The left is thought to be dominant for the processing of positively valenced stimuli (stimuli inducing approach behaviors), & negatively valenced stimuli (inducing withdrawal behaviors) would be processed in the right hemisphere; this is too rigid a belief

A short review on emotion processing: a lateralized network of neuronal networks. Nicola Palomero-Gallagher & Katrin Amunts. Brain Structure and Function, Jul 3 2021. https://rd.springer.com/article/10.1007/s00429-021-02331-7

Abstract: Emotions are valenced mental responses and associated physiological reactions that occur spontaneously and automatically in response to internal or external stimuli, and can influence our behavior, and can themselves be modulated to a certain degree voluntarily or by external stimuli. They are subserved by large-scale integrated neuronal networks with epicenters in the amygdala and the hippocampus, and which overlap in the anterior cingulate cortex. Although emotion processing is accepted as being lateralized, the specific role of each hemisphere remains an issue of controversy, and two major hypotheses have been proposed. In the right-hemispheric dominance hypothesis, all emotions are thought to be processed in the right hemisphere, independent of their valence or of the emotional feeling being processed. In the valence lateralization hypothesis, the left is thought to be dominant for the processing of positively valenced stimuli, or of stimuli inducing approach behaviors, whereas negatively valenced stimuli, or stimuli inducing withdrawal behaviors, would be processed in the right hemisphere. More recent research points at the existence of multiple interrelated networks, each associated with the processing of a specific component of emotion generation, i.e., its generation, perception, and regulation. It has thus been proposed to move from hypotheses supporting an overall hemispheric specialization for emotion processing toward dynamic models incorporating multiple interrelated networks which do not necessarily share the same lateralization patterns.

Looking into the future: the need for hemispheric functional-equivalence hypotheses

Function-location meta-analyses have been applied in an attempt to quantitatively integrate results from multiple studies belonging to a specific cognitive or emotional domain. E.g., in a meta-analysis of over 100 functional magnetic resonance imaging studies addressing the mechanisms underlying processing of emotional faces (Fusar-Poli et al. 2009a, b), the authors first tested regional activation differences for an effect of laterality independently from the valence of stimulus, and found the components of the emotion network to be bilaterally activated, thus providing no support for the right-hemispheric dominance hypothesis. The authors then searched for possible lateralization patterns based on both the motivational and the drive variants of the valence lateralization hypothesis. When testing for the emotional valence of the stimulus, a laterality was only to be induced by the processing of faces expressing negative emotions. However, contrary to what is predicted by the model, the activation was localized in the left hemisphere. Finally, when grouping stimuli according to their corresponding approach/withdrawal category, a left-lateralized activation was found in the inferior frontal gyrus during the processing of faces encoding approach emotions, and right-lateralized activations occurred in the medial frontal and middle frontal gyri during the processing of faces encoding withdrawal emotions. A meta-analysis addressing the neuroanatomical structures underpinning emotional experiences demonstrated that the basic emotions happiness, sadness, fear, anger and disgust are associated with distinct regional brain activation patterns (Vytal and Hamann 2010). A lateralization could only be associated with the processing of fear, since most prominent clusters are located in the right cerebellum and insula, as well as bilaterally in the amygdala. For each of the remaining basic emotions, largest activation clusters were found in both the left and right hemisphere (Vytal and Hamann 2010). Specifically, happiness is associated with activations in the right superior temporal gyrus and the left anterior cingulate cortex, sadness with clusters in the left caudate nucleus and medial frontal gyrus, as well as in the right inferior frontal gyrus. Anger is associated with activations of the left inferior frontal gyrus and right parahippocampal gyrus, and disgust with bilateral insular activations (Vytal and Hamann 2010). Finally, results of a multi-center study evaluating functional connectivity in resting state functional magnetic resonance imaging scans from over a thousand subjects also highlight the existence of both left- and right-dominant intrinsic connectivity hubs rather than that of a global hemispheric lateralization in the human brain (Nielsen et al. 2013). In this context, it has been postulated, that the right-hemispheric dominance and the valence lateralization models may reflect different aspects of emotion processing, thus highlighting the need to move away from the concept of an overall hemispheric specialization and to elaborate on the hypothesis that emotions are the result of activations in networks which are interrelated, but may have differential lateralization patterns (Fusar-Poli et al. 2009a; Killgore and Yurgelun-Todd 2007; Neumann et al. 2008).

Along such lines of argument, a hemispheric functional-equivalence hypothesis has recently been formulated to explain lateralization associated with the perception of emotional and neutral faces (Stankovic 2021). It is a dynamic model proposing the existence of an initial default setting in which the brain would be right-biased in emotional and neutral face perception, and this lateralization pattern would be maintained as long as environmental task demands remain low. However, since emotion perception should be viewed as a multi-layered phenomenon, increasing task demands would result in a redistribution of activity among the hemispheres as an adaptive mechanism to ensure continued accurate and prompt responses (Stankovic 2021). Since environmental requirements are known to modulate psychological modulators, this hypothesis would also explain how altered conditions such as acute stress could even result in a reversed lateralization. By proposing the functional-equivalence of both hemispheres, the model also accounts for intersubject variability in lateralization patterns, as it has been demonstrated that not all individuals display the asymmetry predispositions identified at the population level (Frasnelli and Vallortigara 2018).

Finally, a recent data-driven meta-analysis revealed that the perception, experience and expression of emotion are each subserved by a distinct large-scale network (Morawetz et al. 2020). Furthermore, three of these networks are composed of left-lateralized of bilaterally activated areas, whereas the fourth one contains left-lateralized, right-lateralized and bilateral activations. This is particularly interesting, given that the hemispheric functional-equivalence hypothesis of emotional face perception assumes an initial right-biased lateralization (Stankovic 2021), whereas the network that Morawetz et al. (2020) found to be associated with the perception of emotion (albeit not specifically in facial expressions) exhibits left-lateralized or bilateral activations. It thus appears necessary to not only abandon hypotheses supporting the concept of an overall hemispheric specialization, but to also move away from a global model of lateralization in emotion processing.