Monitoring Scalp Coupling of fNIRS Optodes

This section describes the step-by-step procedure for using PHOEBE during the preparation of a subject for an fNIRS experiment. Instrument-specific tasks are noted inline.

1. Obviously, the first step is to fit the subject with an fNIRS headgear which geometrical layout is consistent with the loaded probe. Originally, PHOEBE was developed as a tool to facilitate placing all optodes in headgear at once, thus saving substantial time. However, if no time constraints exist, optodes can be placed individually after parting hair underneath.

2. Launch the instrument software, perform an instrument calibration to optimize light emission and detection levels on all optodes and start streaming data via LSL. See instructions for specific instruments.

3. Launch PHOEBE and click the START MONITOR button on the Monitor panel. All optodes are now colored according to the following scheme:

   • GREEN (sufficient scalp coupling): these optodes do not need to be adjusted.

   • RED (insufficient scalp coupling): these optodes need to be adjusted manually until their color turns green or possibly yellow (see below). An improved scalp coupling is usually achieved by wiggling the optode in attempt to penetrate the hair layer, or by manually parting the hair underneath.

   • YELLOW (undetermined scalp status): in certain cases, the coupling status of an optode cannot be determined when paired to one or more red optodes in an optical channel(s) (see our paper for more details and an example). To resolve the status of yellow optodes, the neighboring red optodes must be adjusted first. As soon as red optodes become coupled to the scalp and their color turns green, the actual coupling status of all linked yellow optodes will be determined, hence turning green or red.

NOTE: Depending on the subject’s hair characteristics (e.g., braids, thick and dark hair), it is possible that not all optodes will be satisfactorily coupled to the scalp even after prolonged adjustment.