Simultaneous recording of EEG and BOLD responses Why and How

Synopsis Motivation and perspectives Technical Setup EEG data processing The gradient artifact Technical prerequisites: synchronization Artifact removal and data quality The ballistocardiographic artifact Current studies Conclusions

Motivation and perspectives 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 scale synchrony neural firing rates

Motivation and perspectives Basic applications fMRI-informed source reconstruction parametric designs and EEG-fMRI covariation single-trial coupling of EEG and fMRI

Motivation and perspectives Higher order models compound neural mass and hemodynamic models joint ICA parallel ICA

Motivation and perspectives Clinical relevance??? Original Motivation: Mapping epileptic zones Recent „clinical“ research: Movement disorders (cortical myoclonus) Brain-computer interfaces (Biofeedback)

Motivation and perspectives Measurement techniques and applications separate recordings of EEG and fMRI (two sessions) interleaved recordings (EEG in “silent periods”) simultaneous recordings (both modalities continuously measured)

Motivation and perspectives Continuous/simultaneous measurements: temporal correlation of EEG and fMRI avoidance of order effects semi-optimized design strongly degraded signal quality (especially EEG) contaminated EEG contaminated EEG raw „clean“ EEG raw „clean“ EEG

Combined EEG – fMRI Recordings Actual Status Hard- and Software Technical Setup Combined EEG – fMRI Recordings Actual Status Hard- and Software

EEG-Recording Technical Setup System Components (BrainAmp MR plus, Brain Products GmbH): EEG amplifier unit, 32 channel, fMRI approved (GE, Bruker, Siemens and Phillips scanner), accumulator driven 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 Sync-Box (Frequency divider), synchronization between MR scanner and EEG data recording EEG-Data acquisition computer + Recording Software BrainAmp I/O USB Adapter, interface between all other components

Technical Setup EEG cap

Technical Setup EEG Amplifier

Stimulation Modes Technical Setup Visual Stimulation: Stimulation Computer (Presentation) -> Beamer -> Ground Glass -> Mirror (800x600 pixel) -> Subject Auditory Stimulation: Stimulation Computer (Presentation) -> Audiometer -> Audio Amplifier -> MR compatible stereo Head Phones -> Subject Tactile Stimulation: Stimulation Computer (Presentation) -> pneumato-tactile Stimulator -> 8 (finger) membranes -> Subject Components which are inside the MR measurement chamber are emphasized in green

Tactile Stimulation Technical Setup driven by compressed air up to eight independent output channels integrated TTL trigger control unit

MRI compatible opto-electrical Response Unit Technical Setup 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 Response Unit

Triggering / Synchronization Technical Setup Triggering / Synchronization (Hardware) Trigger Generators: Stimulation Computer: event coding and timing via Presentation port codes Response Unit: response coding trigger 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) 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 fMRI Scanner MR chamber Electronic Sync preAmp Stimulation Audio Amplifier Opto-elect Transducer Pneumato-tactile Stimulator Response Buttons Clips Membranes Head Phones Beamer fMRI Scanner Electronic EEG-Amplifier Volume Trigger Sync preAmp I/O-USB Adapter Sync Box EEG Recording

Online Recording Setup Technical Setup Online Recording Setup

Combined EEG – fMRI Recordings Data quality Technical Setup Combined EEG – fMRI Recordings Data quality

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

EEG data correction 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

corrected EEG with sluggishly fixed electrode EEG data correction corrected EEG with sluggishly fixed electrode contaminated EEG raw „clean“ EEG corrected EEG

EEG data correction

EEG data correction The ballistocardiographic artifact movement of conductive material in static magnetic field cardiac-related axial head motion pulsatile movement of the scalp electromagnetic induction due to blood flow

EEG data correction The ballistocardiographic artifact correction via subtraction of channel-specific templates Problems: biological artifact – high degree of variability template stability over time – motion induced changes

BCG artifact – after template subtraction EEG data correction BCG artifact – after template subtraction BCG artifact

EEG data correction

EEG data correction The ballistocardiographic artifact further improvements may be obtained via: removal of residual BCGA via ICA Optimal Basis Set (OBS – channelwise temp. PCA) OBS - ICA

EEG data correction BCG artifact – after additional ICA filtering BCG artifact – after template subtraction

EEG data correction BCG artifact – after additional ICA filtering BCG artifact – after template subtraction

EEG data correction The ballistocardiographic artifact 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

additional ICA filtering EEG data analysis subtraction only additional ICA filtering

EEG data analysis

EEG data analysis amplitude time

EEG data analysis 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