Positive emotions are mainly processed in the left hemisphere of the brain

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Research into the neuronal basis of emotion processing has so far mostly taken place in the laboratory, i.e. in unrealistic conditions. Bochum-based biopsychologists have now studied couples in more natural conditions.

Using electroencephalography (EEG), they recorded the brain activity of romantic couples at home while they cuddled, kissed or talked about happy memories together.

The results confirmed the theory that positive emotions are mainly processed in the left half of the brain.

A group led by Dr. Julian Packheiser, Gesa Berretz, Celine Bahr, Lynn Schockenhoff and Professor Sebastian Ocklenburg from the Department of Biopsychology at Ruhr-Universität Bochum describes the results in the journal Scientific Reports, published online on 13 January 2021.

In previous studies on the neuronal correlates of emotion processing, feelings were usually triggered in subjects by presenting certain images or videos in the laboratory.

“It was unclear whether that really reflects how people experience and act out feelings,” says Julian Packheiser. “Ultimately, emotions comprise not only the perception of feelings but also their expression.”

Mobile EEG equipment enables measurements at home

This is why, in the current study, the researchers measured the brain waves of 16 couples in positive emotional situations in their own homes. A measurement of this kind would not be possible using conventional EEG systems as the movements created during kissing, cuddling or gesticulation produce artifacts in the data.

“We have used a mobile EEG system that records not only brain waves but also the movement patterns of the subjects,” explains Julian Packheiser. This allowed the team to control the artifacts in the data.

Positive situations, above all emotional situations involving kissing and talking, were associated with greater activity in the frontal areas of the left half of the brain.

The study thus confirmed the results from laboratory investigations and what is known as the valence model of emotional lateralisation, which states that positive emotions tend to be processed in the left half of the brain and negative emotions in the right.

During the study, the researchers evaluated the EEG data in a certain frequency range, the alpha frequency band between 8 and 13 Hz and the beta frequency band between 13 and 30 Hz.


Recognizing emotions and being able to simulate them—a process generally referred to as facial mimicry—are important facets of human social functioning. These elements are vital in human life. Newborn infants already show a preference for faces and face‐like stimuli (Johnson, 2005), and facial mimicry is considered to be an automatic process (Dimberg, Thunberg, & Grunedal, 2002) that supports a quick understanding of the emotionality of “the other” in social interaction (Niedenthal, 2007).

Thus, simulation and mimicry of facial emotion expressions are a human fundamental ability that plays a key role in attending to and interpreting other’s facial expressions in human interaction and communication.

However, not all people are blessed with such ability. There are patients who can be assumed to encounter limitations in simulating facial expressions, due to impaired facial functioning such as facial paresis. In line with this idea, there is compelling evidence that impaired facial functioning undermines social functioning, emotional life, and mental health (Cross, Sheard, Garrud, Nikolopoulos, & O’Donoghue, 2000; Fu, Bundy, & Sadiq, 2011; Guntinas‐Lichius, Straesser, & Streppel, 2007; Ishii et al., 2011; Nellis et al., 2017; Ryzenman, Pensak, & Tew, 2005).

Because of the association between facial dysfunction and social and emotional factors of quality of life, it is especially relevant to understand whether facial dysfunction in patients impacts specific aspects of facial emotion processing. This study was designed to address this issue and to provide a first test to explore whether facial emotion processing might be impaired in specific patient group that suffers from facial dysfunction—patients with a Vestibular Schwannoma.

Vestibular Schwannoma (VS) refers to a unilateral brain tumor also referred to as an acoustic neuroma (Weinberger & Terris, 2015). Typical clinical symptoms are hearing loss on the affected side, tinnitus, as well as disequilibrium (Weinberger & Terris, 2015).

Because a VS is located near the facial nerve, surgical removal of it can cause a degree of unilateral paresis in the patient. To examine the potential disturbing impact of VS on facial emotion processing, we used a well‐documented method that tests a specific facet of facial emotion perception, namely hemispheric lateralization of facial emotion processing.

Theories regarding hemispheric lateralization of facial emotion processing generally consider two main viewpoints. Whereas the right hemisphere hypothesis states that all emotions are, generally, processed in the right hemisphere, the valence hypothesis states that the left hemisphere is dominant in processing positive emotions, and that the right hemisphere is dominant in processing negative emotions (e.g., Bourne, 2010).

Support for both viewpoints exist. For instance, right—compared to left—hemispheric processing of facial emotional expressions has often been reported to relate to better discrimination, recognition, and stronger perceived emotionality (e.g., Bourne, 2010). Furthermore, right hemisphere deficiencies have been shown to relate to difficulties in emotional facial expression recognition, as well as with difficulties in general social and emotional functioning (e.g., Meletti et al., 2003); Murray et al., 2015).

However, other studies show a more varied picture, providing evidence for both the right hemisphere as well as the valence hypothesis (e.g., Wyczesany, Capotosto, Zappasodi, & Prete, 2018). For example, a recent study in which behavioral, that is, as well as electrophysiological data were collected of participants while they viewed faces presented in either the left or right visual field, or in both, the behavioral data were more in support of the valence hypothesis, while the electrophysiological data were more in line with the right hemisphere hypothesis (Prete, Capotosto, Zappasodi, & Tommasi, 2018).

All in all, while the main evidence appears to suggests that the right hemisphere generally plays a more important role in emotion processing than the left hemisphere (Murray et al., 2015), evidence is definitely not conclusive and it is suggested that the two main hypotheses regarding the hemispheric lateralization of emotion processing are not mutually exclusive (Prete et al., 2018).

Therefore, though this study is mainly focused on examining possible differences in lateralization of emotion processing between VS patients with and without facial paresis, we will examine the overall lateralization—in line with the right hemisphere hypothesis—as well as possible differences in lateralization based on valence—in line with the valence hypothesis.

The current’s study addresses the lateralization of hemispheric processing by a method that has been extensively used in previous research: The chimeric faces test, a behavioral test of facial emotion processing which presents a face with an emotional expression in one half of the face and a neutral expression in the other half of the face.

The image of the face is presented centrally, with the emotional facial expression thus being presented either in the left or the right visual field. This test examines whether there exists a bias in the observer considering the perception of emotional expressions presented in the left compared to the right visual field (e.g., Bourne, 2010; Bourne & Gray, 2011; Levy, Heller, Banich, & Burton, 1983).

Hemispheric lateralization of emotion processing concerns the bias people tend to show in perceiving emotional expressions shown in the left or the right visual field as more emotional, or to recognize them more accurately depending on the visual field in which they are portrayed (Bourne, 2010; Murray et al., 2015).

Considering that the information that is shown in the left visual field initially is received and processed by the right brain hemisphere, a left visual field bias is interpreted as support for the notion that the right hemisphere is more strongly involved in emotion processing than the left hemisphere (Bourne, 2006).

The role of the facial muscles of the observer in relation to hemispheric lateralization has been partly examined in healthy individuals as well as patients with mild unilateral facial paralysis (Blom, Aarts, & Semin, 2019; Korb et al., 2016) First, a recent study (Blom et al., 2019) using the chimeric faces test reported typical left visual field bias on perceived emotionality, but this visual field bias did not directly emerge in facial muscle activation.

Furthermore, a study testing patients with acute, subacute or chronic unilateral facial paresis found that patients with a left versus right facial paresis processed emotional expression of happiness and anger equally. Interestingly, patients with a left facial paresis processed happy expressions more accurately when presented in the right versus left visual field, indicating a somewhat complicated relationship between facial paresis and emotional processing of others’ expressions (Korb et al., 2016). In short, although suggestive, the research conducted so far does not give a clear picture about the role of facial muscles in perceiving emotionality in facial expressions of others.

The current study aims to enhance the understanding of the possible role of facial mimicry in perceived emotionality by examining the impact of being limited in one’s facial functioning on emotion processing of hemispheric lateralization. First of all, while the left visual field bias—in line with the right hemisphere hypothesis—has often been observed in healthy individuals, we aim to replicate this typical bias effect in a sample of patients with VS. Additionally, we test for possible differences in bias based on the valence of the emotional expression.

If the patients show a left versus right visual field bias for positive versus negative facial expressions, this would relate to the valence hypothesis. Most importantly, however, we examined the role of facial functioning in hemispheric lateralization of emotion processing by comparing VS patients with and without facial paresis, as well as by examining the association between hemispheric lateralization of emotion processing and the degree of facial dysfunction as measured by the House Brackmann Grading scale (HBG; House, 1985). If facial functioning plays an important role in this, patients’ facial functioning should be related to the visual field bias.

DISCUSSION AND CONCLUSION
The current study was aimed at examining hemispheric lateralization of facial emotion processing by means of the chimeric faces test in Vestibular Schwannoma patients with and without facial paresis. First of all, we replicated the left visual field bias in this patient sample, meaning that when an emotional expression was depicted in the left visual field, rather than in the right visual field, the face was perceived as being more emotional.

This left visual field bias showed to be somewhat stronger for positive (happy) than for negative (angry) facial expressions. Our findings are, therefore, in line with the right hemisphere hypothesis, and not with the valence hypothesis. No difference in this bias showed based on the mere presence or absence of a facial paresis, nor did it show to be associated with the specific degree of facial functioning of the patients.

Furthermore, exploratory analyses revealed no relationship between the side of the facial paresis and the visual field bias. Lastly, no difference showed between the visual field bias of VS patients and a healthy control sample. All in all, VS patients with and without a facial paresis show the same type of hemispheric lateralization of facial emotion processing as has been reported in nonpatient samples and thus do not appear to differ in this facet of emotion processing.

The current findings suggest that facial functioning and facial mimicry are not vital for hemispheric lateralization of facial emotion processing. These findings are in line with previous research, showing no direct association between emotion processing of other’s expressions and facial muscle activity in healthy participants (Blom et al., 2019).

Another recent related study, however, reported that individuals with left facial paresis showed an opposite error pattern compared to individuals with a right facial paresis when detecting whether a happy facial expression first appeared in the left versus right visual field (Korb et al., 2016).

There are some differences between the Korb et al. and our study that could explain this apparent disparity in findings. First, the present study examined and compared VS patients with and without a facial paresis (matched on various factors), while Korb et al. tested a varied group of patients with a facial paresis (including patients with acute—less than 6 weeks—to chronic—more than 4 months paresis).

Second, we examined differences between patients with or without facial paresis in lateralization of perceived emotionality of facial expressions, while Korb et al. (2016) tested whether patients with a left or right facial paresis were able to detect in which visual field a happy facial expression first appeared.

Though speculative, then, differences in patient groups and task measurements might have produced different findings between the studies as a result of tapping into different aspects of emotion processing (e.g., detection of emotion in faces vs. perceiving emotionality in faces).

Considering that the current study does provide a strong replication of the left visual field bias, a finding in line with numerous previous studies showing the occurrence of this hemispheric bias in facial emotion perception, and the absence of facial paresis effects suggests that processes other than facial mimicry play a more important role here.

First of all, the perceived emotional intensity of emotional facial expressions could involve a neural network that is distinctive from mimicking the emotional expression itself. Perceived intensity of emotion has been associated with a network implicating more rudimentary subcortical processing and related to activity of the amygdala and nucleus accumbens (e.g., Gainotti, 2012; Phan et al., 2004), whereas the act of mimicking facial expressions involves more cortical processing related to motor simulation of facial expressions, the posterior cingulate cortex, and medial temporal lobe structures (Schilbach, Eickhoff, Mojzisch, & Vogeley, 2008).

Accordingly, a possible explanation for the current findings might be that encoding the emotionality of another person’s facial expression might occur (partly) independent from the mere mimicry of the facial expression itself. Furthermore, a recent study showed that the recognition of facial expressions can be achieved via two routes, namely by relying mainly on visual information and by sensorimotor information such as facial mimicry (de la Rosa, Fademrecht, Bülthoff, Giese, & Curio, 2018).

Extrapolating those findings to the current study would suggest that hemispheric lateralization of facial emotion processing might be a process that relies more on visual and subcortical information processing, rather than on sensorimotor information processing involved in simulating the facial expressions of others.

Though our findings could be interpreted as evidence against the role of facial mimicry in emotion processing, we would like to stress here that the findings reported in this study do not necessarily go against the important function of facial mimicry. Other information—such as the visual (de la Rosa et al., 2018)—can sometimes provide sufficient input in order to complete emotion processing tasks, hence reducing the “need” for facial mimicry for certain tasks (e.g., Arnold & Winkielman, 2019).

For example, while facial mimicry did show to relate to the valence of the chimeric faces in a previous study (Blom et al., 2019), it did not show to relate to the visual field in which the expression was shown. Hence, though the facial muscles might react to the facial expressions shown in the paradigm used in the present study, participants apparently can judge the emotionality of presented faces without relying on the sensorimotor route.

Relatedly, the task utilized by Korb et al. (2016) might have relied more on the sensorimotor route than the current studies’ task, hence providing a different account for their reported findings somewhat diverging from our present findings.

In closing, although the present study mainly aimed to address the role of facial functioning in emotional processing of facial expressions, we would like to stress that is of equal importance to study different facets of emotion processing in patients with a facial paresis, as well as in patients with cerebellar damage.

Other studies have for example reported differences in emotion perception and regulation in individuals with cerebellar damage (e.g., Houston et al., 2018). We, therefore, believe that future studies could examine this further by use of additional tasks that have previously been proven insightful for individuals with facial paresis and/or cerebellar damage.

We wish to note here that the current study is part of a larger project that examined possible differences in emotion processing of facial expressions as well as perceived quality of life, social function, and emotion between VS patients with and without facial paresis. This project aims to provide a first step in obtaining a more complete picture of emotion processing and emotion regulation in patients by using several experimental tasks as well as questionnaires (see Blom, Aarts, Wever, Kunst, & Semin, 2020; Blom, Aarts, Kunst, Wever & Semin, manuscript under review).

The current study is one of the few experimental studies on facial emotion processing in patients with a facial paresis, and patients with a VS in particular. Knowledge on emotion processes that are and that are not affected in VS patients’ with and without facial paresis informs health practitioners regarding the care they could provide patients with respect to their wellbeing. Although the present study suggests that facial paresis is not associated with impaired lateralization of emotion processing, future studies could focus on other types of facial emotion processing to further the understanding of the possible impact of a facial paresis on emotion processing.

reference link : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7375079/


Source: RUB

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