Adding event markers to fNIRS data by hand - Nirx

Friday, 07 de May de 2021
Adding event markers to fNIRS data by hand - Nirx

Although event markers are typically obtained in recorded data by means of external trigger signals, the NIRStar software also allows the user to set event markers by sending triggers manually during the course of the experiment. For both the NIRScout and the NIRSport, a manual marker can be recorded by either clicking the respective button or by pushing the corresponding keys on the keyboard (i.e. F1,F2,…F8).

For the NIRSport, four marker numbers are available, allowing the user to distinguish between different types of event. For NIRScoutX systems, eight manual markers are available. Note that each marker can be used as many times as the user desires.

Different trigger displays: For a NIRSport system, four manual markers are available (left), whereas for the NIRScoutX eight manual markers are available (right).

While adding event markers manually may not be the most precise way to discriminate between experimental conditions, it is very useful for marking unexpected experimental events (e.g. motion artifacts, changes in environmental light, etc.). 

A lab journal, or the experimental notes window in the setup screen, may be used in conjunction. Importantly, when sending triggers by hand, the users should ensure that the event markers that are used, are not already assigned to experimental conditions.

For Aurora fNIRS we currently advice to use the *.nirs file for data analysis. If no event markers have been recorded during the experiment, the data folder will still contain a file that is intended to log temporal information of the experimental session, but it will be empty. Below more explanation is given for the *.evt file and *.nirs file, respectively.

The *.evt file records the event marker channel code and frame number in a format compatible with NIRx NIRSport and NIRScout systems. Each row of the tab-separated table corresponds to a recorded trigger event. The first column contains the frame number at which a trigger was received. Columns 2-9 record the trigger markers, for up to 8 trigger input channels. ‘1’ indicates that an input received a trigger signal. Because NIRScout and NIRSport systems have a maximum of 4 digital input lines, columns 7-10 are always 0.

*.evt file, digital input 1 (DI1) was triggered during frame 18, DI4 was triggered during frame 27, DI15 during frame 31

Although other data files in the Aurora fNIRS data folders contain information about triggers too, we currently advice the user to use the *.nirs datafile created. The *.nirs file is a matlab file that can be seen as the most basic unit for Homer2 analysis. The file will contain several fields, of which one field is SD. SD is a structured variable which contains both information regarding triggers as well as information regarding the source/detector geometry (i.e. the montage).

Trigger information is stored inside the s variable. The s variable specifies the time points and conditions (event markers) of stimulus onset, with <number of time points> and <number of conditions> as dimensions.

While the user may add information to this file manually, analysis platforms also contain tools to edit event marker information. For example, in the nirsLAB software, an editor is available that allows the user to modify information of the *.evt file, or add information in case the file is empty (i.e. no event markers were recorded).

 Reference / Original

Based on NIRx Trigger Manual R2.1 – 2018-11-22

The content published here is the exclusive responsibility of the authors.


César Noronha