Functional asymmetry of emotions in primates: new findings in chimpanzees

In the past 15 years, there have been a number of studies conducted on asymmetries in the perception and production of facial expressions in human and non-human primates as a means of inferring hemispheric specialization for emotions. We review these studies to assess continuity and discontinuity between species in these emotional processes. We further present new data on asymmetries in the production of facial expressions in a sample of captive chimpanzees. Objective measures (hemimouth length and area) and subjective measures (human judgement's of chimeric stimuli) indicate that chimpanzees' facial expressions are asymmetric, with a greater involvement of the left side of the face (right hemisphere) in the production of emotional responses. Left hemimouth was bigger than the right in the facial expressions of pant-hooting, play, and silent bared-teeth (p < 0.05) and it extended laterally more than the right in the categories of pant-hooting, silent bared-teeth, and scream face (p < 0.05). Human judges also reported that the left side of the faces was emotionally more intense in the case of the play and silent bared-teeth categories (p < 0.01). Thus, chimpanzees, like humans and some other non-human primates, show a right hemisphere specialization for facial expression of emotions, which suggests that this functional asymmetry is homologous in all these species.     

Lateralisation effect in comprehension of emotional facial expression: A comparison between EEG alpha band power and behavioural inhibition (BIS) and activation (BAS) systems

Asymmetry in comprehension of facial expression of emotions was explored in the present study by analysing alpha band variation within the right and left cortical sides. Second, the behavioural activation system (BAS) and behavioural inhibition system (BIS) were considered as an explicative factor to verify the effect of a motivational/emotional variable on alpha activity. A total of 19 participants looked at an ample range of facial expressions of emotions (anger, fear, surprise, disgust, happiness, sadness, and neutral) in random order. The results demonstrated that anterior frontal sites were more active than central and parietal sites in response to facial stimuli. Moreover, right and left side responses varied as a function of emotional types, with an increased right frontal activity for negative, aversive emotions vs an increased left response for positive emotion. Finally, whereas higher BIS participants generated more right hemisphere activation for some negative emotions (such as fear, anger, surprise, and disgust), BAS participants were more responsive to positive emotion (happiness) within the left hemisphere. Motivational significance of facial expressions was considered to elucidate cortical differences in participants' responses to emotional types.     

Contralateral reinnervation in Bell's palsy. A case report.

Congenital lesion of the left facial nerve trunk has been investigated in a 30-year-old woman. Upon stimulation of the left facial nerve, no response was evoked in any of the mimic muscles. The patient was able to perform voluntary contractions of the left corrugator glabellae muscle and of the left corner of the orbicular oral muscle; these contractions were recorded electromyographically. The stimulation of the right facial nerve elicited responses in the referred left mimic muscles. This circumstance indicated their activity to be due to contralateral reinnervation by the intact facial nerve. The blink reflex examination evoked no response on the left side, while the blinik reflex on the right side was elicited with both its components, both upon ipsi- and contralateral stimulation.     

Direction of hair whorl and handedness

Although facial symmetry correlates with facial attractiveness, human faces are often far from symmetrical with one side frequently being larger than the other (Kowner, 1998). Smith (2000) reported that male and female faces were asymmetrical in opposite directions, with males having a larger area on the left side compared to the right side, and females having a larger right side compared to the left side. The present study attempted to replicate and extend this finding. Two databases of facial images from Stirling and St Andrews Universities, consisting of 180 and 122 faces respectively, and a third set of 62 faces collected at Abertay University, were used to examine Smith's findings. Smith's unique method of calculating the size of each hemiface was applied to each set. For the Stirling and St Andrews sets a computer program did this automatically and for the Abertay set it was done manually. No significant overall effect of gender on facial area asymmetry was found. However, the St Andrews sample demonstrated a similar effect to that found by Smith, with females having a significantly larger mean area of right hemiface and males having a larger left hemiface. In addition, for the Abertay faces handedness had a significant effect on facial asymmetry with right-handers having a larger left side of the face. These findings give limited support for Smith's results but also suggest that finding such an asymmetry may depend on some as yet unidentified factors inherent in some methods of image collection.     

The enigma of facial asymmetry: is there a gender-specific pattern of facedness?

Although facial symmetry correlates with facial attractiveness, human faces are often far from symmetrical with one side frequently being larger than the other (Kowner, 1998). Smith (2000) reported that male and female faces were asymmetrical in opposite directions, with males having a larger area on the left side compared to the right side, and females having a larger right side compared to the left side. The present study attempted to replicate and extend this finding. Two databases of facial images from Stirling and St Andrews Universities, consisting of 180 and 122 faces respectively, and a third set of 62 faces collected at Abertay University, were used to examine Smith's findings. Smith's unique method of calculating the size of each hemiface was applied to each set. For the Stirling and St Andrews sets a computer program did this automatically and for the Abertay set it was done manually. No significant overall effect of gender on facial area asymmetry was found. However, the St Andrews sample demonstrated a similar effect to that found by Smith, with females having a significantly larger mean area of right hemiface and males having a larger left hemiface. In addition, for the Abertay faces handedness had a significant effect on facial asymmetry with right-handers having a larger left side of the face. These findings give limited support for Smith's results but also suggest that finding such an asymmetry may depend on some as yet unidentified factors inherent in some methods of image collection.     

Left-handers' smiles: asymmetries in the projection of a posed expression

Asymmetries in the expression of a posed smile and in a relaxed facial expression were observed in 24 left-handers. Neither writing position nor familial sinistrality predicted the variance of the results. Left-handers were found to smile more with their left than right side of face; an asymmetry which had previously been observed in right-handers. If anything, left-handers' smiles were more asymmetric, though in the same direction, than right-handers. When relaxed, however, the left-handers' face was judged more unhappy on its right than left side--a reversal of the direction of asymmetry previously noted in right-handers' relaxed expressions. No single neurological or psychological theory accounts for these results; it is suggested that hand preference may exert a myotonic effect which is reflected in judgements of relaxed facial expressions.     

The perception of positive and negative facial expressions in unilateral brain-damaged patients: A meta-analysis

How the brain is lateralised for emotion processing remains a key question in contemporary neuropsychological research. The right hemisphere hypothesis asserts that the right hemisphere dominates emotion processing, whereas the valence hypothesis holds that positive emotion is processed in the left hemisphere and negative emotion is controlled by the right hemisphere. A meta-analysis was conducted to assess unilateral brain-damaged individuals’ performance on tasks of facial emotion perception according to valence. A systematic search of the literature identified seven articles that met the conservative selection criteria and could be included in a meta-analysis. A total of 12 meta-analyses of facial expression perception were constructed assessing identification and labelling tasks according to valence and the side of brain damage. The results demonstrated that both left and right hemisphere damage leads to impairments in emotion perception (identification and labelling) irrespective of valence. Importantly, right hemisphere damage prompted more pronounced emotion perception impairment than left hemisphere damage, across valence, suggesting right hemisphere dominance for emotion perception. Furthermore, right hemisphere damage was associated with a larger tendency for impaired perception of negative than positive emotion across identification and labelling tasks. Overall the findings support Adolphs, Jansari, and Tranel (2001) model whereby the right hemisphere preferentially processes negative facial expressions and both hemispheres process positive facial expressions.     

Asymmetry of facial expression in spontaneous emotion

The observation that emotional expressions are more intense on the left side of the face is consistent with other evidence of the importance of the right hemisphere in emotional communication. However, the question has been raised whether it is truly spontaneous emotional expressions or only posed facial displays that show a left-sided asymmetry. We surreptitiously examined facial asymmetry during spontaneous emotional expressions as subjects remembered happy or sad experiences. These were contrasted with the subjects' posed expressions of happy or sad emotions. Both of these procedures resulted in more intense expressions on the left side of the face. The left-sided advantage was stronger during the spontaneous than the posed displays, and was observed for both happy and sad emotions.     

The Development of Facedness

Thirty-two children of both sexes, ranging in age from 6 to 13 years, were photographed while posing or imitating happiness, sadness or surprise. Full-face photographs which were considered by independent judges to express the intended emotions were submitted to a split-face recombination procedure that created two composites from each face, one from the left side and the other from the right. Independent observers found the left composites to be more expressive than the right m the older group (age 12–13), but not in the younger groups, who evinced no left-right asymmetries. This finding applied similarly to the three emotions, and did not depend on an age-related change in the capacity for emotional expression.Further, left-right asymmetries in facial expression, where present, did not correlate with difference in size between the two halves of the face, and were found for both the upper and lower parts of the face. It is concluded that the advantage of the left hemiface for emotional expression, which is typical of adults, is the result of growth and development. While this facial asymmetry is likely to depend on an hemispheric asymmetry favoring the right side of the brain for the volitional control of the facial musculature, the neural mechanisms of the phenomenon of facedness are still largely obscure.     

Facial asymmetry while posing positive and negative emotions: support for the right hemisphere hypothesis

The purpose of this report was to examine facial asymmetry during the expression of positive and negative emotions. In addition, methodological factors in the study of facial asymmetry were considered. Subjects were 16 neurologically-healthy right-handed adult males, videotaped while posing eight facial expressions (positive and negative) under two conditions (verbal command and visual imitation). Separate asymmetry ratings of the two sides of the face were made by judges viewing normal or mirror-reversed versions of the videotape. There were no effects of valence, condition, or videotape orientation on the asymmetry ratings, and, in general, expressions were produced significantly more intensely on the left than the right side of the face. This finding is consistent with the hypothesis that the right cerebral hemisphere is dominant for the expression of facial emotion of both valences.     

Hemiface mobility and facial expression asymmetry

Thirty-seven right-handed college-aged males and females were assessed for facial asymmetry during emotional expression and for nonemotional hemiface mobility. Objective, subjective, and undirected measures of facial mobility were obtained, separately for the upper and lower face. While judges rated mobility of the lower face as left-sided, subjects declared themselves to be as facile on the right as on the left side of the face. When asked to move a side of the lower face, subjects moved the right side more frequently than the left. For the upper face, none of the measures of mobility were significantly left- or right-sided. Facial expression asymmetries (which were observed to be left-sided) were not significantly related to any measures of hemiface mobility.     

睫状神经营养因子对面神经损伤修复大鼠面神经核内STAT3磷酸化的影响

目的 探讨重组人睫状神经营养因子（ciliary neurotrophica factor．CNTF）对面神经损伤修复大鼠面神经核运动神经元STAT3活性的影响。方法 成年大鼠面神经切断后行端端吻合．局部给予CNTF．以生理盐水为对照。术后7d．运用抗磷酸化STAT3抗体做免疫印迹（immuobloting，IB）以检测面神经核抽提物STAT3的磷酸化变化。结果 局部给予CNTF组大鼠面神经核内p-STAT含量较对照组高（P〈0．05）。结论 局部给予重组CNTF可增强面神经损伤修复大鼠面神经核内STAT3的磷酸化。     

Right-sided facial asymmetry in infant emotional expression

In normal adults, emotional expressions tend to be more intense on the left side of the face, while in preschool-age children, no significant bias in facial asymmetry has been found. We examined facial asymmetries during smiling and distress in 59 infants studied longitudinally at 6.5, 10 and 13.5 months of age. In these infants, asymmetric expressions showed a bias toward greater intensity on the right side of the face. The study of infant facial expression may provide clues to the maturation of the cortical control of emotional responses.     

Cortico-bulbar fibers to orofacial muscles: recordings with enoral surface electrodes

A new recording technique was developed to eliminate current problems on recording transcranial evoked facial muscle responses. A fork-shaped device equipped with 2 pairs of Ag/AgCl-electrodes was inserted enorally at the buccinator muscle level. Advantages offered by this method comprise clearly defined negative deflection of the compound muscle action potential, lack of relevant volume conduction from adjacent muscles, reliability of amplitude criteria, absence of interfering stimulus artifacts, easy achievement of preactivation, and noninvasive recording by surface electrodes. In 43 healthy subjects transcranial magnetic stimulation evoked contralateral responses at a mean latency and mean amplitude of 10.3±1.1 ms and 1.6±1.1 mV, respectively on the right side of the face and of 9.9±1.0 ms and 1.6±1.1 mV, on the left side of the face. Ipsilateral cortical evoked responses were observed in 29 and 25 subjects (left and right side of the face) at a mean latency and amplitude of 10.7±2.5 ms and 0.8±0.5mV, respectively on the left side of face and of 11.9±3.2 ms and 1.1±1.2 mV, on the right side of face. No responses were obtained in 2 and 4 subjects (left and right side of the face), and could not be assessed due to simultaneous facial nerve stimulation in 12 and 14 subjects (left and right side of the face).     

AAEM case report #26: Seventh cranial neuropathy

AAEM CASE REPORT #26. A 25-year-old man with acute, bilateral facial palsies is presented. He had a lymphocytic meningitis, history of tick bites, and lived in an area endemic for Lyme disease, which was ultimately confirmed by serology. Electrodiagnostic investigation included facial motor nerve study, blink reflex and electromyography of facial muscles, which were indicative of a neurapraxic lesion on the right and an axonopathic lesion on the left. The clinical course was consistent with these findings as the right side fully recovered and the left remained plegic. The clinical features of Lyme associated facial neuritis are reviewed, as is the electrodiagnostic evaluation of facial palsy.     

Is trustworthiness lateralized in the face? Evidence from a trust game

A turn of the head can be used to convey or conceal emotion, as the left side of the face is more expressive than the right. As the left cheek moves more when smiling, the present study investigated whether perceived trustworthiness is lateralized to the left cheek, using a trust game paradigm. In Experiment 1, participants were asked to share money with male and female “virtual partners.” Left–left or right–right composite faces were used to represent the partners. There were no differences in the amount shared based on composite face, suggesting trustworthiness is not lateralized in the face. However, there was a robust effect whereby female partners were perceived to be significantly more trustworthy than males. In Experiment 2, the virtual partners presented either the left or the right cheek prominently. As in Experiment 1, the amount shared with the partners did not change depending on the cheek presented. Interestingly, female partners were again sent significantly more money than males. We found no support for lateralized trustworthiness in the face, suggesting that asymmetries in the face are not large enough to influence trustworthiness judgements. Instead, more stable facial features, such as sex-typical characteristics, appear to influence perceived trustworthiness.     

Right perceptual bias and self-face recognition in individuals with congenital prosopagnosia

The existence of a drift to base judgments more on the right half-part of facial stimuli, which falls in the observer's left visual field (left perceptual bias (LPB)), in normal individuals has been demonstrated. However, less is known about the existence of this phenomenon in people affected by face impairment from birth, namely congenital prosopagnosics. In the current study, we aimed to investigate the presence of the LPB under face impairment conditions using chimeric stimuli and the most familiar face of all: the self-face. For this purpose we tested 10 participants with congenital prosopagnosia and 21 healthy controls with a face matching task using facial stimuli, involving a spatial manipulation of the left and the right hemi-faces of self-photos and photos of others. Even though congenital prosopagnosics performance was significantly lower than that of controls, both groups showed a consistent self-face advantage. Moreover, congenital prosopagnosics showed optimal performance when the right side of their face was presented, that is, right perceptual bias, suggesting a differential strategy for self-recognition in those subjects. A possible explanation for this result is discussed.     

The effect of left and right poses on the expression of facial emotion

Research, using composite facial photographs has demonstrated that left-left composites are more emotionally expressive than are right-right composites. The present study investigated whether hemifacial asymmetries in expression are apparent in photographs, that feature one side of the face more than the other. Photographs were taken of the models who turned their heads: (a) 15 degrees to the left, (b) 15 degrees to the right or (c) faced directly towards the camera. It was predicted that left hemiface and midline photographs would be judged as more emotionally expressive than right hemiface photographs, where the left hemiface is less prominent. Three hundred and eighty-four participants viewed photographs of the three posing conditions, and rated each photograph along an emotional expressivity scale. Midline and left hemiface portraits were rated as more emotionally expressive than were right hemiface portraits. To investigate whether this effect was caused by observer's aesthetic/perceptual biases, mirror-reversed versions of the three posing conditions were included. Left hemiface and midline portraits were rated as more emotionally expressive, irrespective of whether they were mirror-reversed. It was concluded that head turns of just 15 degrees can bring about significant changes in the perceived emotionality. The relevance of these findings to painted portraits, which feature the left hemiface more than the right, is discussed.     

Left-sided oral asymmetries in spontaneous but not posed smiles

To investigate possible facial asymmetries during the production of posed and spontaneous smiles, the displacement of various reference points on the mouth were measured as subjects produced both kinds of smiles. Strobe cameras were used in combination with a computer-based analysis to record the smiles of left- and right-handed males and females. The analysis revealed that the left side of the mouth moved more than the right side during spontaneous but not posed smiles, supporting the notion that the right hemisphere may play a special role in emotional expression. This asymmetry was most apparent in left-handed females and right-handed males. These sex and handedness differences are discussed with reference to apparent inconsistencies in previous research on asymmetries in emotional expression.     

Differences in asymmetry of facial expression between left- and right-handed children

Ten left-handed children unselected for cerebral dominance and ten right-handed children, 8–10 yr old, posed for facial photographs while mimicking several emotional expressions. Left side and right side facial composites were prepared and shown to 30 raters. The majority of right-handed children were judged to be left facial dominant, while the left-handers were judged to be mainly indeterminate and right facial dominant. These findings indicate a difference in hemifacial emotional expression between left- and right-handed children.