2. Brain signals recorded with EEG and MEG
Understanding the neural mechanism underlying the EEG/MEG
signal and knowing about the possibilities and limitations of the
methods has a large impact on design and performance of a
successful study.

3. The origin of the neuroelectric / neuromagnetic signal

4. The origin of the neuroelectric / neuromagnetic signal

5. The origin of the neuroelectric / neuromagnetic signal
Ramon y Cajal

6. The origin of the neuroelectric / neuromagnetic signal

7. Intra-cellular current flow
Transmembrane current flow
Intracellular current flow
Extracellular current flow
The intracellular current
flow generates an
external
electromagnetic field

8. Source activity: The dipole moment
T
¡e
¡ e
dl
I
c
Dipolemoment:
q = I · dl
(Am, nAm)

9. Source activity: The dipole moment
Dipole moment of a
T single neuron:
¡e 0.2 . . . 0.5 pAm
¡ e
dl e.g.:
I I=0.5nA, dl=1mm
c
Dipolemoment:
q = I · dl
(Am, nAm)

10. Source activity: The dipole moment
Dipole moment of a
T single neuron:
¡e 0.2 . . . 0.5 pAm
¡ e
dl e.g.:
I ¡e I=0.5nA, dl=1mm
¡ e
n·I
c
Dipolemoment:
q = I · dl
(Am, nAm) Dipolemoment:
q = n · I · dl

11. Source activity: The dipole moment
Dipole moment of a
T single neuron:
¡e 0.2 . . . 0.5 pAm
¡ e
dl e.g.:
I ¡e I=0.5nA, dl=1mm
¡ e
n·I MEG/EEG evoked
c response:
1 . . . 100 nAm
n=2000 . . . 500,000
synchronously active
neurons
Dipolemoment:
q = I · dl
(Am, nAm) Dipolemoment:
q = n · I · dl

12. Source of the MEG: – Anatomical organization in columnar structures
FROM: Hutsler and Galuske Trends in Neuroscience, 2003, 26:429-435
Neurons in the neocortex are organized in a hierarchy of micro-
and macro-columns.

13. The neural columns are aligned perpendicular to the cortical surface

14. Coil configuration: first order gradiometer

15. Whole head MEG system

16. Not all sources appear equally in the MEG

17. Not all sources appear equally in the MEG
A dipole tangential to the skull produces a
strong magnetic field outside the head.
A radial source may be missed in the MEG

18. The human magnetoencephalogram

19. The averaged auditory evoked response
single trial data
1
2
3
4
5
6
7
0 200 400 600 800 1000
Time (ms)

20. The averaged auditory evoked response
single trial data
1
2 averaged data
3
n=1
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

21. The averaged auditory evoked response
single trial data
1
2
3
n=2
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

22. The averaged auditory evoked response
single trial data
1
2
3
n=4
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

23. The averaged auditory evoked response
single trial data
1
2
3
n=8
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

24. The averaged auditory evoked response
single trial data
1
2
3
n=16
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

25. The averaged auditory evoked response
single trial data
1
2
3
n=32
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

26. The averaged auditory evoked response
single trial data
1
2
3
n=64
4
5
6
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

27. The averaged auditory evoked response
single trial data
1
2
3
n=128
4 P2
P1
5
6 N1
0 200 400 600 800 1000
7 Time (ms)
0 200 400 600 800 1000
Time (ms)

28. Magnetic field waveforms of auditory evoked responses
600 fT
700 ms

29. Magnetic field waveforms of auditory evoked responses
300
200
100
fT
0
−100
−200
0.0 0.2 0.4 0.6 0.8 1.0
sec
300
200
100
fT
0
−100
600 fT
−200
700 ms

30. Auditory evoked responses
E
cortical responses

31. Why do we have positive and negative response components?
FROM: Niedermeyer and Lopes da Silva
Two factors decide about the polarity of the response:
1. The nature of synaptic connection: excitatory or inhibitory.
2. The location of synaptic contact: apical or basal.
Generally, subsequent waves are generated in different micro circuits.

32. Event related responses
Early responses are strictly time-locked to the stimulus (exogenous
components)
Later responses are time-locked to internal processing (endogenous
components)
trade off around 250 ms (?)

33. The first human MEG recording
David Cohen, Jim Zimmerman, MIT, 1971
single channel SQUID sensor

34. The first human MEG recording
David Cohen, Jim Zimmerman, MIT, 1971
single channel SQUID sensor

35. The first human MEG recording
David Cohen, Jim Zimmerman, MIT, 1971
single channel SQUID sensor

36. The first human MEG recording
David Cohen, Jim Zimmerman, MIT, 1971
single channel SQUID sensor

37. The first human MEG recording
David Cohen, Jim Zimmerman, MIT, 1971
single channel SQUID sensor
Hans Berger, 1929

38. The first human MEG recording
David Cohen, Jim Zimmerman, MIT, 1971
single channel SQUID sensor

39. Beta oscillations 15-30 Hz
Beta oscillations have been first observed in the motor system.
Beta increased during preparation for a movement.
Beta decreased at initiation of the movement.
and beta increased again at the end of the movement
Beta oscillations are involved in sensorimotor integration
Modulation of beta oscillation have been found in the auditory and
visual system.

40. Gamma oscillations 30-80 Hz
Gamma oscillation have been first observed as a short burst after
stimulus onset in the visual modality - also with auditory and
somatosensory stimulation.
There is a large interest in gamma oscillation because of a strong
theoretical framework related to feature binding, attention,
consciousness ...
Gamma oscillations always increase in the active state

41. The micro circuit underlying gamma oscillations

42. early gamma oscillation are
time (phase) locked to the
stimulus and can be detected
in the averaged sgnal
Endogenous gamma
oscillations are less strictly
time (phase) locked to the
stimulus. The signal is
canceled out in the average.
Instead we can analyze the
event related changes in the
magnitude of oscillation.

43. Event related changes in oscillatory activity
120
2
γ2 100
0
80 Time-frequency
-2
analysis of the MEG
2
γ1 50 signal
Signal Power Change (dB)
40 0
Change in signal
Frequency (Hz)
30 -2 strength relative to an
28
2 inactive pre-stimulus
β 24
interval
20 0
16 -2 The signal changes
14 are often termed
3
α 12 ’Event related
0
10 synchronisation (ERS)’
8 -3 and ’Event related
8 12 desynchronisation
7 6
6 (ERD)’
0
θ 5
4 -6
3 -12
-0.5 0 0.5 1 1.5 2
Time (s)

44. Synchrony between gamma oscillations
Source Strength (nAm) 100
50
0
-50
-100
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Time (s)

45. Synchrony between gamma oscillations
Source Strength (nAm) 100
50
0
-50
-100
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Time (s)

46. Synchrony between gamma oscillations
20
Source Strength (nAm)
10
0
-10
-20
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Time (s)

47. Synchrony between gamma oscillations
10
Source Strength (nAm)
0
-10
-20
-30
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Time (s)

48. Synchrony between gamma oscillations
20
Source Strength (nAm)
10
0
-10
-20
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Time (s)

49. Synchrony between gamma oscillations
Source Strength (nAm) 10
0
-10
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Source Strength (nAm)
10
0
-10
0.4 0.5 0.6 0.7
Time (s)

50. Synchrony between gamma oscillations
Source Strength (nAm) 10
0
-10
-0.4 -0.2 0 0.2 0.4 0.6 0.8 1
Source Strength (nAm)
10
0
-10
-0.2 -0.1 0 0.1
Time (s)

51. Analysis of oscillatory activity
Phase locked responses (averaging, phase statistics)
Event related changes in signal magnitude (ERS, ERD)
Coherence between sensor signals and between source signals
Event related changes in coherence
Analysis of coupling between frequency bands (gamma - theta)
Steady-state approaches