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

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

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

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 Stimulationtactile

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

Thank you for your attention!

See you next week,

to introduce a way how to analyse your data!