Pre-processing and the role of horses in the history of M/eeg vladimir Litvak

Pre-processing and the role of horses in the history of M/EEG

• Vladimir Litvak

The *.mat file contains a struct, named D, which is converted to an meeg object by spm_eeg_load.

• The *.mat file contains a struct, named D, which is converted to an meeg object by spm_eeg_load.

• The *.dat file is memory-mapped and linked to the object.

• Special functions called ‘methods’ provide a simple interface for getting information from the object and updating it and ensure that the header data remain consistent.

Now lets take a step back

MEG:

• MEG:

• Requires quite complex sensor representation including locations and orientations of the coils and the way MEG channels are derived from the sensors.

• Sensor representation is read automatically from the original dataset at conversion.

• EEG:

• Presently only requires electrode locations. In the near future will also include a montage matrix to represent different referencing arrangements.

• Usually electrode locations do not come with the EEG data.

• SPM assigns default electrode locations for some common systems (extended 10-20, Biosemi, EGI – with user’s input).

• Individually measured locations can be loaded; requires co-registration.

Coordinate systems can differ in their origin, units and orientation.

• Coordinate systems can differ in their origin, units and orientation.

• MNI coordinates are defined using landmarks inside the brain.

• Advantage: locations can be related to anatomy
• Disadvantage: co-registration to a structural scan is required

• Head coordinates are defined based on the fiducials. Commonly used for MEG, but the definition differs between different MEG systems.

• Advantage: once the location of the head is expressed in head coordinates, it can be combined with sensor locations even if the subject moves.
• Disadvantage: requires fiducials; if the fiducials are moved, the coordinate system changes.

• Device coordinates are defined relative to some point external to the subject and fixed with respect to the measuring device.

• Advantage: head locations can be compared between different experiments and subjects.
• Disadvantage: head location needs to be tracked.

In SPM8

• In SPM8

• Before co-registration

• MEG sensors are represented in head coordinates in mm.
• EEG sensors can be represented in any Cartesian coordinate system. Units are transformed to mm.

• After co-registration

• MEG sensor representation does not change. The head model is transformed to head coordinates.
• EEG sensors are transformed to MNI coordinates.

Definition: Cutting segments around events.

• Definition: Cutting segments around events.

• Need to know:

• What happens (event type, event value)

• When it happens (time of the events)

• Need to define:

• Segment borders

• Trial type (can be different triggers => single trial type)

• Note:

• SPM8 only supports fixed length trials (but there are ways to circumvent this).

• The epoching function also performs baseline correction (using negative times as the baseline).

High-pass – remove the DC offset and slow trends in the data.

• High-pass – remove the DC offset and slow trends in the data.

• Low-pass – remove high-frequency noise. Similar to smoothing.

• Notch (band-stop) – remove artefacts limited in frequency, most commonly line noise and its harmonics.

• Band-pass – focus on the frequency of interest and remove the rest. More suitable for relatively narrow frequency ranges.

Re-referencing can be used to sensitize sensor level analysis to particular sources (at the expense of other sources).

• Re-referencing can be used to sensitize sensor level analysis to particular sources (at the expense of other sources).

• For other purposes (source reconstruction and DCM) it is presently necessary to use average reference. This will be relaxed in the future.

• Re-referencing in SPM8 is done by the Montage function that can apply any linear weighting to the channels and has a wider range of applications.