electromagnetic brain mapping physiology of source signals

Sylvain Baillet

McConnell Brain Imaging CentreMontreal Neurological InstituteMcGill University

[[email protected]]

Google it! ’MEG MNI ’

Electromagnetic Brain MappingPhysiology of source signals

mailto:[email protected]

Ongoing background brain activity (MEG)

[1.5-40] Hz

Amplitude scale

Ongoing background brain activity (MEG)

Amplitude scale

[40-350] Hz

Withsubject

Withoutsubject

Hamalainen et al., Rev. Mod. Phys., 1993

Power line contamination

The resting and active brain: frequency power spectrum

delta

theta alpha

beta terra

incognita

gamma

The resting and active brain: frequency power spectrum

State-‐dependent expressions of neural oscilla3ons

Large-scale electrophysiology; oscillatory “brain rhythms”

aroused β

relaxed α

drowsy 𝜃

light sleep (spindles)

deep sleep δ


Raichle M. Brain Connectivity (2011)

?

fMRI - BOLD


aroused β

relaxed α

drowsy 𝜃


deep sleep δ


“Resting-state networks”Buckner RL et al. Ann NY Acad. Sci. (2008)Carhart-Harris R L , Friston K J Brain (2010)

Resting-state networks

Raichle M. Brain Connectivity (2011)

?

fMRI - BOLD


aroused β

relaxed α

drowsy 𝜃


deep sleep δ

Electrophysiological origins of MEG/EEG signals

The neural cell as elementary building block

The neural cell as elementary building block

pyramidal cell as canonical source

Basic neural electrophysiology

apical dentrites

basal dentrites


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +

primary current


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +

return (volume) currents

primary current


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +

primary current


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +

magnetic induction

primary current

magnetometer


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +

magnetic induction

primary current

magnetometer


apical dentrites

basal dentrites

- - - - - - - -- - - - - - - -

++ + + + + + + + + + + +

magnetic induction

primary current

measured induced currents

= +

The equivalent current dipole =

current source model of PSP’s and AP’s

current dipole model

influence of cell morphology on signal strength

radial cell morphology



net current dipole



net current dipole

weaker net currents than from elongated cell morphology

Mass effect of neural ensembles


overlap of multiple PSPs yields stronger contribution to MEG/EEG signalling


overlap of multiple PSPs yields stronger contribution to MEG/EEG signalling

stronger individual currents but poorer temporal overlap of APs


overlap of multiple PSPs yields stronger contribution to MEG/EEG signalling.

~50,000 cells, at minimum

stronger individual currents but poorer temporal overlap of APs

Mass effect of neural ensemblesVoltage-‐sensi3ve ion channels (Llinás, 1988; Hille, 2001)

-‐ Large, detectable PSP sodium spikes

‣ from 10,000 synchronous cells

‣ a possible source of high-‐frequency bursts?


experimental data (micro)MEG

Murakami et al., J. Physiol (2002)

Voltage-‐sensi3ve ion channels (Llinás, 1988; Hille, 2001)

-‐ Large, detectable PSP sodium spikes

‣ from 10,000 synchronous cells

‣ a possible source of high-‐frequency bursts?

mesoscopic distribu3on of currents

cortex

mesoscopic distribu3on of currents

cortex

nuclei

dynamicsa rapid overview

Evidence of low-‐to-‐high frequency coupling of neural oscilla3ons: single cells & assemblies

Contreras & Steriade, J. Neurosci. (1995)

Temporal ji_er of unitary and ensemble responses

Thalamic stimulation Cortical responseContreras & Steriade, J. Neurophys. (1996)

Ji_er in brain responses or s3mulus 3ming yields slower event-‐related components and lower SNR

• Sloppy s3mulus 3ming (ji_er) yields smeared MEG responses

• Physiological ji_er produces similar effects

• The longer the response latency, the longer and smoother the response

Somatosensory evoked fields

Latency

Adapted from L. Parkkonen

Oscilla3onsa scaffold of neural dynamics

Theore3cal framework: Oscillatory Implementa3on of Perceptual Inference

Donhauser, Florin & Baillet, CNS 2014 Schroeder & Lakatos, TINS 2009 Arnal & Giraud, TICS 2012



Slow Inhibitory

Fast InhibitoryExcitatory

Elementary cell assembly



Slow Inhibitory



MEG, EEG, LFP



Slow Inhibitory



E EE

I I

MEG, EEG, LFP



Slow Inhibitory



E EE

I I

MEG, EEG, LFP

δ－α cycles



Slow Inhibitory



E EE

I I

MEG, EEG, LFP

δ－α cycles

𝜸



Slow Inhibitory



E EE

I I

MEG, EEG, LFP

δ－α cycles

𝜸 𝜸



Slow Inhibitory



E EE

I I

MEG, EEG, LFP

δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



E EE

I I

STIM MEG, EEG, LFP

δ－α cycles

𝜸 𝜸 𝜸

E EE

I I δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



STIM MEG, EEG, LFP

E EE

I I δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



SI

FIE

Higher-order region

STIM MEG, EEG, LFP

E EE

I I δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



SI

FIE

Higher-order region

Cortical hub, thalamic relaySTIM MEG, E

EG, LFP

E EE

I I δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



SI

FIE

Higher-order region

Cortical hub, thalamic relay

β burstsSTIM MEG, E

EG, LFP

E EE

I I δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



SI

FIE

Higher-order region


β burstsSTIM STIM MEG, E

EG, LFP

E EE

I I δ－α cycles

𝜸 𝜸 𝜸



Slow Inhibitory



SI

FIE

Higher-order region


β burstsSTIM STIMSTIM MEG, E

EG, LFP

E EE

I I δ－α cycles

𝜸 𝜸 𝜸


A possible mechanism for op3mal sensory processing (efficacy in 3ming, metabolism, downstream processing, etc.)


Slow Inhibitory



SI

FIE

Higher-order region


β burstsSTIM STIMSTIM MEG, E

EG, LFP

Wrap-‐up: dis3nct roles for dis3nct frequencies


δ－α: cycles of regional excitability



β: bursts as expressions of top-down modulations




𝜸: bursts nested in slower rhythms, bottom-up signaling





E EE

I I





E EE

I I

higher 𝜸: PSP spiking ?

Correla3on between low-‐, high-‐frequency ongoing brain rhythms and BOLD

Schölvinck et al., PNAS (2010)

Delta/Theta/Alpha

Beta

Gamma

High-Gamma

MEG Res3ng-‐State Networks

Auditory+

VisualDMN

Dorsal, sensori-motor

fMRI

Florin & Baillet, Neuroimage, 2015

delta

theta alpha

beta terra

incognita

gamma

Cross-‐frequency coupling (CFC): A generic mechanism regula3ng local and long-‐range brain dynamics?

delta

theta alpha

beta terra

incognita

gamma

Cross-‐frequency coupling (CFC): A generic mechanism regula3ng local and long-‐range brain dynamics?

E EEI I

Review

Review

• Signal origins

Review

• Signal origins

๏ currents induced by spa3o-‐temporal overlap of post-‐synap3c poten3als in cell assemblies (cortex and elsewhere)

Review

• Signal origins


๏ possible fast spiking ac3vity

Review

• Signal origins



• Dynamics

Review

• Signal origins



• Dynamics

๏ event-‐related responses as resejng of ongoing ac3vity

Review

• Signal origins



• Dynamics


๏ dis3nct roles of typical frequency bands: net excita3on, bo_om-‐up vs top-‐down signaling, etc

Review

• Signal origins



• Dynamics


๏ dis3nct roles of typical frequency bands: net excita3on, bo_om-‐up vs top-‐down signaling, etc

๏ a field of intense research

electromagnetic brain mapping physiology of source signals

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