6. Infrastructure and tools

The HED infrastructure includes libraries written in Python, Matlab, and JavaScript that support the use of HED in validation and/or applications. This section describes the expected behavior of the HED infrastructure and its integration into other systems such as BIDS.

In general, tools should either explicitly call HED validation to assure that the input tag strings are valid or should make explicit that they assume the HED has already been validated. Most tools will use the latter approach.

See 3.2. Annotation formats for more detailed specifications of HED formats.

See 4. Basic annotation and 5. Advanced annotation for examples and usage.

6.1. Basic tag handling

HED-compliant tools should be able to a handle HED string in its equivalent forms and using various valid syntax as described in this section.

6.1.1. Tag forms


HED-compliant tools should be able to handle tags in long-form, short-form or any valid intermediate-form.

Tools may assume that validated HED tags do not have leading, trailing, or consecutive forward slashes in their names.

In addition to being property formed, validated HED strings will correspond to terms in the schemas under which they were validated.

Tools should not distinguish between variations in case for the same tag term. Only units must have their cases preserved.

Tools may assume that the individual tags within validated HED strings have values of the proper form and that the units, if provided, are consistent with any unit classes

Note: At this time it is not required that terms with specified unit classes always have associated units. However, it is implicitly assumed that if the units are omitted in this case, the value has the default units.

See 3.2.2. Tag forms for more information on tag forms.

6.1.2. Parentheses and commas

Tools may assume that validated HED strings have no duplicates, empty tags, empty groups (parentheses enclosing only whitespace), or mismatched parentheses.

Grouping with parentheses in HED indicates that the tags are associated. Where possible, parentheses should be preserved.


HED-compliant tools should be able to handle arbitrary correctly nested parentheses and correctly distinguish differences in grouping.

6.1.3. Tag ordering

Any ordering of HED tags and HED tag groups at the same level within a HED tag group is equivalent.

Any ordering of top-level HED tags and HED tag groups in a HED string is equivalent.


HED-compliant tools should not rely on the order that HED tags appear within a string or group during processing.

6.1.4. Definitions


HED-compliant tools should be able to expand, shrink, or remove definitions.

HED definitions should only appear in sidecars in dummy entries or in an accompanying definition list. Actual Definition groups should not appear in the HED column of event files.

6.2. File-level handling

Dataset formats such as BIDS (Brain Imaging Data Structure) allow users to provide HED tags in multiple places. For example, BIDS dataset event files often use local codes to identify event markers in tabular (events.tsv) files and then provide dictionaries called JSON sidecars to map local codes to annotations.

The introduction of definitions and temporal scope for HED versions >= 8.0.0 has added additional complexity to validation and processing. Instead of being able to validate the HED string for each event individually, HED validators must now also check consistency across all events in the data-recording.

Tools should make explicit whether they support temporal scope. Tools that support temporal scope should be able to add scoped event information to the Event-context tag group of the intermediate events upon request.

Tools should make explicit whether they support insertion of actual events for Delay tag expansions and for the offsets of Duration tags. This information will allow analysts to call HED tools that support these operations to appropriately modify event files as a preamble to processing if the tool does not support these tags.

6.3. HED support of BIDS

BIDS (Brain Imaging Data Structure) is a widely-adopted specification and supporting tools for organizing and describing brain imaging and behavioral data.

6.3.1. BIDS tabular files

HED has been incorporated into the BIDS standard as the mechanism for annotating BIDS tabular files, which are tab-separated value files whose first line has the column names. A BIDS tabular file has extension .tsv and can be optionally accompanied by a similarly named .json file (i.e., a sidecar) containing metadata for the file.

The most common types of BIDS tabular files are shown in the following table.

Ends with


A row represents



Markers on the timeline of another file.



Metadata about one dataset subject.



Metadata about one dataset recording.



A measurement not associated with an onset.



Properties of a particular sample (e.g., tissue).



A measurement from a particular participant.

HED treats tabular files such as (e.g., events.tsv files) whose first column has the name onset as expressing markers on a timeline. These timeline files allow Onset, Inset, and Offset tags in their annotations and receive additional file-level processing to assure that these tags are properly matched.

Tabular files that do not represent timelines are not permitted to use Onset, Inset, and Offset tags in their annotations. See ONSET_OFFSET_INSET_ERROR for additional information.

The following shows an excerpt from a BIDS event file:

Example: Excerpt from a BIDS events.tsv file.

onset  duration  trial_type  response_time  HED
1.2    0.6       go          1.435          Label/Starting-point, Quiet 
5.6    0.6       stop        1.739          n/a

The first two columns in a BIDS event file are required to be onset and duration, respectively. The onset is the time in seconds of an event marker relative to the start of its corresponding data recording, while the duration represents the duration in seconds of some aspect of the event. The remaining columns in this event file are optional.

BIDS reserves an optional column named HED to contain HED strings relevant for the instance represented by that row. In the above example, the first row HED column contains Label/Starting-point, Quiet, meaning that the event that starts at time 1.2 has these particular annotations in addition to annotations contributed by any relevant sidecars. The HED column in the second row contains n/a, indicating that entry should be ignored. In this case only annotations from applicable sidecars are applied.

HED annotations can also be associated with entries in other columns of the event file through an associated JSON sidecar as described in the next section.

6.3.2. BIDS timeseries

BIDS also includes another type of file with extension .tsv that represents continuous timeseries data. For example, the motion capture _motion.tsv files give samples from channels of a motion capture apparatus, while physiological data files _physio.tsv and _stim.tsv represent continuous physiological recordings such as cardiac and respiratory signals. These tabular files do not have column headers, but rather use other files to define the column names. BIDS currently does not support HED in these types of files.

6.3.3. BIDS sidecars

BIDS also recommends data dictionaries in the form of JSON sidecars to document the meaning of the data in the event files. HEDTools supports BIDS dataset format, where event metadata is contained in compatibly-named sidecars. See the example sidecar in Chapter 3 for an explanation of the different sidecar entries.

6.3.4. Annotation assembly

HED tools are available to assemble the annotations associated with each row in a tabular file using its HED column and the sidecar information associated with other columns of the events file.

For example, the annotations for the first row of the example event file above can be assembled using the example sidecar in Chapter 3 to give the following annotation:

Example assembled HED annotation for one event marker.

Sensory-event, Visual-presentation, (Square, Blue), (Delay/1.435 ms, Agent-action, (Experiment-participant, (Press, Mouse-button))), Label/Starting-point, Quiet

The process is to look up the appropriate row annotation for each column in the sidecar and append these with an annotation in the HED column if available.

6.3.5. HED version in BIDS

The HED version is included as the value of the "HEDVersion" key in the dataset_description.json metadata file located at the top level in a BIDS dataset. HEDTools retrieve the appropriate HED schema directly from GitHub or from locally cached versions when needed.

The following example dataset_description.json specifies that HED version 8.0.0 is used for a dataset called “A wonderful experiment”.

Example: BIDS dataset description using HED version 8.0.0.

   "Name": "A wonderful experiment",
   "BIDSVersion": "1.4.0", 
   "HEDVersion": "8.0.0"

It is possible to include library schema in the HED version specification of the dataset_description.json file as shown by the following example:

Example: BIDS dataset description using HED version 8.1.0 and score library 1.0.0.

  "Name": "A great experiment",
  "BIDSVersion": "1.7.0",
  "HEDVersion": ["8.1.0", "sc:score_1.0.0"]

The version specification indicates that tags from the score library must be prefixed with sc: namespace identifier in dataset HED annotations.

The prefix notation (such as the sc: prefix for the score library in the previous example) is required when more than one schema is used in the annotation. However, namespace prefixes can be used with the standard schema as well as library schemas as illustrated by the following example.

Example: Prefixed standard schema in BIDS dataset description version specification.

  "Name": "A great experiment",
  "BIDSVersion": "1.7.0",
  "HEDVersion": ["st:8.1.0", "score_1.0.0"]

For this specification tags from the standard schema must be prefixed by st:, while tags from the score library are unprefixed. The sc: and st: namespace prefixes are arbitrary (usually short) alphabetic strings chosen by the annotation and are specific to each dataset based on its version specification.


HED-compliant tools must be able to handle multiple schemas and prefixed tags.

6.3.6. HED in the BIDS validator

HED provides a JavaScript validator in the hed-javascript repository, which is available as an installable package via npm. The BIDS validator incorporates calls to this package to validate HED tags in BIDS datasets.

6.3.7. HED python tools

The hedtools package includes input functions that use Pandas data frames to construct internal representations of HED-annotated event files.

HED schema developers generally do initial development of the schema using .mediawiki format. The tools to convert schema between .mediawiki and .xml format are located in the hed.schema module of the hed-python GitHub repository. All conversions are performed by converting the schema to a HedSchema object. Then modules wiki2xml.py and xml2wiki.py provide top-level functions to perform these conversions.