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Standard texts and polarity rules

1. Introduction
2. The signal fields 'label' and 'physical dimension'
    2.1. The standard 'label' structure
    2.2. The standard 'physical dimension' structure
    2.3. List of signals with the corresponding label and dimension
    2.4. Specifications and polarity rules for EXG labels
    2.5. Specifications for respiration labels
    2.6. Dimension prefixes and EEG electrode names
3. Annotations
    3.1. General
    3.2. PSG scoring
    3.3. Linking to signals

The use of standard texts in several EDF+ fields is usefull if it facilitates more automatic and reliable data processing. This page specifies standard texts that correspond to widely accepted definitions in Clinical Neurophysiology such as the 10/20 and 10/10% electrode systems, and in Sleep Medicine such as the R&K and AASM scoring manuals. Additional texts may be developed later and will be kept at this page. The texts are obligatory in EDF+.
        We will try to update the page regularly in order to keep track with current definitions in Clinical Neurophysiology, Sleep Medicine and possibly other specialisms.

2. The signal fields 'label' and 'physical dimension'

Standard texts for the EDF+ header fields 'label' and 'physical dimension' enable EDF+ software to automatically recognize the kind of signal (EEG, EMG and so on), its polarity and dimension, and the location of sensors (such as EEG electrode positions for mapping). These standard texts comply with the EDF specs and therefore do not cause any incompatibility with EDF software. The texts are obligatory in EDF+ but not in EDF.

2.1. The standard 'label' structure
The header field 'label' offers 16 ASCII characters. The standard structure consists of three components, from left to right:

• Type of signal (for example EEG).
• A space.
• Specification of the sensor (for example Fpz-Cz).

2.2. The standard 'physical dimension' structure
The hader field 'physical dimension' offers 8 ASCII characters.  The standard structure that we propose here consists of two components, from left to right:

• Prefix (for example u).
• Basic dimension (for example V).

Powers in a basic dimension  (for instance the basic dimension of a volume is: meters to the power 3) are noted by ^. Examples are m^3 for a volume or m/s^2 for acceleration. Some basic dimensions involve more complicated mathematical expressions, such as for instance in  Kg*m/s^2. Of course, first everything between brackets must be evaluated. Then the evaluation order is:  prefix - powers - multiplication - division. In this example, the evaluation is ((Kg)*m)/(s^2). As another example, Km^2 means (1000m)^2, not 1000(m^2).

These 16- and 8-character fields contain the standardized information about the type of signal and its dimension. Further, non-standardized, information can be stored in the 80-character fields 'Transducer Type' and 'Prefiltering'.
 

2.3. List of signals with the corresponding label and dimension

The standard 'label' contains a 'Type' of signal and a 'Specification' component. The standard 'physical dimension' contains a 'Basic' dimension component that can be preceded by a prefix. The prefix multiplies or divides the basic dimension by factors of 10 till 10^24. Standard prefixes are listed below.

As an illustration of the versatility of EDF and EDF+, the table also shows some signals that have not yet been, but can easily be, stored. In fact, any time-varying signal, from geology to medicine or the stock market, can be stored. We are deliberately keeping the table incomplete for a lot of signals until these would be frequently applied. At that time we would like to use the expertise from professionals in that field of application.
 

Signal	Label (16 ascii)			Physical Dimension (8 ascii)
	Type	Specification	Example	Basic	Example
Length or distance	Dist	any	Dist A'dam-R'dam	m	Km
Area	Area	any	Area pupil	m^2	mm^2
Volume	Vol	any	Vol moon	m^3	Mm^3
Duration	Dur	any	Dur AP	s	ms
Velocity	Vel	any	Vel light	m/s	Mm/s
Mass	Mass	any	Mass body	g	mg
Angle	Angle	any	Angle azimuth	rad, deg	deg
Percentage	%	any	%	%	%
Value (money)	Value		Value	see below	NLG
Electroencephalogram	EEG	see below	EEG Fpz-Cz	V	uV
Electrocardiogram	ECG	see below	ECG	V	mV
Electroöculogram	EOG		EOG horizontal	V	mV
Electroretinogram	ERG		ERG left	V	uV
Electromyogram	EMG	see below	EMG LAT	V	mV
Magneto encephalogram	MEG		MEG
Magneto cardiogram	MCG		MCG
Evoked Potential	EP	see below
Temperature	Temp	any	Temp rectal	K, degC or degF	degC
Respiration	Resp	see below	Resp abdomen
Oxygen saturation	SaO2	any	SaO2 finger	%
Light	Light	any	Light sternum
Sound	Sound	any	Sound trachea
Sound Pressure Level	Sound	SPL	Sound SPL	dB, dBA, dBB, dBC or DBL	dBA
Events	Event	any	Event button

2.4. Specifications and polarity rules for EXG labels

The 'Specification' of an EEG, EP or EMG signal consists of the locations of the two recording electrodes, separated by a '-' (minus) character. The voltage (i.e. signal) in the file by definition equals [(physical miniumum) + (digital value in the data record - digital minimum) x (physical maximum - physical minimum) / (digital maximum - digital minimum)]. This voltage must equal the potential at the first electrode (before the '-' character) minus the potential at the second electrode. For example, if  the 'Specification' is Fpz-Cz (i.e. the standard label reads 'EEG Fpz-Cz      '), then the voltage in the file must be the potential at Fpz minus the potential at Cz. In case of a concentric needle electrode recording, a positivity at the centrally insulated wire relative to the cannula of the needle is stored as a positive value in the file.
        If electrodes are on any of the below-mentioned standard locations then the corresponding names must be used, for instance in 'EEG Fpz-Cz      '. Else any other name is appropriate, like in 'EEG A-B         '. If the electrode locations cannot be accurately specified in short form, like in some EMG recordings, the 'Specification' may be replaced by a less accurate indication such as the name of the muscle.
       In many standard procedures in Clinical Neurophysiology, a relative negativity at the first electrode must be displayed as an upward deflection on the screen. The displaying software must implement any such 'negativity upward' rule by simply upwardly displaying a negative voltage in the file.
       In standard EEG investigations, EEG electrode signals in the file are usually referenced to one, common, electrode, for example A1. The file then contains, in this example, the signals C1-A1, C2-A1, C3-A1, C4-A1, F1-A1, F2-A1, F3-A1, and so on. This enables re-referencing (remontaging) of derivations afterwards and reduces file size. In some cases, the reference electrode is an average over more than one electrode. In that case, define this average between round brackets. For instance, the EEG between C3 and linked earlobes has label 'EEG C3-(A1+A2)/2'. If the reference is unknown, irrelevant (for instance because it is only used temporarily), or makes the signal label exceed its 16 characters, then use the text Ref, for instance in 'EEG C3-Ref      '. If more of such references exist, then use the text Ref1, Ref2, and so on.

The two EMG derivations for leg movement scoring, as described in "The AASM manual for the scoring of sleep and associated events", have specifications "RAT" and "LAT" for the right and left anterior tibialis muscle, respectively. So, the standard labels are "EMG RAT" and "EMG LAT".

If a standard ECG derivation I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5, V6, or -aVR, or V2R, V3R, V4R, V7, V8, V9, or X, Y, Z is recorded, then the 'Specification' of the ECG signal must equal the name of that derivation, for instance resulting in label 'ECG V2R         '.

2.5. Specifications for respiration labels
 

chest

abdomen

oral

nasal

oro-nasal

For example, the standard label for a nasal airflow recording is 'Resp nasal  '. The 'Transducer type' field should make clear whether this was a pressure, thermistor or thermocouple recording.

2.6. Dimension prefixes and EEG electrode names

Left section of table: dimension prefixes that multiply or divide the basic dimension by factors of 10 till 10^24. For example, the dimension uV means microVolt (10^-6 Volt, that is 0.000001 Volt), ms means millisecond and Ms means Megasecond. Note that the prefix 'u' is the standard-ASCII letter 'u', not the extended-ASCII letter 'µ'. Middle section: standard locations for EEG electrodes according to the international 10/20 and 10/10% systems(see Figure). For instance C3 denotes the Central-left electrode at position 3. Right section: the standard basic dimension for  a currency equals the standard abbreviation as used by banks.

Figure. The standardized 10/20 (top) and 10/10% (bottom) electrodesystems. Note that the T3, T4, T5 and T6 electrodes of the 10/20 system have different names (T7, T8, P7 and P8, respectively) in the 10/10% system. Figure from the webversion of the book by Jaakko Malmivuo & Robert Plonsey: Bioelectromagnetism - Principles and Applications of Bioelectric and Biomagnetic Fields, Oxford University Press, New York, 1995", chapter 13.3.

Standard 'Annotation' texts enable EDF+ software to automatically detect apneas, leg movements and so on. They are obligatory in EDF+ and do not apply to EDF. The texts speak for themselves.

3.1. General Annotations
 

3.2. Annotations mainly for PSG scoring

Note that all annotations except the top row must also specify a duration.

Added in accordance to the AASM manual were the Periodic limb movement of sleep (PLMS), Sleep stages N/N1/N2/N3, Respiratory events including Cheyne Stokes breathing (CS breathing), Cardiac events, Bruxism, REM sleep behavior disorder (RBD), and Rhythmic movement disorder (RMD).

Note that the R&K1968 sleep stages 1, 2, 3, and 4 cannot be used when scoring according to the AASM manual. Use the new stages N, N1, N2 and N3 instead.

Note that a limb movement is annotated as 'Limb movement' and not as 'LM'.

The full list:

3.3. Linking annotations to signal channels

Linking signals to specific recording channels can be optionally specified in EDF+ for which each corresponding annotation must be followed by the two-character string '@@', followed by the corresponding standard EDF+ label. Examples are: 'Limb movement@@EMG RAT' or 'EEG arousal@@Fpz-Cz' .