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MEG Experiments Stimulation and Recording Setup

MEG Experiments Stimulation and Recording Setup. Educational Seminar Institute for Biomagnetism and Biosignalanalysis February 8th, 2005. Overview. general structure of an MEG / EEG experiment Triggers , a way of cross-talk between stimulation and data recording

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MEG Experiments Stimulation and Recording Setup

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  1. MEG ExperimentsStimulation and Recording Setup Educational Seminar Institute for Biomagnetism and Biosignalanalysis February 8th, 2005

  2. Overview • general structure of an MEG / EEG experiment • Triggers, a way of cross-talk between stimulation and data recording • types of stimulation and their setup • data acquisition setup

  3. General Structure of an MEG / EEG Experiment several subjects / patients (multiple) sessions (+ structural MRI session) (several) runs  single MEG data set (multiple) trials / epochs  different conditions (several) stimuli / events

  4. Structure of a Run recording can be done in continuous or trial mode

  5. Structure of a Trial trial can contain more than one stimulus / event

  6. Triggers, a way of cross-talk between Stimulation and Data Recording • generated by • stimulation control software (Presentation) • stimulus detection devices (photo diode / sound detector) • subject response unit (response buttons) • used to • synchronize recording and analysing intervals (run / trial onset) • mark stimulus appearance (event timing) • define stimulus type (event coding) • sent to and acquired by the MEG electronic • stored in data set (Stim channel / Markerfile)

  7. Trigger Interface Unit VSM Omega 2000 WC MEG System • total number of trigger input channels: 32 • 16 adjustable lines (bit 1-16, marker 1-16) • each line can be seperately setup to operate as input port for external trigger signals (ADC) or as internal trigger generator (DAC) • connected to photo diode, sound onset detector and response button unit • input can be any TTL signal with at least 10ms duration • 16 remote lines (bit 17-32) • 8 lines used by Presentation Software (marker 17-24)

  8. Triggers / Markers Representation in the Dataset • triggers are stored as the (sum) of their binary pattern in the Stimchannel in the data set (visible during recording) • input on line 1 amplitude on Stim channel = 20 • input on line 17 amplitude on Stim channel = 216 • simultaneous input on line 1 and 17 amplitude on Stim channel = 20 + 216 • triggers are stored as markers in the Markerfile labeled either with a user defined name or with a number according to the activated trigger line • input on line 1 at 0 ms after sync trigger  entry for marker1, delay = 0.0 s • input on line 17 at 16 ms after sync trigger  entry for marker17, delay = 0.016 s • simultaneous input on line 1 and 17 entry for both markers as described above

  9. Specific Aspects on Trigger Setup with the Presentation Software • 8 data lines of the parallel port of the stimulation computer are used for transfering trigger signals to MEG recording unit • value (port code) send to the parallel port encoded as 8 bit binary pattern recommendation: to get a single marker entry in case of less than 9 different conditions port code value should be set to exponentials of 2 (1, 2, 4, 8, 16, 32, 64, 128)

  10. Types of Stimulation • auditory stimulation • visual stimulation • somato-sensory stimulation • tactile (pneumatic stimulator) • electric (Grass stimulator, media-nerve stimulation) • tactile (vibration, braille modules) • During an MEG recording all these types of stimulation can be applied simultaneously (multi-modal).

  11. Auditory Stimulation

  12. Auditory Stimulation (cont´d) • trial and stimulus sequence controlled by Presentation software • to be prepared:soundfiles (.wav) containing trigger determination pulse (18kHz, 2ms) • recommendation: to perform a hearing threshold measurementprior to a recording

  13. Visual Stimulation

  14. Visual Stimulation (cont´d) • trial and stimulus sequence controlled by Presentation software • to be prepared: • image files (.jpeg / .img) containing trigger determination marker • additionally to be adjusted: • position, angle of the projection screen • size of the projection (field of view) • brightness of the signal

  15. Somato-Sensory Stimulationtactile

  16. Somato-Sensory Stimulationtactile (cont´d) • trial and stimulus sequence controlled by Presentation software

  17. Data Acquisition SetupRun Time Protocol Setup • channel selection • sample rate • trial parameters • online filter • head localization • noise reduction Trigger setup 

  18. Data Acquisition SetupRun Time Protocol Setup (cont‘d) • adjustable trigger lines (marker 1-16) • trigger as sync signal • signal attributes • marker naming Presentation trigger lines (marker 17-24) (same attributes as above)

  19. Data Acquisition SetupEEG Recording Setup • select number of electrodes to be recorded • (max. 120 uni- and 8 bipolar channels) • define label and position of electrodes (e.g. from Polhemus scan) • define online high pass filter (default: no filter) • perform impedance check and null offset correction prior to recording

  20. Data Acquisition Setup (cont‘d) prior to a MEG / EEG recording: • EEG cap setup (?) • Polhemus scan (?) • noise measurement under experimental conditions • attach the head localization coils to the subject • connect subject to stimulation equipment • perform a stimulus threshold detection measurement (hearing level) • Start recording

  21. Thank you for your attention! See you next week, to introduce a way how to analyse your data!

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