Ole Jensen 08/06/2005 New Perspectives on Rhythmic...

Ole Jensen 08/06/2005

F.C. Donders Centre for Cognitive Neuroimaging

New Perspectives on Rhythmic Brain Activity

Ole Jensen




Spontaneous Oscillatory Activity


Brain Oscillations

Gibbs and Gibbs, 1950



Calculating power spectra



+ + ... =

Calculating power spectra

Power spectra during rest

Jensen and Vanni (2002) Neuroimage 15:568

Identifying oscillatory sources by minimum current estimates

Discretize the brain volume:Boundary Element Model (BEM)

Find the current distribution which best explains the field

Find the current distribution which best explains the field

Number of Grid Points (~1000)>

Number of Sensors (~100)

Constraint:

Minimize the total currentmin || q ||

Sources of Sensory-Motor Beta ActivityIdentified by MCEfd

Jensen et al. (2005) Neuroimage 26:347-355



Induced Oscillatory Activity



Evoked and Induced Responses

evoked induced

trial 1

trial 2

trial 3

average

trial n

. . .


Ole Jensen



Evoked and Induced Responses

evoked induced

trial 1

trial 2

trial 3

average

trial n

. . .


Ole Jensen



Characterizing Induced Oscillatory Activity Using Wavelets

Morlet wavelet convoled to the signal



Signal:

Amplitude, A:

Phase, φ:

π

−π

0

1


Wavelets scale with frequency

f = 10 Hz :

f = 20 Hz :

f = 40 Hz :

100 ms

A time-frequency representation

Time (s)

Fr e

q (H

z )

Reproduced from Tallon-Baudry and Bertrand (1999) TICS



Example:

Evoked fields and beta oscillations in a “N400” language comprehension task

N400m


planar gradient



Time-Frequency Representations(Planar gradient)


Incongruentsentences:

Ole Jensen

ERFs and Oscillatory Responses Provide Complementary Information

TFRs (beta rebound):ERF (N400m):

15 – 22 Hz0.3 – 1.5 s

0.3 – 0.6 s




Identifying sources using beamforming/spatial filtering

• Linearly Constrained Minimum Variance (LCMV)

• Synthetic Aperture Magnetometry (SAM)

• Dynamical Imaging of Coherence Sources (DICS)

Spatial filtering

The measured field at sensor x(t) given source yr (t) and forward model H(r)

x(t) = H(r) yr (t) + n(t)

H1

H2

H3

H4

H5

x1(t)

x2(t)

x3(t)

x4(t)

x5(t)

yr(t)

sr(t) = Σi=1:n

w(r) x(t)

h1

h2

h3

h4

h5

yr

x1(t)x2(t)

x3(t)

x4(t)

x5(t)

w1r

w2r

w3r

w4r

w5r

sr (t) = w(r)T x(t)

“Strength“ of activity at location r:

sr (t) = w(r)T x(t)



Scanning ....

w(r) = [HT(r) Cov-1(x) H(r)]-1 HT(r) Cov-1(x)

w(r)



Sources of The Beta Increase IdentifiedUsing a Beamformer Technique



Summary

• Oscillatory activity– Spontaneous/ongoing

– Induced/event related

– Modulated by behavior

• Characterization in time and frequency– Power spectra

– Wavelets, FFT-based spectrogram, multitapers

• Identification of sources– Current estimates (MCE/MNE)

– Beamforming techniques



Suggested reading

MEG/oscillations general:• Hari R (2005) Magnetoencephalography in Clinical Neurophysiological Assessment of Human Cortical Functions. In

Electroencephalography: Basic Principles, Clinical Applications, and Related Fields (eds. E. Niedermeyer F. Lopes da Silva). 5th edition. Lippincott Williams & Wilkins

• Hari R, Salmelin R. (1997) Human cortical oscillations: a neuromagnetic view through the skull. Trends Neurosci. 1997 Jan;20(1):44-9.

• Tallon-Baudry C, Bertrand O. (1999) Oscillatory gamma activity in humans and its role in object representation. Trends Cogn Sci. 3:151-162.

• Varela F, Lachaux JP, Rodriguez E, Martinerie (2001) The brainweb: phase synchronization and large-scale integration. Nat Rev Neurosci. 2:229-39.

Power spectra + wavelets• Challis RE, Kitney RI. SE. (1991) The power spectrum and coherence function. Medical & Biological Engineering and Computing.

225-241• Steven W. Smith (1997) The Scientist and Engineer's Guide to Digital Signal Processing California Technical Publishing. http://• Sinkkonen J, Tiitinen H, Naatanen R.(1995) Gabor filters: an informative way for analysing event-related brain activity. J Neurosci

Methods. 56:99-104. • Tallon-Baudry C, Bertrand O. (1999) Oscillatory gamma activity in humans and its role in object representation. Trends Cogn Sci.

3:151-162. Localizing oscillatory sources• Liljestrom, M., Kujala, J., Jensen, O., and Salmelin, R. (2005). Neuromagnetic localization of rhythmic activity in the human brain: a

comparison of three methods. Neuroimage 25:734-745. • Jensen, O. and Vanni, S. (2002) A new method to identify multiple sources of oscillatory activity from magnetoencephalographic

data. Neuroimage 15:568-574 • Vrba J, Robinson SE. (2001) Signal processing in magnetoencephalography. Methods. 25:249-71 Gross J, Kujala J, Hamalainen M,

Timmermann L, Schnitzler A, Salmelin R. (2001) Dynamic imaging of coherent sources: Studying neural interactions in the human brain. Proc Natl Acad Sci U S A. 98(2):694-9.

• Hillebrand A, Singh KD, Holliday IE, Furlong PL, Barnes GR. (2005) Hum Brain Mapp. 25:199-211. A new approach to neuroimaging with magnetoencephalography.

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