How does the human brain process social (versus nonsocial) emotional information? New evidence from an electroencephalography (EEG) study.

The human species is inherently social. From the moment of birth and throughout the entire life span, we depend on social interaction, care, and protection. This notion is nicely reflected by attachment theory and its derivative social defense theory. Both theories emphasize the fundamental human need for social connection and effective cooperation to deal with threat by utilizing the strength of numbers. Such strong reliance on social resources for survival, in combination with a continuous increase in the complexity of our social environment, is even thought to have driven the evolution of our unusually large brains (for body size) – as described by another prominent theory, the social brain hypothesis.

Against the above background, it is likely that the degree of information’s social content may constitute a fundamental and distinct stimulus dimension, which means the human brain may be highly sensitive to the mere presence of social information. Neuroimaging studies using functional magnetic resonance imaging (fMRI) or electroencephalography (EEG), however, have so far only sparsely looked at the specificity of social versus non-social information processing. This particularly holds true for studies also comprising other stimulus dimensions that may be intrinsically salient, as for example negative and/or positive emotional valence as compared to neutral content.

On a more theoretical level, the above question can be situated within the realm of appraisal theories of emotion, and particularly the component process model of emotion. This model suggests that incoming information is only (neurally) processed if it is relevant to the organism at a specific moment in time. The model furthermore suggests that stimulus relevance can be determined in terms of (i) novelty (i.e. suddenness, familiarity, and/or predictability), (ii) intrinsic pleasantness (i.e. negative vs. positive [vs. neutral] valence), and (iii) goal / need relevance (i.e. whether the assessed information accords to or obstructs the current goals and needs of the organism). Despite the intrinsic relevance of social information for humans, the component process model of emotion does not comprise a relevance check in terms of social versus non-social content.

In this EEG study, we sought to clarify how the human brain processes social versus non-social content of incoming information if the latter also contains different intrinsic pleasantness, i.e. is of positive or negative valence or comprises emotionally neutral content. To this end, n= 24 young healthy females were shortly (150 ms) presented with complex visual scenes and their scrambled counterparts while making a scrambled versus intact decision by button press.

The most important finding of our study was that two very early components of visual processing around 80-120 ms and 200-320 ms after stimulus onset showed evidence for an interactive assessment of social content and emotional valence. On the one hand, the first component, also termed P1 – associated with attention allocation during sensory processing in the extrastriate visual cortex – showed augmented amplitudes selectively for social positive (versus non-social positive) content. On the other hand, the second component, also named early posterior negativity (EPN) – reflecting enhanced sensory encoding resulting from involuntary capture of attention – showed larger positivities for social (as compared to non-social) content, with an apparent boost of particularly positive and neutral social information (see Figure 1 below). 

The above findings provide evidence for a very early neural dissociation of social versus non-social content and therefore an additional relevance check. Furthermore, our data suggest that such early social content relevance check is carried out by integrating it with intrinsic pleasantness information from the very beginning of visual processing. What is particularly interesting, is the fact that neural encoding of information that is of a social nature with a positive emotional valence (P1) and later on also neutral content (EPN) appears to be boosted. This boost counteracts a strong and prolonged negativity bias that is evident from about 200 ms and that persists until (at least) 620 ms after stimulus presentation.  

The interactive pattern of social content and emotional valence processing as detected using EEG described above is highly consistent with previous data we have obtained from a study using fMRI. Furthermore, the anatomical source localization of the P1 and EPN effects using EEG strongly overlap with brain activation observed in the fMRI data – particularly for the social > non-social contrast during the EPN (see Figure 2). Such replication of the fMRI effects by EEG therefore further bolsters the validity and generalization of our results.

Taken together, our novel EEG results suggest that relevance detection may occur already as early as around 100 ms after stimulus onset and may combine relevance checks not only examining intrinsic pleasantness/emotional valence, but also social content as a unique and highly relevant stimulus dimension. 

We are currently running a follow-up combined fMRI & EEG study to replicate and further extend the observed early social content by emotional valence interaction effects (mapping of fMRI and EEG data within the same participants).

Both the fMRI and EEG papers described above are published open access and are freely available (distributed under the terms of the Creative Commons Attribution 4.0 International License [http://creativecommons.org/licenses/by/4.0/], which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made).