MEG recording of brain signals

400 fT1.0 s

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MEG introductionBrain Signals

MEG seminarOct 06 2011

Bernhard Ross

Rotman Research Institute

Department of Medical BiophysicsUniversity of Toronto

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Brain signals recorded with EEG and MEG

Understanding the neural mechanism underlying the EEG/MEGsignal and knowing about the possibilities and limitations of themethods has a large impact on design and performance of asuccessful study.

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The origin of the neuroelectric / neuromagnetic signal

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Ramon y Cajal

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Intra-cellular current flow

Transmembrane current flowIntracellular current flowExtracellular current flow

The intracellular currentflow generates anexternalelectromagnetic field

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Source activity: The dipole moment

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Dipolemoment:q = I · dl

(Am, nAm)

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dlAA��

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(Am, nAm)

Dipole moment of asingle neuron:0.2 . . . 0.5 pAme.g.:I=0.5nA, dl=1mm

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6

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dlAA��

I


(Am, nAm)

AA��

n · I

Dipolemoment:q = n · I · dl

Dipole moment of asingle neuron:0.2 . . . 0.5 pAme.g.:I=0.5nA, dl=1mm

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6

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dlAA��

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(Am, nAm)

AA��

n · I

Dipolemoment:q = n · I · dl

Dipole moment of asingle neuron:0.2 . . . 0.5 pAme.g.:I=0.5nA, dl=1mmMEG/EEG evokedresponse:1 . . . 100 nAmn=2000 . . . 500,000synchronously activeneurons

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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.

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The neural columns are aligned perpendicular to the cortical surface

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Coil configuration: first order gradiometer

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Whole head MEG system

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Not all sources appear equally in the MEG

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Not all sources appear equally in the MEG

A dipole tangential to the skull produces astrong magnetic field outside the head.

A radial source may be missed in the MEG

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The human magnetoencephalogram

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The averaged auditory evoked response

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Magnetic field waveforms of auditory evoked responses

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Magnetic field waveforms of auditory evoked responses

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Auditory evoked responses

-cortical responses

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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.

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Event related responses

Early responses are strictly time-locked to the stimulus (exogenouscomponents)

Later responses are time-locked to internal processing (endogenouscomponents)

trade off around 250 ms (?)

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The first human MEG recording

David Cohen, Jim Zimmerman, MIT, 1971single channel SQUID sensor

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Hans Berger, 1929

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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 andvisual system.

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Gamma oscillations 30-80 Hz

Gamma oscillation have been first observed as a short burst afterstimulus onset in the visual modality - also with auditory andsomatosensory stimulation.

There is a large interest in gamma oscillation because of a strongtheoretical framework related to feature binding, attention,consciousness ...

Gamma oscillations always increase in the active state

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The micro circuit underlying gamma oscillations

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early gamma oscillation aretime (phase) locked to thestimulus and can be detectedin the averaged sgnal

Endogenous gammaoscillations are less strictlytime (phase) locked to thestimulus. The signal iscanceled out in the average.

Instead we can analyze theevent related changes in themagnitude of oscillation.

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Event related changes in oscillatory activity

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Time-frequencyanalysis of the MEGsignal

Change in signalstrength relative to aninactive pre-stimulusinterval

The signal changesare often termed’Event relatedsynchronisation (ERS)’and ’Event relateddesynchronisation(ERD)’

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Synchrony between gamma oscillations

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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 signalsEvent related changes in coherenceAnalysis of coupling between frequency bands (gamma - theta)Steady-state approaches

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MEG recording of brain signals

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