Although the human brain is working as a functional and hierarchical organized unity, the two cerebral hemispheres still differ from each other, they are at least up to a certain degree anatomically and functionally asymmetric, which relates to the concept of laterality.
Anatomically, there are a number of brain regions that are more pronounced or more differentiated within one hemisphere as compared to the other, such as for example Heschl’s gyrus (von Economo, 1930), Brocas area (Falzi, Perrone, & Vignolo, 1982) and the Planum temporale (Geschwind & Levitsky, 1968). Furthermore, several functional differences between corresponding areas within the left vs. the right hemisphere have been identified, such as the classical finding of relative left hemispheric dominance for processing information related to language (see Penfield et al., 1959; Binder et al., 1997). Despite these asymmetries or hemispheric differences that are affected and tuned by environmental and genetic factors, it is however important to stress that laterality is rather relative than absolute, with both hemispheres being able to carry out a given task and most functions being accomplished by both cooperating hemispheres. Research about laterality includes behavioural experiments by using techniques such as tachistoscopic stimulus presentation or dichotic listening paradigms, invasive methods such as injection of sodium amobarbital in the carotid artery and brain stimulation as well as various neuroimaging methods such as for example PET or fMRI.
Finally, there are also case studies on neurological patients suffering from brain lesions or commissurotomy. Despite the fact that laterality has been examined for a long time, there is unfortunately still the problem that behaviourally measured laterality does not necessarily mirror or correspond to anatomical or physiological asymmetry.
Regarding the question of hemispheric specialization of emotion processing, a general assumption has often been made based on behavioural data of neurological patients (Adolphs, Damasio, Tranel, & Damasio, 1996; Borod et al., 1998; Borod, Santschi, & Koff, 1997), pointing towards a special role of the right hemisphere, which seems to have a superior processing capacity for emotional stimuli, be they auditory (Erhan, Borod, Tenke, & Bruder, 1998) or visual such as facial expressions (Schweinberger, Baird, Bluemler, Kaufmann, &
Mohr, 2003). Further support for a special role of the right hemisphere comes from
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electrophysiological and functional brain imaging studies, revealing a greater overall activity for the right hemisphere when presented with emotional stimuli (Smith, Meyers, Kline, &
Bozman, 1987; Troisi et al., 1999). Specific activation patterns for specific time segments were demonstrated in different brain regions by asking subjects to generate or mimic basic expressions that have been presented to them (Esslen, Pascual-Marqui, Hell, Kochi, &
Lehmann, 2004). Therefore the general right hemispheric superiority does not only seem to exist for the perception of emotional stimuli, but also for the generation.
Three somewhat different hypotheses concerning the degree to which emotion processing might be lateralized have been proposed. According to the right- hemisphere hypothesis, the right hemisphere generally plays a dominant role in terms of emotion processing (Borod, Kent, Koff, & Mertin, 1989; Hertje, 2001). However, the valence hypothesis suggests a dissociation between positive and negative emotions, with only negative emotions being mediated by the right hemisphere (Lee et al., 2004; Reuter-Lorenz & Davidson, 1981; Schiff
& Lamon, 1989). Favoring the valence- dependant approach, Pizzagalli et al. (Pizzagalli, Regard, & Lehmann, 1999) even identified significantly modulated ERP responses for the left vs. the right hemisphere, depending on valence of the stimulus presented. These differences related to stimulus valence could already be demonstrated at a very early stage of processing and even 10 month old infants already showed an asymmetric brain response towards positive and negative affective stimuli (Davidson & Fox, 1982). A third hypothesis, the approach-withdrawal hypothesis, which is related to the valence hypothesis, states that emotional responses associated with approach behaviour are mediated by the left hemisphere, whereas emotional responses associated with withdrawal behaviour are mediated by the right hemisphere (Canli, Desmond, Zhao, Glover, & Gabrieli, 1998; Hamon & Allen, 1998).
Contradictory results of studies dealing with hemispheric contributions of emotional processing are partly due to the fact that differences in methodological approaches, stimulus- and task- factors as well as variables such as sex, handedness and age of subjects influence the outcomes of a given study to a large extend. Nevertheless, most researchers still concede a somehow important role to the right hemisphere for emotion processing, even though individual proportions or contributions of the two cerebral hemispheres are still debated.
Assuming a general dominance of the right hemisphere in terms of emotion processing and given the fact, that innervation of facial muscles is mainly contralateral (Liscic & Zidar, 1998), the left hemiface which is controlled by the right hemisphere should consequently be more expressive. This has indeed been demonstrated, for example by Dimberg and Petterson (2000), who recorded significantly larger muscle activity of the zygomatic major and the
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corrugator supercilii on the left side of the face as a reaction of subjects having been exposed to emotional expressive faces (Dimberg & Patterson, 2000). Moreover, not only the innervation of facial muscles, but also morphological characteristics of the human face are up to a certain degree asymmetric (Vannuci, Zoccolotti, & Mammucari, 1989), which in turns influences the perception of faces in general as well as the perception of facial expression. It could for example be demonstrated that pictures artificially composed out of right halves of human faces were rated as being more similar to the original face than composites made out of left hemifaces (Gilbert & Bakan, 1973).
Facial asymmetry has usually been taken as a behavioural index of hemispheric specialization for facial expression. Based on the observation of these facial asymmetries, Sackeim, Gur and Saucy (1978) demonstrated that the right hemisphere is not only playing an important role for the reaction to emotional stimuli, but also for the generation of facial expressions. Chimeric faces, consisting only of left hemifaces, were judged as being more emotionally expressive than chimerics consisting only of right hemifaces (Sackeim, Gur, &
Saucy, 1978). This result has often been taken as support for the hypotheses that facial expression of emotion is mainly controlled by the right hemisphere.
Yet, a still unsolved striking implication of that generally assumed right hemispheric dominance for the processing of emotion is the fact that in face-to-face situations, the left hemiface of a person displaying a facial emotion, which is thought to be more expressive, is falling into a perceivers’ right visual field. Due to neuronal wiring and the fact that the right visual field of a perceiver projects to his or her left hemisphere, extensive information coming from the poser ends up in the perceiver’s hemisphere that is thought to be less specialized for emotion processing. In terms of evolutionary principles or biological signification, this loss of information being transmitted between poser and perceiver appears quite paradoxical. On the other hand, having a situation in which the inferior hemisphere is supplied with stronger information in order to compensate for its relative weakness would actually makes sense.
Proceeding on the assumption that hemispheric substrates of the perception of emotional stimuli and those associated with generation of emotional responses are dissociable (Davidson
& Tomarken, 1989), it might be very useful to have a closer look at that general right hemisphere hypothesis and to identify different components and factors influencing the assumed right hemispheric superiority for the processing of facial expressions.
In search of a more differentiated approach in terms of laterality of facial expressions, Indersmitten and Gur (2003) suggested that intensity and efficiency of emotional expression might be dissociable from each other, with a left hemiface advantage for the expression of
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emotional intensity and a right hemiface advantage for efficiency of emotional expression.
Presenting happy, angry, sad and fearful chimeric faces, consisting either of left of right hemifaces, subjects had to provide intensity judgments on a scale from 0 to 100 in intervals of 10. Overall, there has been an advantage of intensity for left composites. In a second task, subjects had to classify these chimerics as fast and as accurate as possible with respect to type of expression, revealing an overall advantage of efficiency for right composites. Indersmitten and Gur concluded from their experiments that the partial double dissociation between the laterality of expression intensity and the laterality of expression efficiency turning out from their experiments, supports the idea of two different processes with two different neural substrates (Indersmitten & Gur, 2003).
If however both cerebral hemispheres are indeed playing different roles in terms of emotion processing, it could actually be a hint to solve the puzzling question of the above mentioned poser- perceiver paradox.