Simultaneous recording of EEG and BOLD responses Why and How

Simultaneous recording of EEG and BOLD responsesWhy and How

1. Motivation and perspectives2. Technical Setup3. EEG data processing

i. The gradient artifact• Technical prerequisites: synchronization• Artifact removal and data quality

ii. The ballistocardiographic artifact4. Current studies5. Conclusions

Synopsis

• Achieving both high spatial and temporal resolution• Shed light on the foundations and interrelations of MEG, EEG and fMRI

Motivation and perspectives

Is there a (partial) correspondence of fMRI and EEG/MEG?• fMRI indirectly inferes neural activity via BOLD-reponse (neurovascular coupling)• EEG/MEG more directly reflect neural activity (apical EPSPs…)

large scalesynchrony

neural firingrates


Basic applications

• fMRI-informed source reconstruction• parametric designs and EEG-fMRI covariation• single-trial coupling of EEG and fMRI



Higher order models

• compound neural mass and hemodynamic models• joint ICA• parallel ICA

Clinical relevance???

Original Motivation:• Mapping epileptic zones

Recent „clinical“ research:• Movement disorders (cortical myoclonus)• Brain-computer interfaces (Biofeedback)


Measurement techniques and applications

• separate recordings of EEG and fMRI (two sessions)• interleaved recordings (EEG in “silent periods”)• simultaneous recordings (both modalities continuously

measured)


Continuous/simultaneous measurements:

• temporal correlation of EEG and fMRI• avoidance of order effects

• semi-optimized design

• strongly degraded signal quality (especially EEG)

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raw „clean“ EEGcontaminated EEGraw „clean“ EEGcontaminated EEG


Combined EEG – fMRI Recordings

Actual Status Hard- and Software

Technical Setup

EEG-Recording

System Components (BrainAmp MR plus, Brain Products GmbH):

1. EEG amplifier unit, 32 channel, fMRI approved (GE, Bruker, Siemens and

Phillips scanner), accumulator driven

2. EEG cap (EASY Cap), 32 channel (plus EOG, ECG), modified 10-20 system, sintered Ag/AgCl sensors, 10 kOhm for EEG cables, 15 kOhm for EOG/ECG cables, 3 different sizes

3. Sync-Box (Frequency divider), synchronization between MR scanner and EEG data recording

4. EEG-Data acquisition computer + Recording Software

5. BrainAmp I/O USB Adapter, interface between all other components

Technical Setup

Technical Setup

EEG cap

Technical Setup

EEG Amplifier

Stimulation Modes

1. Visual Stimulation:

Stimulation Computer (Presentation) -> Beamer -> Ground Glass -> Mirror (800x600 pixel) -> Subject

2. Auditory Stimulation:

Stimulation Computer (Presentation) -> Audiometer -> Audio Amplifier -> MR compatible stereo Head Phones -> Subject

3. Tactile Stimulation:

Stimulation Computer (Presentation) -> pneumato-tactile Stimulator -> 8 (finger) membranes -> Subject

Technical Setup

Components which are inside the MR measurement chamber are emphasized in green

Technical Setup

Tactile Stimulation

• driven by compressed air• up to eight independent output channels• integrated TTL trigger control unit

MRI compatible opto-electrical Response Unit

– 2 response panels (shape is adapted for left and right hand)

– Each panel provides 2 response buttons (best fitting for index and middle finger)

– Response panels are connected to opto-electrical transducers via fiber optical cables (inside MR chamber)

– Response signals are recorded by Stimulation and Recording Software in order being referable during later analysis

Technical Setup

Technical Setup

Response Unit

Triggering / Synchronization

(Hardware) Trigger Generators:

1. Stimulation Computer: event coding and timing via Presentation port codes

2. Response Unit: response coding trigger

3. SyncBox: periodic sync trigger generated from scanner electronic pulse to synchronize the EEG signal sampling by the MR scanner rate (requisite for scanner artefact rejection)

4. fMRI-Scanner: volume trigger representing MR volume scan onset time (used for scanner artefact rejection and event timing in Presentation) All triggers are represented in the recorded EEG data set and one can refer to

them during the subsequent data analysis (artefact rejection, averaging etc.).

Technical Setup

Technical Setup

EEG Recording

EEG-Amplifier

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Technical Setup

Online Recording Setup

Combined EEG – fMRI Recordings

Data quality

Technical Setup

EEG data correction

Major artifacts

• “gradient artifact”• induced currents due to gradient switching

• “ballistocardiographic artifact” • movement of conductive material in static magnetic

field

• vibrations due to active helium pump

The “gradient artifact”

• slice selection: • frequency of slice acquisition• e.g. TR = 2s, 28 slices – 14 Hz (and harmonics)

• spatial encoding within a slice:• usually phase encoding• e.g. 64 × 64 Matrix – 64 × 15 = 960 Hz (not recorded)

EEG data correction

The “gradient artifact”

• technical artifact – rather invariant• correction via subtraction of channel-specific templates

• problem 1: subject motion changes position of cables/electrodes

• foam cushions• problem 2: differential timing of EEG sampling and fMRI

acquisition• EEG/MR Synchronisation – “SyncBox”

EEG data correction

synchronized unsynchronized

EEG data correction


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contaminated EEGraw „clean“ EEGcorrected EEGcorrected EEG with sluggishly fixed electrode

EEG data correction

EEG data correction

The ballistocardiographic artifact

• “ballistocardiographic artifact” • movement of conductive material in static magnetic

field

a) cardiac-related axial head motionb) pulsatile movement of the scalpc) electromagnetic induction due to blood flow

EEG data correction


• correction via subtraction of channel-specific templates

Problems:• biological artifact – high degree of variability• template stability over time – motion induced changes

EEG data correction

BCG artifact

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BCG artifact – after template subtraction

EEG data correction

EEG data correction


• further improvements may be obtained via:• removal of residual BCGA via ICA• Optimal Basis Set (OBS – channelwise temp. PCA)• OBS - ICA

EEG data correction

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BCG artifact – after template subtractionBCG artifact – after additional ICA filtering

EEG data correction


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BCG artifact – after template subtractionBCG artifact – after additional ICA filtering

EEG data correction


• further improvements may be obtained via:• removal of residual BCGA via ICA• Optimal Basis Set (OBS – channelwise temp. PCA)• OBS – ICA

• “automatized” component identification• correlating the raw ECG-trace with time courses of

independent component• correlating BCGA-topography with IC weighting matrix

EEG data correction

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subtraction only additional ICA filtering

EEG data analysis

EEG data analysis

EEG data analysis

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standard fMRI single trial fMRI

Conclusions

Current studies:• Tactile Stop-Signal task (executive functions)• Affective conditioning• Language processing

Planned study:• Resting state/default mode network

Documents

Simultaneous recording of EEG and BOLD responses Why and How