meg experiments stimulation and recording setup educational seminar institute for biomagnetism and...
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MEG ExperimentsStimulation 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
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: 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)
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 = 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
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
2 2 18
3 1, 2 17, 18
7 1, 2, 3 17, 18, 19
8 4 20
32 6 22
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
1. auditory stimulation
2. visual stimulation
3. somato-sensory stimulation
• tactile (pneumatic stimulator)
• electric (Grass stimulator, media-nerve stimulation)
• tactile (vibration, braille modules)tactile (vibration, braille modules)
During an MEG recording all these types of stimulation can be applied simultaneously (multi-modal).
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 (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 Stimulationtactile (cont´d)
• trial and stimulus sequence controlled by Presentation software
Data Acquisition SetupRun Time Protocol Setup
• channel selection
• sample rate
• trial parameters
• online filter
• head localization
• noise reduction
Trigger setup
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)
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
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