Transcript 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
 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: 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
recommendation:
port code
Presentation
lines active on
parallel port
marker‘s entry
MEG data set
to get a single marker
entry in case of less than 9
different conditions port
code value should be set to
exponentials of 2
2
2
18
3
1, 2
17, 18
7
1, 2, 3
17, 18, 19
(1, 2, 4, 8, 16, 32, 64, 128)
8
4
20
32
6
22
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)
During an MEG recording all these types of stimulation can be
applied simultaneously (multi-modal).
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!