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  types of stimulation and their setup  data acquisition setup

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

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

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

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)

Trigger Interface Unit VSM Omega 2000 WC MEG System total number of trigger input channels: 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)

Triggers / Markers Representation in the Dataset triggers are stored as the (sum) of their binary pattern in the Stim channel in the data set (visible during recording) input on line 1  amplitude on Stim channel = 2 0 input on line 17  amplitude on Stim channel = 2 16 simultaneous input on line 1 and 17  amplitude on Stim channel = 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 = s simultaneous input on line 1 and 17  entry for both markers as described above

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 port code Presentation lines active on parallel port marker‘s entry MEG data set , 217, 18 71, 2, 317, 18, 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)

Types of Stimulation

Auditory Stimulation

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 measurement prior to a recording

Visual Stimulation

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

Somato-Sensory Stimulation tactile

Somato-Sensory Stimulation tactile (cont´d) trial and stimulus sequence controlled by Presentation software

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

Data Acquisition Setup Run 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)

Data Acquisition Setup EEG 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

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

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