How to Find Effects of Stimulus Processing on Event Related Brain Potentials of Close Others when Hyperscanning Partners

The overall goal of this experiment is to assess the sensitivity of the participants'brain activity to their partner's processing of image stimuli. Reciprocally, the brain activity of the partner is also assessed while presenting stimuli to the participant. This method answer the question of whether brain-to-brain interactions exist and provides a potential basis for the similarity of qualia assumed across individuals, particularly across those close to each other.

Simultaneous EEG recording should detect the emissions of one brain and their impact on the partner's brains in real time, given the precise temporal resolution of EEG. Though this method provides insights into human consciousness, it can also be applied to other organisms that have a cerebral cortex which can be recorded using EEG. To begin this protocol, first recruit pairs of close participants such as close friends, siblings, or spouses, ages 18 to 35.

Inform interested pairs that they will be separately screened to ensure only close participants are included. Make sure that they meet all other inclusion criteria as specified in the manuscript accompanying this video, such as right-handedness and university-level education before scheduling their visit to the lab. Upon their arrival, obtain informed consent and then have each participant separately fill out the electronic Friendship Eligibility Questionnaire without being able to communicate with each other.

After this, have the participants also fill out a criteria eligibility questionnaire and the McGill Friendship Questionnaire. Review this questionnaire to assess their attitude about their relationship and exclude any partners who fail to reach the minimum score of 13 correct answers. Then escort the partners to the adjacent EEG recording rooms.

Turn on the stimulus presentation computer as well as the EEG acquisition computer. Then, start the EEG acquisition application and set the status of EEG channels to Impedance Check. Have each participant sit at a designated computer desk in the adjacent rooms separated by glass.

Keep the curtains open so that each participant can talk to his or her partner through the glass while being fitted with the EEG cap, to reinforce the likelihood that they will feel other's presence during the entire experiment. Next, fit and set up the EEG cap for each participant. For the full setup procedure, please see the protocol and video from JoVE article 52082.

Each participant is connected to his or her own separate set of amplifiers, each having their own ground and reference electrodes. Just prior to the experiment, instruct the participants to try to feel the presence of their partner during the entire testing period and mention to them that this will be checked after the experiment. Then, draw the curtains on both sides of the glass window to prevent any visual communication, dim the lights, and close the door of each subject's room.

It is critical to emphasize to the participants that they try to feel the presence of their partner during the presentation of the images, even when they cannot see each other because they are separated by the glass and curtain. Type in the appropriate command for the given stimulus sequence to run the image presentation. Then, on the EEG data acquisition interface, start recording EEG for both participants when they are presented the visual stimuli simultaneously.

The on-screen directives will first instruct the participants to memorize the images and tell them that they will see different images from those their partner sees. Then, the stimulus sequence begins. This will initiate the serial presentation of images from the International Affective Picture System or IAPS.

In each trial, images are simultaneously shown to each participant. However, within the block of stimuli, the trials vary randomly so that the images are consistent with the stimulus instruction in only half of the trials. This creates two conditions that are either consistent or inconsistent with the earlier instructions that they both will be seeing different images than their partner.

Once the stimulus presentation is complete, stop recording, remove and clean the EEG caps and save the recorded data onto a USB drive for analysis. After the participants have cleaned their hair, have them separately complete a debriefing questionnaire so they cannot feel each other's presence when they are filling out this questionnaire. Here they report the degree to which they felt the presence of their partner during the experiment and they specify during which part they felt this way and for how long.

To begin data processing, first open the data processing software and EEG lab in the command interface. Then import the data file by selecting File and making the subsequent selections as seen on screen here. Split the channels into Human1 and Human2, with one for each participant, and then choose the desired data file for analysis.

Now create an EEG Event List, which consists of a list of entries that correspond to the types of stimuli that were used during the experiment, the same image believed to be different and the different image believed to be different. Next, extract the bin-based epochs, with each epoch consisting of a single Event-Related Brain Potential or ERP that spans from negative 204 milliseconds to 1, 000 milliseconds, where zero corresponds to the onset of the visual stimulus. After performing artifact detection, compute the ERPs of each participant for the consistent and the inconsistent condition by averaging all the corresponding EEG epochs.

Finally determine if at least 12 subjects meet the following criteria, they felt in the presence of each other during the image presentations, had mean voltage ERPs within the 75-to 150-millisecond time window for the consistent condition that were more negative at the F8 electrode site than those of the inconsistent condition. Compute the grand averages for the ERP sets in each condition and plot the resulting waveforms for the critical-consistent versus control-inconsistent conditions. Typical good ERPs of a single participant can be seen here.

Each waveform represents a single EEG electrode, most with well-defined ERP components. The red lines correspond to the inconsistent condition and the black lines correspond to the consistent condition. Here we see typical poor ERPs of the single participant.

Note that many of the signals are characterized by flat-lining and, at other electrodes, many ERP components are not very well defined. This figure shows the grand average of the ERPs of the 27 subjects who felt together during more than 50%of the IAPS trials. The black lines correspond to the critical-consistent category and the red lines correspond to the control-inconsistent category.

This figure shows the average of ERPs of the individuals who felt together for more than 50%of the trials and for whom the consistent condition was more negative at the F8 electrode during the 75-to 150-millisecond time window. There is a noticeable difference between consistent, black, and the inconsistent, red, conditions. The consistent condition is less positive than the inconsistent condition in the Late Posterior Positivity time window for most electrodes.

Once mastered, this technique can be done in 2 1/2 hours if it is performed properly. While performing this procedure, it is especially important to make sure that the participants meet all the criteria and to remind them to focus on feeling the presence of their partner during the entire experiment. After watching this video, you should have a good understanding of how to perform simultaneous EEG scans of pairs of participants.