Development and Cross-Species Continuity
Claims about evolutionary continuity are implicit in any theory that views emotions from the vantage point of adaptive actions. Given the remarkable similarity between the brains of higher primates including humans, much is to be learned from cross-species comparisons. And the area where adaptive actions are maybe the most comparable is that of the social environment. Dealing with the good, the bad, and the ugly signals coming from conspecif- ics is the daily bread of higher species. In the course of evolution the repertoire of facial displays evolved in parallel with species-specific social interactions (Burrows et al., 2009; Parr et al., 2005). Hence, although many aspects of processing emotional expressions may be conserved across primate species, the differences between humans and monkeys may primarily be reflected in neural pathways involved in social cognitive processes (Brothers, 1989; Joffe & Dunbar, 1997; Parr et al., 2005). Neuroscientific research in this area is still in its infancy. Since the advent of functional neuroimaging, facial expressions have been the favorite stimulus class for studying emotion processing in the human brain, and insights from animal research have strongly influenced the interpretation of findings in humans. However, a direct comparison of processing emotional expressions between species has not been reported yet, and the literature on how the primate brain evolved to deal with emotional cues remains largely speculative (Ghazanfar & Santos, 2004).
To compare directly the processing of facial emotion signals and critical cues between species, we used event-related fMRI in monkeys and human dynamic faces using a factorial design with dynamic facial expression (fear and chewing), species (human and monkey), and configuration (intact versus mosaic scrambled). We used fear as an emotional condition, because this is the most widely studied expression in neuroimaging studies of each species. With the factorial design we could study which areas responded preferentially to conspecific emotional expressions by contrasting them with heterospecific expressions in both species. Our data reveal differences in neural processing of emotional facial expressions between humans and monkeys and argue for a more unique role of human STS in facial emotion perception than previously documented. First, although human and monkey STS are both responsive to dynamic faces, we found that human but not monkey STS showed significant activity differences between emotional and nonemotional dynamic facial expressions. Second, we provided evidence for further functional specialization within human STS along a posterior to anterior axis. Posterior STS responded to emotional expressions independent of species and the emotion effect in the right posterior STS (rSTSp) fell within a face-selective region. In contrast, the response in more anterior middle part was highly selective for the emotional human faces and was outside face-selective areas. The well- known right-hemisphere advantage for facial expressions found in human brains is not found in monkey. This shift to the right hemisphere is possibly a consequence of the appearance of the predominantly left-hemisphere-based language ability in humans (De Winter et al. 2015).
Studies of whole-body emotion expressions are even scarcer. In a pilot fMRI study with rhesus monkeys, we used whole bodies with blurred faces as stimuli, and we compared brain activation for passive viewing of neutral, anger, and fear whole-body pictures (de Gelder & Partan, 2009). We found that the strongest fMRI responses in these body-sensitive areas were obtained by viewing threatening body postures. The specific expression- sensitive voxels we observed in that study are a subset of two larger body- sensitive areas. This clearly indicates that the threat signals are more salient than fear signals. Interestingly, these findings are in agreement with the role of human STS for processing bodily signals of threat, and they are also consistent with the fact that STS is an important gateway to the AMG. This selectivity for angry body postures suggests that anger, at least anger expressed in the body posture, may be more salient and more socially relevant than neutral or fear expressions. Of course, anger expressions also functions as threat signals, but one expects them to function differently for the observer than fear signals. Where fear expressions signal the presence of a cause for fear, anger expressions present that cause directly, as often they themselves the cause of the fear! This suggests that the reaction to an anger stimulus, more specifically to the threat presented by observing an anger expression, is likely to be much more specific and focused than the threat indicated by a fear expression. Certainly, when the fear signal comes from the face, it may refer to any number of social or environmental threats. Again, there is likely to be less ambiguity when the fear signal comes from the whole body. For example, a whole-body fear reaction is likely to be specific for what causes it. Our defensive fear reaction to an attack by a spider or a dog mobilizes different postural reflexes than an attack by a burglar. Unfortunately there are no data available to support this intuition. Once emotional behavior at the scale of the whole body is considered, it may be easier and more fruitful to engage in cross-species comparisons to understand, more specifically than is now the case, how detailed action schemes associated with some of the major emotions and emotional contexts are entrenched in our biology.
In conclusion, this chapter first reviewed a series of arguments in favor of substantially extending and enriching current theories on human emotions by adding investigations of bodily expressions. Subsequently, it highlighted the importance of new research on bodily expressions for theories that consider emotions to be closely linked to adaptive action. Finally, it discussed some recent studies to illustrate the potential of bodily expression research for neuropsychological investigations as well as for clinical research. We return to all these issues in detail in the next chapters.