EEG / MEG: Experimental Design & Preprocessing

EEG / MEG: Experimental Design & Preprocessing Lone Hørlyck Marion Oberhuber

Experimental Design Technology Signal Inferences Design Limitations Combined Measures Preprocessing in SPM8 Data Conversion Montage Mapping Epoching Downsampling Filtering Artefact Removal Referencing Outline

Technology | Signal | Inferences | Design | Limitations | Combined Measures Oscillations • alpha (3 – 18Hz): awake, closed eyes • beta (18 – 30Hz):awake, alert; REM sleep • gamma (> 30Hz):memory (?) • delta (0.5 – 4 Hz):deep sleep • theta (4 – 8Hz):infants, sleeping adults

Technology | Signal | Inferences | Design | Limitations | Combined Measures EP vs. ERP / ERF • evoked potential • short latencies (< 100ms) • small amplitudes (< 1μV) • sensory processes • event related potential / field • longer latencies (100 – 600ms), • higher amplitudes (10 – 100μV) • higher cognitive processes

Technology | Signal | Inferences | Design | Limitations | Combined Measures ERS & ERD • event related synchronization • oscillatory power increase • associated with activity decrease? • event related desynchronization • oscillatory power increase • associated with activity increase? long time windows, not phase-locked

observe: time course … amplitude … distribution across scalp … differences in ERP infer: timing … degree of engagement … functional equivalence … of underlying cognitive process Technology | Signal | Inferences | Design | Limitations | Combined Measures Inferences Not Based On Prior Knowledge

Technology | Signal | Inferences | Design | Limitations | Combined Measures Inferences Based On Prior Knowledge An “ERP component is scalp-recorded elec-trical activity that is generated in a given neuroanatomical module when a specific computational operation is performed.” (Luck 2004, p. 22)

Technology | Signal | Inferences | Design | Limitations | Combined Measures Design Strategies • focus on specific, large, easily isolable component • use well-studied experimental manipulations • exclude secondary effects • avoid stimulus confounds (conduct control study) • vary conditions within rather than between trials • avoid behavioral confounds

Technology | Signal | Inferences | Design | Limitations | Combined Measures Sources of Noise in EEG • EEG activity not elicited by stimuli • e.g. alpha waves • trial-by-trial variations • articfactual bioelectric activity • eye blinks, eye movement, muscle activity, skin potentials • environmental electrical activity • e.g. from monitors

Technology | Signal | Inferences | Design | Limitations | Combined Measures Signal-to-Noise • noise said to average out • number of trials: • large component: 30 – 60 per condition • medium component: 150 – 200 per condition • small component: 400 – 800 per condition • double with children or psychiatric patients

Technology | Signal | Inferences | Design | Limitations | Combined Measures Limitations • ambiguous relation between observed ERP and latent components • signal distorted en route to scalp • arguably worse in EEG than MEG (head as “spherical conductor”) • MEG: application restrictions • patients with implants • poor localization (cf. “inverse problem”)

Technology | Signal | Inferences | Design | Limitations | Combined Measures The Best of All – Combining Techniques? • MEG & EEG • simultaneous application • complementary information about current sources • joint approach to approximate inverse solution … and how about fMRI?

Technology | Signal | Inferences | Design | Limitations | Combined Measures The Best of All – Combining Techniques? • MEG & fMRI • no simultaneous application • co registration (scalp-surface matching) • use structural scan: infer grey matter position to constrain inverse solution • run same experiment twice: use BOLD activation map to bias inverse solution

Technology | Signal | Inferences | Design | Limitations | Combined Measures Summary – General Design Considerations • large trial numbers, few conditions • avoid confounds • focus on specific effect, use established paradigm • take care when averaging • combined measures?

Technology | Signal | Inferences | Design | Limitations | Combined Measures Summary – Specific EEG Considerations • amplifier and filter settings • sampling frequency • number, type, location of electrodes • reference electrodes • additional physiological measures?

Technology | Signal | Inferences | Design | Limitations | Combined Measures Summary – Specific MEG Considerations • amplifier and filter settings • sampling frequency • equipment and participant compatible with MEG? • need to digitize 3D head or recording position?

Experimental Design Technology Signal Inferences Design Limitations Combined Measures Preprocessing in SPM8 Data Conversion Downsampling Montage Mapping Epoching Filtering Artefact Removal Referencing Outline

PREPROCESSING Raw data to averaged ERP (EEG) or ERF (MEG) using SPM 8

Conversion of data • Convert data from its native machine-dependent format to MATLABbased SPM format *.mat (data) *.bdf *.bin *.eeg • ‘just read’– quick and easy • define settings: • read data as ‘continuous’ or as ‘trials’ • select channels • define file name *.dat (other info)

Downsampling • Sampling frequency: number of samples per second taken from a continuous signal • Data are usually acquired with a very high sampling rate (e.g. 2048 Hz) • Downsampling reduces the file size and speeds up the subsequent processing steps (e.g. 200 Hz) • SF should be greater than twice the maximum frequency of the signal being sampled

Montage and referencing • Identify vEOG and hEOG channels, remove several channels that don’t carry EEG data; • Specify reference for remaining channels: • single electrode reference: free from neural activity of interest • average reference: Output of all amplifiers are summed and averaged and the averaged signal is used as a common reference for each channel

Epoching • Cut out chunks of continuous data (= single trials) • Specify time window associated with triggers [prestimulus time, poststimulus time] • Baseline-correction: automatic; the mean of the prestimulus time is subtracted from the whole trial • Segment length: at least 100 ms for baseline-correction; the longer the more artefacts • Padding: adds time points before and after each trial to avoid ‘edge effects’ when filtering For multisubject/batch epoching in future

Epoching

Filtering • EEG data consist of signal and noise • Some noise is sufficiently different in frequency content from the signal. It can be suppressed by attenuating different frequencies. • Non-neural physiological activity (skin/sweat potentials); noise from electrical outlets • SPM8: Butterworth filter • High-, low-, stop-, bandpass filter • Any filter distorts at least some part of the signal • Gamma band activity occupies higher fequencies • compared to standard ERPs

Reassignment of trial labels

Adding electrode locations • Not essential because SPM recognizes most common settings automatically (extended 10/20 system) • However, these are default locations based on electrode labels • Actual location might deviate from defaults • Individually measured electrode locations can be imported and used as templates Change/review 2D display of electrode locations 1. Load file 2. Change/review channel assignments • 3. Set sensor positions • Assign defaults • From .mat file • From user-written locations file

Artefact Removal • Eye movements • Eye blinks • Head movements • Muscle activity • Skin potentials • ‘boredom’ (alpha waves)

Artefact Removal • It’s best to avoid artefacts in the first place • Blinking: avoid contact lenses; have short blocks and blink breaks • EMG: make subjects relax, shift position, open mouth slightly • Alpha waves: more runs, shorter length; variable ISI; talk to subjects • Removal • Hand-picked • Automatic SPM functions: • Thresholding (e.g. 200 μV): 1st – bad channels, 2nd – bad trials • No change to data, just tagged • Robust averaging: estimates weights (0-1) indicating how artefactual a trial is

Excursus: Concurrent EEG/fMRI • MR gradient artefact: • Very consistent because it’s caused by the scanner • Averaged artefact waveform template is created and substracted from EEG data • Ballistocardiogram (BCG) artefacts: • Caused by small movements of the leads and electrodes following cardiac pulsation • Much less consistent • Subtracting basic function from data • SPM8 extension: FAST; http://www.montefiore.ulg.ac.be/~phillips/FAST.html

Signal averaging • S/N ratio increases as a function of the square root of the number of trials • It’s better to decrease sources of noise than to increase number of trials

Visualization, stats, reconstruction, …

References • Ashburner, J. et al. (2010). SPM8 Manual. http://www.fil.ion.ucl.ac.uk/spm/ • Hermann, C. et al. (2004). Cognitive functions of gammaband activity: memory match and utilization. Trends in Cognitive Science, 8(8), 347-355. • Luck, R. L. (2005). Ten simple rules for designing ERP experiments. In T. C. Handy (Ed.), Event-related potentials: a methods handbook. Cambridge, MA: MIT Press. • Otten, L. J. & Rugg, M. D. (2005). Interpreting event-related brain potentials. In T. C. Handy (Ed.), Event-related potentials: a methods handbook. Cambridge, MA: MIT Press. • Rippon, G. (2006). Electroencephalography. In C. Senior, T. Russell, & M. S. Gazzaniga (Eds.), Methods in Mind. • Rugg, M.D. & Curran, T. (2007). Event-related potentials and recognition memory. Trends in Cognitive Science, 11(6), 251-257. • Singh, K. D. (2006). Magnetoencephalography. In C. Senior, T. Russell, & M. S. Gazzaniga (Eds.), Methods in Mind. • MfD slides from previous years(with special thanks to Matthias Gruber and Nick Abreu for their EEG signal illustrations)

Thank You! … and next week: contrasts, inference and source localization

EEG / MEG: Experimental Design & Preprocessing