The processing of stimuli largely and automatically ...

The processing of stimuli largely and automatically depends on whom we are with

J. Bruno Debruille,1,2,3 * Maud Haffar,1,3 Sheila Bouten,1 Ursula Hess,4 & Hugo Pantecouteau,1,5

Affiliations: 1

Research Center of the Douglas Institute, Montréal, Canada.

2

Department of Psychiatry, McGill University, Montréal, Canada.

3

Department of Neurosciences, McGill University, Montréal, Canada.

4

Department of Psychology, Humboldt University, Berlin, Germany.

5

École Normale Supérieure, Lyon, France

* Corresponding author: [email protected] Abstract: Our immediate reactions to events can depend on which close others are present. The production of behaviors that fit those of well-known others reveals an ability at anticipating their reactions. It shows that we can process stimuli not only from our perspective but also from their perspective. Looking for the mechanisms of this social mode of processing, we found that the mere presence of a close other next to participants has a major impact on their event-related brain potentials (ERPs). These ERPs were evoked by images presented for simple memorization. They largely depended on whether participants were sitting side by side with a close one or with a stranger. Partners had larger N300s and N400s and smaller late posterior positivities than strangers. On the other hand, over the right prefrontal cortex, ERPs differed as early as 100 ms post stimulus onset. Discussion of these results led to suggest that stimulus processing could be initially performed in all learned perspectives. It would then depend on the assumed perspective of the close one present, with fewer aspects of the stimulus eventually reaching consciousness in such a presence.

When faced with a happy event, some members of a family rejoice while others temper their enthusiasm. Had each of these people been alone or with strangers, they would have reacted differently as one feels and responds to an event according to the close others present at the time1,2. This variety of reactions among people who know each other well suggest that the way stimuli are processed could differ according to whom the subject is with. This is possible since with prior experiences of emotions and behaviors of close others in various situations one can anticipate some of their reactions to new events. Thus, one can process stimuli not only according one’s own perspective but also, and at the same time, according to the assumed perspective of close others. Such simulations would enrich processing, as they permit to realize more aspects of an event. However, to account for the above described variety of reactions to an event, there would then have to be a rapid selection of a way of being and reacting that fits the anticipated reactions of others and one’s own position relative to them. On the contrary, the variety of the immediate reactions to an event among close ones could be due to a distribution of cognitive processes. Having experienced that some people are better at processing particular aspects of situations might, after a while, prevent this processing when we are in their presence, automatically leaving it to their expertise. This would impoverish our experience of the event but would also account for the reactions’ variety. Both possibilities imply that, when placed in the presence of close others, the brain would not process stimuli as it processes them when we are alone or in the presence of unknown people, that is, of people whose reactions to events have not yet been experienced. There would be a switch to a “social mode” of processing. To the best of

our knowledge, no prior study has tested the existence of such a switch. More specifically, we found no work focusing on the brain activity elicited by the presentation of stimuli, which would have explored whether this activity could depend on whom participants are with. This was the goal of the present study. We recorded the eventrelated brain potentials (ERPs) of subjects who were accompanied by a close other and compared them to those of subjects who were accompanied by a person they never met before. At each trial of the experiment, each member of each of these pairs of subjects was presented with a picture from the international affective picture system (IAPS3) at the same time as the other member of the pair. The participants’ explicit task was to memorize each stimulus separately at its presentation. In order to assess the degree of automaticity of the switch to a social mode of processing when subjects are in the presence of a close one, two block-conditions were used, one where participants were told that they will be presented with the same stimuli as the other person (i.e., the believesame condition) and another where they were told they will be presented with different stimuli (i.e., the believe-different condition). If ERP indices of the switch were found in both conditions and thus even when these processes are irrelevant because the other person is actually seeing something else, this would be strong evidence that social processing of stimuli occurs automatically in the presence of close others, independent of situational goals. During all trials, participants were sitting side by side in front of the screen (Fig. 1 A). They could see each other in the very periphery of their visual field. Participants in closely related pairs could thus feel close to their partner.

Figure 1 A Photo of the experimental setup showing how each of the two members of each pair was tested. The tip of the black arrow coincides with the vertical piece of cardboard that was perpendicular to the flat computer screen, separating it into two halves. This prevented participants from seeing the stimulus presented to his/her partner on the other half. B Timing of the presentation of the IAPS stimuli used for the 2 conditions. The believe-different condition is used as an example. Believe-same

Believe-different

Figure 2. Grand averages (GAs) of the event-related brain potentials (ERPs) elicited by the stimuli of the international affective picture system (IAPS). Participants had to wait for the offset of the stimulus to blink. Red lines correspond to the 32 participants tested in the presence of a close other, black lines, to the 34 tested with a stranger. A Condition where participants believed the other member was presented with the same stimuli, B with different stimuli. These GAs were 2 to 4 μV more negative for partners than for strangers at most of electrode sites. These “closeness effects” started a bit before 200 ms post stimulus-onset and lasted until the end of the brain potentials evoked by the stimulus offset, around 1175 ms (Fig. 3). They appeared to be largest at the central electrode site, Cz. In contrast, they seemed quasi absent at occipital (O1 and O2), right occipitotemporal (T6) and left frontal (F7 and Fp1) sites. Strikingly, at the right prefrontal (Fp2) electrode, they were in the reverse direction and started earlier, around 100 ms post onset.

A: Subtraction waveforms

B: Spline interpolated isovoltage maps of the subtractions in each of the time-window of measure.

Figure 3. Subtraction of the grand averages (GAs) of the event-related brain potentials (ERPs) of the 34 participants tested with a stranger from the GAs of the ERPs of the 32 participants tested with a close other. A Red waveforms: believe-same condition; black waveforms: believe-different condition. B Scalp maps. First row: beliefsame condition, 2nd row: belief-different condition. First column: 100-200 ms post stimulus-onset time-window, 2nd column: 200-350 ms (N300), 3rd: 350-550 ms (N400), 4th: 650-950 ms (late parietal positivity, LPP). The mixed-model ANOVA with closeness (partners vs. strangers) as a betweenand with belief as a within-subject factor for the 100-200 ms post stimulus-onset timewindow revealed that the early part of the closeness difference observed at the right orbito-frontal site (Fp2) in the reverse-direction was significant (F1,64 = 15.97, P = 0.00017). The same type of ANOVA run on the 200 to 350 ms (N300) measures made at the sagittal subset of electrodes (SoE) found the main effect of closeness also significant (F1,64 = 11.12, P = 0.0014), as well as the ANOVA conducted for the parasagittal-SoE (F1,64 = 5.45, P = 0.023), which also revealed that the effect differed across anteroposterior sites and hemiscalps (F6,384 = 7.15, P = 0.00044). The ANOVA performed for

the lateral-SoE also confirmed the effect (F1,64 = 7.20, P = 0.009). Within the 350-550 ms (N400) time window, the three ANOVAs showed an effect at the sagittal- (F1,64 = 16.09, P = 0.00016), parasagittal- (F1,64 = 9.37, P = 0.003) and lateral-SoE (F1,64 = 10.80, P = 0.002). At the parasagittal one, the effect interacted with anteroposterior sites (F6,384 = 7.37, P = 0.0014) and with sites and hemiscalps (F6,384 = 6.91 P =0.00065), which was also the case at lateral-SoE (F4,256 = 7.88, P=0.002). Within the 650-950 ms (P3b-P600 or late parietal positivity) time window, we found a closeness effect at the sagittal-, (F1,64 = 16.51, P=0.00013), parasagittal- (F1,64 = 7.03, P=0.010) and lateral-SoE (F1,64 = 8.14, P=0.006), with an interaction with electrode (F6,384 = 6.37, P=0.003) and with electrode and hemiscalp (F6,384 = 4.32, P=0.011) at the parasagittal SoE, which was also observed at the lateral-SoE (F4,256 = 5.74, P=0.006). The belief of seeing a stimulus identical or different from those seen by the other member of the pair did not interact with the closeness effect. Being with a close other induced much larger negative ERP components (N300s and N400s) and smaller LPPs than being with a stranger. This social switch involved well-known ERP indexes of cognitive processing, suggesting that cognition largely and automatically depends on the person we are with. When in the presence of someone else, the reactions of whom we have experienced in many situations, events are processed very differently. The large spreading of the ERP difference over the scalp is consistent with modulations of several aspects of higher cognitive processes. In contrast, the similarity of ERPs at occipital electrodes sites (i.e., at O1/2, Fig. 1) in the early time window (up to 200m post-onset) suggests that the processing of the physical features of the visual stimuli used did not depend on whom participants were with.

One possible explanation of the effects, namely the influence of a general factor, such as attention, may be excluded. It cannot provide a coherent account of the effects found although being with a close other could have made subjects more distracted, or, on the contrary, more attentive to stimuli by making them more alert. For instance, had the larger N300s and N400s of close others than strangers signaled greater attention, as these ERPs can4,5, they would have been accompanied by larger LPPs. This was not the case whereas the amplitude of this latter component is very well-known to be positively correlated with the amount of attentional resources allocated to the processing of the stimuli6,7. Moreover, close others would also have had greater early ERPs, such as the P1 and the N1, as they are also enhanced by attention8,9. This was not the case. Thus, the ERPs effects found reveal modulations of the particular processes indexed by each of the components involved. Emotion did not have a major role in the ERP differences obtained here either, although processing the affectively laden IAPS stimuli together with a close other could have generated more emotions than processing them with a stranger, as found in2. Had such an emotion effect been responsible for a notable portion of the ERP effects, it would have been greater when partners knew they were presented with the same stimuli and minimal when they knew they were presented with different stimuli, as in2. This was not the case. The belief of being presented with the same versus different stimuli did not interact with the closeness effect. Moreover, none of the ERP effects found match those generated by emotion10. Taken together with the prior exclusion of attention, this also permit to discard a type of emotion, namely anxiety, which could have been greater when

sitting next to a stranger than when with the partner. In effect, anxiety goes with greater levels of arousal. Lastly, the encoding in episodic memory, which was required by the task, is also unlikely to have played a significant role in the ERP effects found here. Deeper encoding is indexed by more positive ERPs for stimuli that are subsequently recognized11,12, the socalled Dm ERP effect. However, when using stimuli that are not related to prior stimuli, as was the case here, the Dm effect only impacts ERPs later than the N400 time window13. In contrast, the present effect started as early as 200 ms post onset, involved both the N300 and the N400 time windows and appears largest during the first one. Moreover, if neither attention nor emotion was affected, as suggested above, there would be no reason for strangers to perform a deeper memory encoding than partners. The results thus clearly demonstrate a major impact of the person we are with on our cognitive processes. They provide a strong support for a switch to a social mode of stimulus processing that occurs automatically when we know we are in the presence of a close one. At first sight, the smaller amplitudes of the LPPs found in partners than in strangers appears to go with the idea of a distributed cognition where the processing of some aspects of the stimulus would be automatically left to the close one. Indeed, having discarded effects of attention, these smaller LPP amplitudes suggest that partners placed a smaller amount of new information into consciousness/working memory than strangers. In effect, the LPP indexes such an amount and is smaller when fewer new information access consciousness. For instance, when the meaning of words used as stimuli is not consciously perceived, these stimuli elicit no LPP14,15, and, the smaller the number of

memory episodes a stimulus can re-activate, the smaller the voltages of the LPP it elicits16. Nevertheless, the alternative idea, namely that of a processing of stimuli performed not only in the perspective of the participant but also in the assumed perspective of the close other could still be correct. In effect, as mentioned, this theory has then to include rapid selections of the subject’s reactions so that they can fit the anticipated reactions of others and one’s own position relative to them. These selections could prevent some aspects of the stimulus from reaching consciousness and be responsible for the smaller LPPs observed in partners. As a matter of fact, the initial processing could even be performed in all the perspectives in which the subject can process the stimulus. This would provide a much larger palette from which the reaction the most adapted to the situation can be selected. Then, if the subject is in the presence of a close other, later processes would focus only on the products of the initial processing that fit the assumed perspective of that particular person. Other stimulus aspects would tend to be discarded. This focus would not occur with strangers, more aspects of the stimulus would enter consciousness, accounting for their larger LPPs. The greater negativities found in partners than in stranger prior to the LPPs may help to see whether the social mode of processing found corresponds to various perspectives or to a distributed cognition. These ERPs differences were maximal at central scalp sites. They reveal a modulation of the N400 ERP itself and thus an impact of the person we are with on our preconscious semantic processing. In effect, there is a general agreement on the idea that the N400 indexes such processes14,15,17. The present results thus confirm that the processing of the meaning of a stimulus in a situation

depends on the people present in that situation. Moreover, the greater N400s of partners signal that stimulus processing was more difficult than that of strangers, as N400s have systematically be related to greater processing difficulties17,

for a review

. This difficulty

clearly goes against the distributed version of the social processing mode. In effect, leaving some aspects of the processing to others should make things simpler for the subject. Greater N400s of partner thus provide support for the multiple perspective version. Nevertheless, as mentioned, this version also has to include the selections of a way of being that fits the anticipated reactions of others and one’s own position relative to them. Based on a limited number of studies showing that N400s could index inhibition processese.g.,

18,19

, one could speculate that the greater N400s of partners index these

selections. They would then be responsible for the reduction of the amount of semantic information entering the content of consciousness. This would prevent us, for instance, from realizing the funny aspects of an event that is negatively affecting the condition of a close other. Greater negativities were also found in partners than in strangers during the N300 time window. Their interpretation can benefit from the works20-23 supporting that the negative ERP deflections maximal around 300 ms at central scalp sites, such as the Nogo N2, index the inhibition of motor representations. Since24, several studies have shown that complex stimuli, such as words and objects, automatically and preconsciously activate the representations of the motor actions they are associated with. These activations occur early, that is, before the occurrence of the processes indexed by these N300s25-27. On the other hand, embodied cognition studies28 have shown that these action representations are involved in the coding of meaning. The larger N300s activity found in

partners than in strangers could thus index selection processes similar to those indexed by the N400, but for this particular type of representations. The striking resemblance of the ERPs of partners and strangers before the N300s at all electrode sites, except FP2, suggests that the early processing of the stimuli was similar. This similarity brings further argument against the distributed version of social processing. In effect, if stimulus processing was only partial when the subject is in the presence of a close other, it should differ from stimulus processing when in the presence of a stranger. The similarity of early ERPs does not support either a multiple perspective version that would occur only when in the presence of a close one. Having eliminated those possibilities leaves us to support only the idea that whether with a close other or with a stranger, subjects would initially process stimuli in all the perspectives they learned. Thus, to sum up, the present results favor a view where the early processing of the stimulus would always be performed in all learned perspectives. Then, if in the presence of a close one, selection processes indexed by the N300 and N400 would limit the amount of information entering consciousness. On the other hand, as mentioned, the earliest ERP effect observed in the present study was a larger negativity observed at the right prefrontal site (Fp2) for strangers than for partners. It began around 100 ms post onset. This electrode-site is close to the orbitofrontal cortex. Lesions of this region tend to be associated with behaviors that do not fit social rules, despite intact knowledge of these rules29. One possibility is that the greater negativity for strangers originates from this brain region and indexes processes that compensate for some of the inhibitions one would have in the presence of close others.

These compensatory processes would allow for appropriate behaviors when a person is not with anyone (s)he knows but with a stranger. Stimuli occurring in that context could automatically trigger such compensatory processes, which would last for the entire duration of the stimulus, as suggested here by the fact this early-starting frontalnegativity remains constant until stimulus offset ERP (Fig 3A).

To conclude, the results reported here demonstrate important and automatic changes of our cognitive processes when in the presence of close others. They open new avenues of research in psychology, starting with the testing of the interpretations proposed above. On the other hand, the results challenge the idea that we can remain ourselves when we are in the company of close others.

Methods Participants 32 right-handed participants (25 F, 7 M, mean age: 23.1 years, SD=3.4) were first recruited by selecting 16 pairs of siblings, or friends and couples who knew each other for at least 3 year. Thirty-four subjects (24 F, 10M, mean age: 22.05 years, SD=2.46) who did not know each other were recruited later. All participants learned about the experiment through classified ad websites. They all had a university degree or were in the process of completing it. They spoke fluent English and had completed, or were in the process of completing, a university degree. They had normal or glasses-corrected to normal vision. Potential participants were excluded if they consumed more than twelve drinks of alcoholic beverages per week or if they used recreational drugs, except if they used marijuana less than once per week. They were also excluded if they had a history of psychiatric disorder, took medication related to such a disorder, or if one of their first degree relatives had a history of schizophrenia or bipolar disorder. All these inclusionand exclusion-criteria were checked by an eligibility questionnaire.

Consent Each participant read and signed an informed consent form accepted by the Douglas Institute Research and Ethics Board, which focused on the second aim. This board, which follows the principles expressed in the declaration of Helsinki, also approved the experiment (Douglas REB #12/12). All methods were performed in accordance with the relevant guidelines and regulations. Data were anonymized, which did not distort scientific meaning.

Stimuli Stimuli were 280 images selected from the International Affective Picture System (IAPS13). They included several striking ones, to ensure the maintenance of participants’ attention during the tasks. The experiment consisted of two blocks, each beginning by the statement mentioning whether participants will be presented with the same or with different stimuli. The order of presentation of these blocks was counterbalanced across pairs of participants. We used two different sets of 140 IAPS stimuli. The allocation of each set to each block was also randomized across subject pairs. In the block in which different pictures were seen by the two members, the 70 pictures presented first to a subject were used for the second part of the block for the other subject of the pair and vice versa.

Procedure Friends were sit side by side and introduced together to the procedure. They read the informed consent form and fill out a demographic questionnaire together. Electrode caps were placed on their scalp and impedances adjusted while they could talk to each other. In contrast, strangers were introduced to the procedure separately and did not receive the electrode caps together. Once sit side by side in front of the computer screen, they were told not to talk to each other when. There, all participants could see the other member of the pair in their very peripheral vision field without moving their eyes. Nevertheless, even if they moved their eye and/or move their heads, they could not see the part of the screen the other member was watching (Figure 3A illustrates this unusual

setting). Each of the two members of each pair faced one half of the same computer screen. This screen divided into two halves by a vertical piece of cardboard. Participants were told to try to memorize each picture. As on Figure 3B, each stimulus was presented for 1000 ms simultaneously to the two members and was followed by a white screen with a black fixation cross, the duration of which randomly varied between 790 and 1500 ms to prevent the development of a contingent negative variation before the next trial.

Data acquisition The electro-encephalogram (EEG) was recorded from 28 electrodes mounted on an elastic cap (Electro-Cap International) and placed according to the modified expanded 10–20 system30. For each participant of each pair, these electrodes were grouped into three subsets: sagittal (Fz, Fcz, Cz and Pz), parasagittal (F3/4, Fc3/4, C3/4, Cp3/4, P3/4, and O1/2), and lateral (F7/8, Ft7/8, T3/4, Tp7/8 and T5/6). There was a separate set of amplifiers for each participant. The right earlobe was used in each subject as the reference for his/her set of amplifiers while the ground was taken from an electrode two centimeters ahead of Fz. For both sets of amplifiers, high- and low-pass filter halfamplitude cut-offs were set at 0.01 and 100 Hz, respectively, using an additional 60 Hz electronic notch filter. EEG signals were amplified 10,000 times and digitized online at a 256 Hz sampling rate and stored in a single file with 56 (28 × 2) channels.

Data processing and measures Each trial whose EEG epoch was contaminated by eye movements, excessive myogram, amplifier saturations or analog to digital clipping was removed offline by

setting automatic rejection criteria. Electrodes for which analog to digital clipping exceeded a 100 ms duration and electrodes for which amplitude exceeded +/- 100 mV were discarded when these excesses were within the -200 to +1000 ms. The baseline was set prior to the onset of the stimulus, from -200 to 0 ms. Averages were calculated for each block and each subject in a 1400 ms time window, beginning 200 ms before the onset of the stimulus and lasting for 1200 ms after the stimulus onset. Following averaging, each file was divided into two files, each containing the ERPs of a single subject. The ERPs of each of the 32 subjects for each belief condition (belief-same vs. belief-different) were then computed and measured independently of the pair of participants they initially belong to. Based on the a priori hypotheses mentioned in the introduction section, we focused on the late positive component (P3b-P600), the N300 and the N400. To measure their amplitude, we computed the mean voltages of ERPs in the 650 to 950 ms, the 350 to 550 and the 200-350 ms time windows for all electrodes, all subjects and each of the two conditions.

Analyses Mixed-model repeated-measures ANOVAs were run to analyze these measures using a multivariate approach. They had closeness (partners vs. strangers) as the betweensubject factor. For the sagittal subset of electrodes, they had belief (belief that stimuli were the same vs. belief they were different) and antero-posterior (Fz vs. Fcz vs. Cz vs. Pz electrode) as within-subject factors. For the parasagittal and lateral subset of electrodes, a fourth within-subject factor, hemiscalp (right vs left), was included. The

Greenhouse and Geisser procedure was used when required to compensate for heterogeneous variances, in which case only the corrected p values are given.

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Acknowledgements This study was supported by the grant 194517-03 from the Natural Sciences and Engineering Research Council of Canada allocated to the corresponding author. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Author contributions J. B. D. wrote the research project that was accepted by the Research and Ethics Board of the Douglas Institute, designed the experiment, partly interpreted the data and wrote the introduction and the discussion of the manuscript. S. B. built the stimulus sequences, recruited the participants, tested them, run the first analyses of the data and wrote the method and the result section of the paper. M. H. and H. P. computed new averages using EEGLab and ERPLab, measured these averages, analyzed the new measures to verify the absence errors and helped to understand the results. Authors had theoretical discussions about social synchrony and perspective taking with U. H. who helped describing and interpreting the results in the terms of social science and reviewed the manuscript. Competing financial interests statement The authors declare that they have no competing financial statement

Materials and correspondence Raw (=EEG) data of all subjects in the EEGLab format (a MATLAB plugin), ERPs of each condition of each subject in the ERPLab format, Excel tables of mean-voltage measures in each time-window for each subject and each condition and SPSS output files

of the mixed-model ANOVAs are available on request to the first author. This author will respond to readers’ enquiries and requests for any materials.