Conducting Hyperscanning Experiments with Functional Near-Infrared Spectroscopy

By measuring brain activities of two or more people simultaneously, this method can shed light on the neurobiological mechanisms underlying social interactions. Functional near-infrared spectroscopy, or fNIRS, is well suited for conducting such hyperscanning experiments because it measures brain hemodynamic response with a high sampling rate and can be applied in relatively natural settings. While this protocol shows how to conduct simultaneous fNIRS recordings of adult-child dyads, it can be adapted to different dyadic constellations such as romantic partners and to different experimental tasks.

Begin by choosing a cap size that is the same size or slightly larger than the participant's head circumference. Cut 15 holes with a diameter of approximately 15 millimeters each, arranged in a horizontal three-by-five grid into the forehead area of each of two raw EEG caps. Ensure that the holes are spaced 30 millimeters from each other in any direction, that the middle column of holes is located in the center of the forehead.

In order to make the caps more comfortable and minimize pressure marks, attach soft foam material at the inner side of the holder grid between the probe sockets and at the edges. Next, mount an empty three-by-five probe holder grid to each of the modified EEG caps such that the holder grid itself is placed on the inside of the cap and the holder sockets stick in the holes. Then, gently insert the probes into the appropriate holder sockets on the grid such that only the first ridge of each probe is mounted in the socket which results in one clicking sound.

Open the probe set monitor window at the nearest measurement system and select two probe sets arranged in a three-by-five grid each, one for the participating child and one for the adult. Ensure that the probe arrangements of the two caps corresponds to the arrangements in the probe set window. Next, switch on the laser diodes of the nearest measurement system 30 minutes before measuring so that the system reaches a stable operating temperature.

Set all the necessary options at the nearest measurement system. Make sure that the device is set to event-related measurement and that the RS232 serial input necessary for receiving triggers from the experimental paradigm is active. Then, prepare the experimental paradigm by starting the technical computing software that serves as base for the Psychophysics Toolbox extensions and setting the current directory to the folder that the paradigm is saved in.

Finally, place two chin rests in front of the computer screen to prevent head movements during the experiment. Begin by escorting the two participants into the testing room and explaining the experimental set-up. Then, seat the participants next to each other in front of the computer screen.

Adjust the height of the chin rests such that both participants sit comfortably. Provide instructions and administer practice trials of both the cooperative and the competitive game. Next, measure and mark the Fpz point according to the 10-20 system, which is 10%of the distance between nasion and inion on each participant's head.

Then turn off the laser, and place the caps with the probes carefully on the participants'heads. Ensure that the middle probe of the bottom row is placed on Fpz and the middle probe column is aligned along the sagittal reference curve. Place the fiber strings on the holder arm attached to the nearest measurement system so that they hang loosely without contact with the participant or chair and that they do not pull on the caps.

Next, push each probe further into its socket until the small white nose in the center of the top of the probe casing is visible. Then, turn the laser on again, and test the signal quality by clicking on the Auto Gain button in the probe set monitor window of the nearest measurement system. If a channel is marked in yellow, meaning insufficient signal, gently put the hair underneath the surrounding probe tip aside.

Make probe adjustments, and click the Auto Gain button again to see if signal quality has improved or is marked in green. Once the signal quality is satisfactory, place a towel over the participants'hands so that they cannot see the hand movements of their respective game partner. Then, start the experiment.

Then, after the experiment is complete, save the data and export the raw light intensity data as a text file by clicking on the Text File Out button. Finally, clean the probes, probe holders, and chin rests with ethnol, and wash the caps with mild detergent. As a measure of brain-to-brain synchrony, the wavelet coherence is calculated between the fNIRS signals of interacting subjects.

Wavelet coherence coefficients are visualized using a color map ranging from blue, little or no coherence, to red, meaning strong or maximum coherence. Results show a strong coherence throughout the experiment in a high-frequency band until a period length of about one second, which likely results from the cardiac rhythms of the subjects. Additionally, a strong coherence appears in a lower frequency band between the two-and eight-second period length.

Coherence in this low frequency range likely reflects a synchronization of brain activities of both subjects during the task. While attempting this procedure, it's important to double-check that the caps are placed correctly on the participants'heads and to ensure a good signal quality. After its development, this technique paved the way for researchers in the field of social neuroscience to explore brain-to-brain synchrony in a variety of different tasks, examining behavior correlates and associations to characteristics of the individuals and their relationship.