This protocol reveals inter-brain synchrony in the collaborative dyads in the naturalistic environment by the fNIRS hyperscanning technique. The significant advantage of this technique is it allows investigation of real-time dynamics of two or more interacting brains. Before beginning the experiment, start the near infrared spectroscopy system for achieving a stable operating temperature, then adopt an elastic swimming cap to place the optode-holder grid and prepare the NIRS caps as described in the text manuscript.
For a channel with insufficient signals, use the lighted fiber optic probes to move the hair under the probe's tip to one side. Once the operating temperature stabilizes, ensure that the trigger receiver is active and set the measurement mode to event-related measurement. Prepare the lighted fiber optic probe to move hair aside.
Instruct the participants regarding the details of NIRS measurement methods and not to look directly into the laser beams as the laser beam may be harmful to the participant's eyes. Make the participants sit face to face to make sure they can see each other directly and adjust the chair-to-table distance to make the participants sit comfortably. Turn on the laser button, put the four optical fiber bundles loosely on the holder's arms without contact with the participants or chairs and place the cap with the probe sets on the participants'heads, ensuring that the three-by-five probe sets cover the forehead of the participant and the four-by-four probe sets cover the left temporoparietal cortex, then carefully push each spring-load probe further into the probe socket so that the probe's tip touches the scalp of the participant.
Then allow two participants to co-learn the learning materials with five minutes rest state, which serves as the baseline. First, to check the quality of the signal, click the auto gain in the probe set monitor window of the functional NIRS machine, and then mark the channel's poor signal in yellow and the sufficient signal in green and repeat this procedure until all the channels are marked green. After the experiment, click on the Text File Out to export the raw light intensity data and save the data as a text file.
Use the three-dimensional digitizer to determine the locations of emitters, receivers, and other references for each participant, then obtain the MNI coordinates of the recording channels using the commercially available numeric computing platform. The data from each participant in each channel were analyzed. The optodensity in channel 33 for a certain dyad was visualized.
The data were extracted using the wavelet-based motion artifacts removing method, alone and along with the principal components analysis. The difference between the curves suggests that PCA is efficient in removing non-neural signals. The wavelet transform coherence matrix was visualized.
The color map varied from blue to yellow, representing inter-brain synchrony values ranging from zero to one, where one denotes the largest coherence between two signals and zero denotes no coherence. In addition, significant coefficients were also obtained, the strong coherence around one hertz representing the dyad's cardiac rhythm coherence. The comparison between the observed T value and the distribution of random T value shows significant results in identified FOI.
It's important to make sure the probe set covers the region of interests and try to reduce the deviation.
