Working With Additional Files

Module information
Modules	BESA Research Basic or higher
Version	6.1 or higher

Binary format (*.foc, *.fsg)

Select Binary High Resolution or Binary Compressed Format to output segments in binary BESA format. If the file already exists, the segment will be appended. Thus, it is possible to create a file combining several segments of interest in a compact form. BESA Research will only allow you to append segments if the number of channels and the sampling interval in source and target files are the same.

In Binary Format all channels (scalp, intracranial, polygraphic, MEG) and file events in the selected time range are exported. Note: The channels are filtered according to the current filter settings.

Select Binary High Resolution to retain the resolution of the processed data. This is the preferred binary format for small amplitude signals such as averages. Select Binary Compressed Format to store raw data using the original resolution or to obtain the space savings of compression (see File/Export and Append Data/Convert.. above). This is the preferred binary format for raw data.

ASCII vectorized format (*.avr)

Select ASCII vectorized Format to output segments in BESA ASCII format, one channel (all time points) per line.

The Format is as follows:

The first of two header lines contains the following data descriptors (6 descriptors, the values shown are only examples):

Npts= 200	number of sampled points in each channel
TSB= -500	time sweep begin [ms]. Time of first data point relative to zero of epoch
DI= 5	digitization or sampling interval [ms]
SB= 2	scaling bins/microvolt in file = number corresponding to 1 microvolt
SC= 50	scaling calibration, used for setting magnitude of display in BESA
Nchan= 27	number of channels
SegmentName= 60dB	an optional label describing the data

The second line of the header contains a label for each channel, e.g.

O1 Oz P3 T5 T3 C3 F7 F3 Fp1 Fz Cz Pz Fp2 F4 F8 C4 T4 T6 P4 Fpz O2 M2 M1 F10 F9 T10 T9

Each of the subsequent Nchan lines of the file contains values for all Npts time points in floating point or scientific format. For more details about scalp electrodes, see chapter Working With Electrodes and Surface Locations/Electrodes/Electrode Conventions.

A second (older) version of the format (written by BESA versions 1, 2 and 3) omits the Nchan=xx information in the first line, and there is no second header line. Labels must be defined elsewhere. See Channel definition file conventions and formats and Reading MEG files in ASCII format. In the older versions, only scalp channels were exported, and the data were average referenced.

ASCII Multiplexed format (*.mul)

Select ASCII Multiplexed Format to output segments one time point (all channels) per line.

The ASCII Multiplexed Format is as follows:

The first of two header lines contains similar information to that of the ASCII vectorized file:

TimePoints= 200 Channels= 27 BeginSweep[ms]= -500.00 SamplingInterval[ms]= 5.000 Bins/uV= 1.000 SegmentName=Condition1

Note that the item SegmentName is missing if no segment comment is specified when writing a segment to file.

If an epoch of a continuous EEG is exported in ASCII multiplexed format, the first header line contains the additional item Time, which indicates the daytime of the first sample in the exported segment:

TimePoints= 200 Channels= 27 BeginSweep[ms]= 0.00 SamplingInterval[ms]= 5.000 Bins/uV= 1.000 Time=22:02:53 SegmentName=Segment1

The second line of the header contains labels for each channel, which may be either the original channel names, or the names of the channels of the current montage, e.g.

O1 Oz P3 T5 T3 C3 F7 F3 Fp1 Fz Cz Pz Fp2 F4 F8 C4 T4 T6 P4 Fpz O2 M2 M1 F10 F9 T10 T9

Each subsequent line contains values for all 'Channels' at one time point, in floating point or scientific format. Values are given for the current or the original montage, selected as described above.

Labels for source montages have the following form: TAr-L.

• The first two letters indicate the head region:
  • TA: temporal-anterior
  • TP: temporal-posterior
  • TB: temporal-basal
  • FL: fronto-lateral
  • FP: fronto-polar
  • PC: posterior-central
  • OL: occipito-lateral
  • OC: occipito-central
  • CL: centro-lateral
  • CC: central

• The small letter indicates in part the orientation: r=radial, t=tangential, and in part the relative location of the basal temporal source: l=lateral, m=mesial.
• The final letter after the hyphen indicates L=left, M=middle, R=right.

Channel definition file conventions and formats

BESA Research can read 3 types of file to define channels. These are identified by different extensions:

• channel definition files containing labels and, optionally, channel types (ASCII, 1 label /line): *.ela
• channel definition files containing coordinates and, optionally, channel types and labels (ASCII): *.elp. This is the format of the file written by the Channel Configuration Dialog.
• channel definition files stored by older versions of BESA Research (binary format): *.elb. This format can still be read, but is no longer written by BESA Research.

BESA Research stores and retrieves the channel configuration after editing in binary files. If you open a data file, BESA Research will search for the related channel information in the following sequence:

1. For all data files with the extension .eeg, .cnt and .foc, check in the additional database file in the data directory with the same basename as the data file and the extension .fst, whether a channel file has been associated previously
2. Check in the db subdirectory whether a channel file has been associated previously
3. Check if labels are defined in the header of the data file
4. Search for a corresponding binary channel definition file with the same basename as the data file in the data folder (xxxx.elb)
5. Search for a corresponding channel definition file with the same basename as the data file containing labels in the data folder (xxxx.ela)
6. Search for a corresponding channel definition file with the same basename as the data file containing labels and/or coordinates in the data folder (xxxx.elp)
7. Search for a file named default.elb, default.ela or default.elp (in this order)) in the data folder
8. Search for a file named default.elb, default.ela or default.elp one directory above the data folder (e.g. ..\default.elb)
9. Check if the new data file is of the same type and has the same number of channels as the preceding data file in the list. If this is the case, the electrode configuration of the previous file will be assumed. This will avoid having to load or edit the electrode configuration more than once, if you load several data segments of the same subject from separate files.
10. If no channel definition file is found, or digitization points (in files *.sfp, *.cot, *.pos, *.pmg) are found in files with the same basename as the data file, the "Channel and digitized surface point information" dialog box is opened, allowing you to specify file names.

Channel Label Files (*.ela)

Files containing a list of channel labels are an alternative to editing electrode configurations. They can be edited using a standard text editor. Electrode label files (.ela) require a sequence of lines corresponding to the sequence of channels in the data. Each line contains one label and an optional identifier. The format is ' [Identifier] {Label} ', ('Identifier' can be omitted if the electrode label defines the type of signal)

Identifiers can be one of:

EEG -- scalp electrode

SCP -- scalp electrode

POL -- polygraphic channel

PGR -- polygraphic channel

ICR -- intracranial electrode

MEG -- MEG sensor

REF -- reference electrode (this can only occur once, and must be the last item in the file)

For example:

Fz   (scalp electrode, coordinates assigned by default.ecd)

Cz   (scalp electrode, coordinates assigned by default.ecd)

......

VEOG   (vertical EOG, Polygraphic type is assigned by default)

Exx   (xx=01, 02.. electrode number, Polygraphic type is assigned by default)

......

EEG xx (scalp electrode, coordinates must be assigned either by default.ecd or by a surface point (+.sfp) file. An alternative to the "EEG" prefix is "SCP".

......

POL XX  (identifier sets Polygraphic type -- an alternative to the "POL" prefix is "PGR")

......

ICR A01  (identifier sets Intracranial type to electrode A01 - do not use A1!)

ICR A02  (identifier sets Intracranial type to electrode A02 - do not use A2!)

ICR A03  (identifier sets Intracranial type to electrode A03)

......

MEG M01  (identifier sets MEG type to electrode M01 - do not use M1!)

MEG M02  (identifier sets MEG type to electrode M02 - do not use M2!)

......

REF Cz  (the label is assigned to the electrode reference, no channel is associated with this entry. This must be the last line of the file!)

Channel spherical coordinate files (*.elp)

These files follow the same rules as the channel label files, with the addition of spherical coordinates (theta, phi) that follow the labels of EEG and MEG channels. Labels can also be omitted. In this case, BESA Research will assign labels according to the nearest coordinate defined in the default.ecd file. To indicate that the coordinates have been assigned, the label will have a tick, e.g. Fz' instead of Fz.

Channels of other types (polygraphic, intracranial) are defined exactly as in the channel label (*.ela) file.

cot (Head center) file

Function: to redefine the center of the head for the sphere used in dipole models. If the cot file has the same base name as the data file, it is read automatically by BESA Research. If the coordinates deviate by more than 1 mm from the previously defined head center, a window is opened, asking if the new values should be adopted. This mechanism is turned off if the data have been coregistered to MRI (see online help chapter "MRI Coregistration"), and an MRI Coregistration File (*.sfh) has been associated with the data.

BESA Research uses any head surface points (e.g. electrode locations), excluding those on the lower part of the face, to compute the sphere center automatically. The cot file is used if you want to override the automatic calculation. A mechanism is provided which allows to pass a location from the MRI (viewed by BrainVoyager) to the Source Module and save the resulting location as a cot file.

Format: one set of coordinates (x y z). These are followed by either "DC" or "HC", which specify whether these coordinates are in Device or Head Coordinates.

Units: must be in meters!

(Note: In special cases, a fifth value, the head radius, may follow. This is used when reading simulated MEG data from the DipoleSimulator program. When this value is set, BESA Research uses the specified head radius and head center and does not fit a sphere to the head surface points. and does not create an ellipsoid transformation).

Force BESA Research to use a completely spherical model without creating an ellipsoid: Write "DipoleSimulator" or "Phantom" on the second line of the file. Under these circumstances, 100% correspondence between DipoleSimulator and BESA Research is achieved. This is also required for dipole fitting on MEG phantom recordings.

The cot file has also been extended for reading CTF MEG files. Documentation for these extensions is found in the CTF help file.)

pos or pmg (MEG sensor coordinate) file

Function: to define coordinates of MEG sensors. Our convention is to save magnetometer information in *.pmg, and gradiometer information in *.pos. In practice, BESA Research doesn’t mind which extension is used — the distinction between gradiometers and magnetometers is based on the number of values on each line in the file.

Format: one sensor per line.

Magnetometers (*.pmg): label (optional), six coordinates per line (location, orientation)

e.g. for BTi:

Channel 'A1': -0.0019193 0.0304846 0.1081738 0.1188222 0.2394208 0.9636177

Gradiometers (*.pos): label (optional), nine coordinates per line (location of primary sensor, location of secondary sensor, orientation).

The program decides whether gradiometers are planar or axial based on the distance between the primary and secondary sensor locations and the center of the head.

e.g. for Neuromag:

0.108510 -0.000143 -0.044954 0.108510 0.000463 -0.028766 0.999999 0.001450 0.000000

Labels in these files are ignored.

See chapter Data reading rules for MEG.

sfn (surface point name) file

Function: to match up digitized coordinates with channels that are defined as EEG electrodes and to define labels for additional digitized head surface points (e.g. MEG coils, etc.).

Format: one label per line.

Contains labels of surface points in the order of digitization. If fiducials are defined, these should be on the first three lines, with the labels 'FidT9', 'FidT10', 'FidNz' or 'FidLPA', 'FidRPA', 'FidNAS'.

If electrodes are defined in the data file, the labels of each electrode as defined in the data file (or in its associated ela, elp, or elb file) must be present!

The case of labels is not important (e.g. 'Fp1' will match with 'fp1').

The sfn file need not exist if labels are defined in the sfp file.

See chapter Data reading rules for EEG.

sfp (surface point coordinate) file

Function: to define coordinates of digitized points on the head surface. The order of points must match with the order in the sfn file, or if no sfn file is present, labels must be included in the sfp file.

Note: If the digitized points include electrodes, the channel labels must correspond to the labels of the digitized points. The sequence of labels in channels and surface point coordinate file need not be the same – the allocation is performed by label matching. Channel labels may be defined in the data file, or they may be assigned using channel definition files (*.ela, *.elp, *.elb).

Format: one set of coordinates (x, y, z) per line. Coordinate units must be either meter, centimeter or millimeter (BESA Research will perform a plausibility check automatically to determine which units are used). If a label is present this can precede or come after the three coordinate values.

If fiducials are defined, these should be on the first three lines. BESA Research will simulate fiducials if none are defined, but it is preferable to record these locations along with the other head surface points.

If there are MEG sensors, the same coordinate systems must be used in the sfp file and the pos/pmg file!

If labels are defined in the sfp file rather than in an sfn file, labeling rules apply as for the sfn file.

Example for the sfp file format:

BESA Research will check the coordinates for plausibility. If coordinates are more than 30° away from the expected location on the sphere there will be an error message. Such errors are usually due either to incorrect labeling or to a digitization error.

See chapter “3D Coordinates for Precise Analysis / Data reading rules for EEG”.

Generic File Format

This reader, incorporated into the GenericBesa.dll file, allows to read simple multiplexed or vectorized data formats, if you know the structure of the data format.

What you have to do

• With a text editor, write information about the data file you want to read into BESA Research into a text file, the Generic Header.
• Save the edited text in the same subdirectory as the data file.
• Mechanism A: The generic header contains the data file name. With BESA Research, navigate to the file you just edited, and open it. The data should then be read into BESA Research.
• Mechanism B: Alternatively, navigate to the data file. The reader will check if there is a generic header in the same subdirectory, and use that to try to open the file. You have two options:
  • Specific: if the file has the same basename as the data file, and the extension “.generic”, this file will be used.
  • Generalized: if a specific file is not found, the reader will look for the file “BESA.generic” in the same subdirectory.

Important note: We recommend using mechanism A, using a header with the extension ".generic". When opening the data file in BESA, select the generic header. Mechanism B sometimes fails when opening the data file in BESA Research, because one of the other readers in BESA may erroneously interpret the file as their "own" data format, sometimes leading to a crash.

Format of the Generic Header

The first line must consist of the text: “BESA Generic Data” (without the inverted commas).

Subsequent lines must contain the following parameters, in any order (note that the parameters are case insensitive):

nChannels=nnn: The number of channels

sRate=fff: The sampling rate (samples/sec)

format=type: One of short, int, float, double, ASCII. If the format is ASCII, the parameter nSamples must be specified as well (see below)

The following parameters are optional (values in square brackets denote optional parameters):

nSamples=nnn: The number of time samples in the data. If this value is 0, or the line is omitted, then use the file size to estimate the number of samples.

file = : The data file name, without path information. If this is omitted, you can only read the data with mechanism B (see above). This line must be included if you want to read the data with mechanism A.

DataOffset = nnn: Offset of data in bytes for binary data, in lines for ASCII data (default = 0).

Factor=fff [range]: Data values are multiplied by this factor to obtain µV values (default = 1). Optional parameters can be appended to define a channel range, e.g. 1-3. Thus, this command can be used multiply, to define different scaling factors for different channels. If only one channel is specified, use on number only, e.g. 5.

SwapBytes = ccc: One of off or on (default = off). If the data block originated from Unix or Mac, this will need to be on. (binary data only)

Prestimulus=fff: Prestimulus interval in milliseconds.

Label=ccc: Segment label.

Trigger = chan….: Channel number containing triggers. Without further parameters, the values are read directly as digital trigger values. Other parameters are described below, for the case where the trigger channel contains analog signals.

nBlocks=nnn: The data are epoched. This specifies the number of equal sized blocks in the data. In BESA Research, each block will be separated by a segment boundary. The number of samples in each epoch is computed from the total number of samples divided by nBlocks.

nEpochs=nnn: Same as nBlocks.

EventFile=name: Load events from an event file, using BESA's event file (*.evt) format. See below for a description of how to prepare this file.

Order=type: One of multiplexed, vectorized. The default is multiplexed (i.e. channels fastest). Specify vectorized if your data are ordered so that all time samples for channel 1 are followed by all time samples from channel 2, etc.

Orientation=type: Same as Order.

Arrangement=type: Same as Order.

Trigger events

This section describes how the reader can be used to encode trigger events when the trigger channel contains analog signals. In this case, a Trigger command is required for each target code in BESA Research.

Syntax:

Trigger = chan  code  fromLevel  toLevel  timerange  deadtime

chan is the channel number on which to find the trigger

code is the trigger number that the reader will assign (must be positive!)

fromLevel is the value in mV defining the lower range for trigger detection

toLevel is the value in mV defining the upper range for trigger detection. If this is “-“, then only fromLevel needs to be exceeded for the trigger to be detected.

timerange is the range in milliseconds to define a trigger. The reader will search for the maximum deviation from baseline within the range to find the level that will define the trigger.

deadtime defines the time after detecting a trigger during which no further trigger with this code can be detected. This does not affect other codes. Also, if the voltage level stays at a level corresponding to a code, the trigger is only defined at the onset of this voltage level.

Multiple lines are required if different trigger codes and different trigger channels are required, one for each new code.

Notes

Channel labels: The data channels are labeled E1, E2, E3,…, and they are initially classified by BESA Research as polygraphic. As with other BESA Research data files, use a *.ela file (or *.elp, optionally combined with *.sfp) to redefine labels and channel types.

Data formats:

• Short: 16-bit
• Int: 32-bit
• Float: 32-bit
• Double: 64-bit
• ASCII: Decimal numbers separated by spaces or tabs (engineering format also permitted)

Prestimulus interval and label: If either of these are defined, BESA Research reads the data in to define an averaged data segment. The label is displayed, and a vertical dotted line marks timepoint zero. If no prestimulus interval is defined, a zero prestimulus interval is assumed.

Future changes

Possible developments:

• Read channel labels

If any of these changes are particularly important to you, please contact support@besa.de and let us know.

Event File

The event file is a text (ASCII) file containing a header line and subsequent lines, with one event description per line.

Each line contains four parameters:

1. latency (units specified by the header, can be µs, ms, s)
2. code (defines the type of event: trigger, comment, marker, pattern, average segment, data break segment)
3. parameter (depends on the event type, e.g. trigger code)
4. label (label assigned to the event)

Header Line:

The header line contains four values. The first specifies the time units, e.g. Tmu specifies microseconds.

    Tmu    Code    TriNo    Comnt

Tms specifies milliseconds. Tsec specifies seconds.

Event Code and Parameter 3 (TriNo):

Code specifies the event type:

1 = trigger -- TriNo specifies the trigger number

2 = comment

3 = marker

11-15 = patterns 1-5

21 = artifact on

22 = artifact off

31 = epoch on

32 = epoch off

41 = segment onset -- TriNo is a time string that specifies date and time, in the format YYYY-MM-DDTHH:MM:SS, e.g. 2010-04-26T15:30:20.31 (note: seconds are a decimal number).

42 = average segment onset -- TriNo is a number specifying the prestimulus baseline of the subsequent average in microseconds. TriNo (parameter) is 0 for markers, comments, artifacts, and epochs.

Comment

The event label. This is not used for markers, artifacts, and epochs.

Example of event file:

A simple way to generate example files is to export events from BESA (ERP/Save Events As...).

Tmu         Code     TriNo     Comnt

0              42     100000    Ave: 25 avs  

10000000  2       0            Comment at 10s

20000000  41     26-04-2010T15:30:20.000   TestSeg2

21000000  3       0

22000000 1        99          Trigger - 99

This specifies an average segment starting at the beginning of the file, with a prestimulus interval of 100 ms, a comment at 10 s, a new segment specifying date and time at 20 s, a marker at 21 s, and a trigger with code 99 at 22 s.

Examples

The following reads ASCII multiplexed data that were previously exported from BESA Research:

BESA Generic Data

nchannels = 64

srate = 100

nsamples = 10000

dataoffset = 2

format = ASCII

file = name.mul

factor = 1

With the sampling rate of 100 Hz and 10000 samples, this represents 100 s of 64-channel data. The first two lines of the data are skipped.