Neuropsychological aspects of facial asymmetry during emotional expression: a review of the normal adult literature

Posed versus spontaneous facial expressions are modulated by opposite cerebral hemispheres

Clinical research has indicated that the left face is more expressive than the right face, suggesting that modulation of facial expressions is lateralized to the right hemisphere. The findings, however, are controversial because the results explain, on average, approximately 4% of the data variance. Using high-speed videography, we sought to determine if movement-onset asymmetry was a more powerful research paradigm than terminal movement asymmetry. The results were very robust, explaining up to 70% of the data variance. Posed expressions began overwhelmingly on the right face whereas spontaneous expressions began overwhelmingly on the left face. This dichotomy was most robust for upper facial expressions. In addition, movement-onset asymmetries did not predict terminal movement asymmetries, which were not significantly lateralized. The results support recent neuroanatomic observations that upper versus lower facial movements have different forebrain motor representations and recent behavioral constructs that posed versus spontaneous facial expressions are modulated preferentially by opposite cerebral hemispheres and that spontaneous facial expressions are graded rather than non-graded movements.

Subliminal and supraliminal processing of facial expression of emotions: brain oscillation in the left/right frontal area

The unconscious effects of an emotional stimulus have been highlighted by a vast amount of research, whereover it remains questionable whether it is possible to assign a specific function to cortical brain oscillations in the unconscious perception of facial expressions of emotions. Alpha band variation was monitored within the right- and left-cortical side when subjects consciously (supraliminal stimulation) or unconsciously (subliminal stimulation) processed facial patterns. Twenty subjects looked at six facial expressions of emotions (anger, fear, surprise, disgust, happiness, sadness, and neutral) under two different conditions: supraliminal (200 ms) vs. subliminal (30 ms) stimulation (140 target-mask pairs for each condition). The results showed that conscious/unconscious processing and the significance of the stimulus can modulate the alpha power. Moreover, it was found that there was an increased right frontal activity for negative emotions vs. an increased left response for positive emotion. The significance of facial expressions was adduced to elucidate cortical different responses to emotional types.

Male Facial Anthropometry and Attractiveness

The symmetry and masculinity of the face are often considered important elements of male facial attractiveness. However, facial preferences are rarely studied on natural faces. We studied the effect of these traits and facial metric parameters on facial attractiveness in Spanish and Colombian raters. In total, 13 metric and 11 asymmetry parameters from natural, unmanipulated frontal face photographs of 50 Spanish men were measured with the USIA semiautomatic anthropometric software. All raters (women and men) were asked to rank these images as potential long-term partners for females. In both sexes, facial attractiveness was negatively associated with facial masculinity, and preference was not associated with facial symmetry. In Spanish raters, both sexes preferred male traits that were larger in the right side of the face, which may reflect a human tendency to prefer a certain degree of facial asymmetry. We did not find such preference in Colombian raters, but they did show stronger preference for facial femininity than Spanish raters. Present results suggest that facial relative femininity, which is expected to signal, eg good parenting and cooperation skills, may be an important signal of mate quality when females seek long-term partners. Facial symmetry appears unimportant in such long-term mating preferences.

Unconscious processing of emotions and the right hemisphere

This survey takes into account the unconscious aspects of emotions and the critical role played in them by the right hemisphere, considering different acceptations of the term 'unconscious'. In a preliminary step, the nature of emotions, their componential and hierarchical organization and the relationships between emotions and hemispheric specialization are shortly discussed, then different aspects of emotions are surveyed: first are reviewed studies dealing with the unconscious processing of emotional information, taking separately into account various lines of research. All these studies suggest that unconscious processing of emotional information is mainly subsumed by a right hemisphere subcortical route, through which emotional stimuli quickly reach the amygdala. We afterwards inquire if a right hemisphere dominance can also be observed in automatic emotional action schemata and if 'non-removed preverbal implicit memories' also have a preferential link with the right hemisphere. Finally, we try to evaluate if the right hemisphere may also play a critical role in dynamic unconscious phenomena, such as anosognosia/denial of hemiplegia in patients with unilateral brain lesions. In the last part of the review, the reasons that could subsume the right hemisphere dominance for unconscious emotions are shortly discussed.

Hemispheric asymmetry in the processing of negative and positive words: a divided field study

Research on the lateralisation of brain functions for emotion has yielded different results as a function of whether it is the experience, expression, or perceptual processing of emotion that is examined. Further, for the perception of emotion there appear to be differences between the processing of verbal and nonverbal stimuli. The present research examined the hemispheric asymmetry in the processing of verbal stimuli varying in emotional valence. Participants performed a lexical decision task for words varying in affective valence (but equated in terms of arousal) that were presented briefly to the right or left visual field. Participants were significantly faster at recognising positive words presented to the right visual field/left hemisphere. This pattern did not occur for negative words (and was reversed for high arousal negative words). These results suggest that the processing of verbal stimuli varying in emotional valence tends to parallel hemispheric asymmetry in the experience of emotion.

Holding-side influences on infant's view of mother's face

Recently we found that adult children whose mothers had had a right-arm preference for holding infants have a reduced left bias for recognising faces, suggesting that they are less well right-hemisphere lateralised for perceiving faces. One possible explanation of this finding is that early visual exposure to faces is suboptimal for right-held infants. To test this idea, we asked mothers to pick up a doll with an inbuilt camera in its face and to start bottle-feeding it. The results showed that less was visible of the face of mothers who held the doll on their right arm in comparison to those who held the doll on their left arm: From the right arm, the mother's left half of the face was less visible when the mothers were looking up and their right half of the face was less visible when they were looking at the doll. These results suggest that right-held infants receive suboptimal information from faces. Because early face exposure is important for face-processing development, the suboptimal face exposure probably experienced by right-held infants may have consequences for their ability to recognise faces and facial emotion later in life.

Consistent left gaze bias in processing different facial cues

While viewing faces, humans often demonstrate a natural gaze bias towards the left visual field, that is, the right side of the viewee's face is often inspected first and for longer periods. Previous studies have suggested that this gaze asymmetry is a part of the gaze pattern associated with face exploration, but its relation with perceptual processing of facial cues is unclear. In this study we recorded participants' saccadic eye movements while exploring face images under different task instructions (free viewing, judging familiarity and judging facial expression). We observed a consistent left gaze bias in face viewing irrespective of task demands. The probability of the first fixation and the proportion of overall fixations directed at the left hemiface were indistinguishable across different task instructions or across different facial expressions. It seems that the left gaze bias is an automatic reflection of hemispheric lateralisation in face processing, and is not necessarily correlated with the perceptual processing of a specific type of facial information.

From symmetry to asymmetry? The development of smile

The left side of the face has been reported to be more expressive than the right side in human adults. The developmental origin of this lateralization, however, is unknown. Chimeric still composite photos of smiling faces of 0-5 months (n=61), 6-12 months (n=67) old infants, 3-8 years old children (n=78), and 32 adults (>18 years) were rated for intensity by 90 judges. The results showed left-hemiface dominance in adults only, and not in infants or children. It can be speculated that this early functional symmetry may evolutionarily increase attractiveness and elicits secure bonding and care.

Processing emotion from the eyes: a divided visual field and ERP study

The right cerebral hemisphere is preferentially involved in recognising at least some facial expressions of emotion. We investigated whether there is a laterality effect in judging emotions from the eyes. In one task a pair of emotionally expressive eyes presented in central vision had to be physically matched to a subsequently presented set of eyes in one or other visual hemifield (eyes condition). In the second task a word was presented centrally followed by a set of eyes to left or right hemifield and the participant had to decide whether the word correctly described the emotion portrayed by the laterally presented set of eyes (word condition). Participants were a group of undergraduate students and a group of older volunteers (> 50). There was no visual hemifield difference in accuracy or raw response times in either task for either group, but log-transformed times showed an overall left hemifield advantage. Contrary to the right hemisphere ageing hypothesis, older participants showed no evidence of a relative right hemisphere decline in performance on the tasks. In the younger group mean peak amplitude of the N170 component of the EEG at lateral posterior electrode sites was significantly greater over the right hemisphere (T6/PO2) than the left (T5/PO1) in both the perceptual recognition task and the emotional judgement task. It was significantly greater for the task of judging emotion than in the eyes-matching task. In future it would be useful to combine electrophysiological techniques with lateralised visual input in studying lateralisation of emotion with older as well as younger participants.

The effects of mothers' past infant-holding preferences on their adult children's face processing lateralisation

Face processing development is negatively affected when infants have not been exposed to faces for some time because of congenital cataract blocking all vision (Le Grand, Mondloch, Maurer, & Brent, 2001). It is not clear, however, whether more subtle differences in face exposure may also have an influence. The present study looked at the effect of the mother's preferred side of holding an infant, on her adult child's face processing lateralisation. Adults with a mother who had a left-arm preference for holding infants were compared with adults with a mother who had a right-arm holding preference. All participants were right-handed and had been exclusively bottle-fed during infancy. The participants were presented with two chimeric faces tests, one involving emotion and the other one gender. The left-arm held individuals showed a normal left-bias on the chimeric face tests, whereas the right-arm held individuals a significantly decreased left-bias. The results might suggest that reduced exposure to high quality emotional information on faces in infancy results in diminished right-hemisphere lateralisation for face processing.

Hemispheric Asymmetries during Processing of Immoral Stimuli

Evolutionary approaches to dissecting our psychological architecture underscore the importance of both function and structure. Here we focus on both the function and structure of our neural circuitry and report a functional bilateral asymmetry associated with the processing of immoral stimuli. Many processes in the human brain are associated with functional specialization unique to one hemisphere. With respect to emotions, most research points to right-hemispheric lateralization. Here we provide evidence that not all emotional stimuli share right-hemispheric lateralization. Across three studies employing different paradigms, the processing of negative morally laden stimuli was found to be highly left-lateralized. Regions of engagement common to the three studies include the left medial prefrontal cortex, left temporoparietal junction, and left posterior cingulate. These data support the hypothesis that processing of immoral stimuli preferentially engages left hemispheric processes and sheds light on our evolved neural architecture.

Right hemisphere dominance for emotion processing in baboons

Asymmetries of emotional facial expressions in humans offer reliable indexes to infer brain lateralization and mostly revealed right hemisphere dominance. Studies concerned with oro-facial asymmetries in nonhuman primates largely showed a left-sided asymmetry in chimpanzees, marmosets and macaques. The presence of asymmetrical oro-facial productions was assessed in Olive baboons in order to determine the functional cerebral asymmetries. Two affiliative behaviors (lipsmack, copulation call) and two agonistic ones (screeching, eyebrow-raising) were recorded. For screeching, a strong and significant left hemimouth bias was found, but no significant bias was observed for the other behaviors. These results are discussed in the light of the available literature concerning asymmetrical oro-facial productions in nonhuman primates. In addition, these findings suggest that human hemispheric specialization for emotions has precursors in primate evolution.

The right-hemisphere and valence hypotheses: could they both be right (and sometimes left)?

The two halves of the brain are believed to play different roles in emotional processing, but the specific contribution of each hemisphere continues to be debated. The right-hemisphere hypothesis suggests that the right cerebrum is dominant for processing all emotions regardless of affective valence, whereas the valence specific hypothesis posits that the left hemisphere is specialized for processing positive affect while the right hemisphere is specialized for negative affect. Here, healthy participants viewed two split visual-field facial affect perception tasks during functional magnetic resonance imaging, one presenting chimeric happy faces (i.e. half happy/half neutral) and the other presenting identical sad chimera (i.e. half sad/half neutral), each masked immediately by a neutral face. Results suggest that the posterior right hemisphere is generically activated during non-conscious emotional face perception regardless of affective valence, although greater activation is produced by negative facial cues. The posterior left hemisphere was generally less activated by emotional faces, but also appeared to recruit bilateral anterior brain regions in a valence-specific manner. Findings suggest simultaneous operation of aspects of both hypotheses, suggesting that these two rival theories may not actually be in opposition, but may instead reflect different facets of a complex distributed emotion processing system.

Asymmetries of the human social brain in the visual, auditory and chemical modalities

Structural and functional asymmetries are present in many regions of the human brain responsible for motor control, sensory and cognitive functions and communication. Here, we focus on hemispheric asymmetries underlying the domain of social perception, broadly conceived as the analysis of information about other individuals based on acoustic, visual and chemical signals. By means of these cues the brain establishes the border between 'self' and 'other', and interprets the surrounding social world in terms of the physical and behavioural characteristics of conspecifics essential for impression formation and for creating bonds and relationships. We show that, considered from the standpoint of single- and multi-modal sensory analysis, the neural substrates of the perception of voices, faces, gestures, smells and pheromones, as evidenced by modern neuroimaging techniques, are characterized by a general pattern of right-hemispheric functional asymmetry that might benefit from other aspects of hemispheric lateralization rather than constituting a true specialization for social information.

Validation of regression-based myogenic correction techniques for scalp and source-localized EEG

EEG and EEG source-estimation are susceptible to electromyographic artifacts (EMG) generated by the cranial muscles. EMG can mask genuine effects or masquerade as a legitimate effect-even in low frequencies, such as alpha (8-13 Hz). Although regression-based correction has been used previously, only cursory attempts at validation exist, and the utility for source-localized data is unknown. To address this, EEG was recorded from 17 participants while neurogenic and myogenic activity were factorially varied. We assessed the sensitivity and specificity of four regression-based techniques: between-subjects, between-subjects using difference-scores, within-subjects condition-wise, and within-subject epoch-wise on the scalp and in data modeled using the LORETA algorithm. Although within-subject epoch-wise showed superior performance on the scalp, no technique succeeded in the source-space. Aside from validating the novel epoch-wise methods on the scalp, we highlight methods requiring further development.

Asymmetrical facial expressions in portraits and hemispheric laterality: a literature review

Studies of facial asymmetry have revealed that the left and the right sides of the face differ in emotional attributes. This paper reviews many of these distinctions to determine how these asymmetries influence portrait paintings. It does so by relating research involving emotional expression to aesthetic pleasantness in portraits. For example, facial expressions are often asymmetrical-the left side of the face is more emotionally expressive and more often connotes negative emotions than the right side. Interestingly, artists tend to expose more of their poser's left cheek than their right. This is significant, in that artists also portray more females than males with their left cheek exposed. Reasons for these psychological findings lead to explanations for the aesthetic leftward bias in portraiture.

Unconscious processing of facial affect in children and adolescents

In a previous study, with adults, we demonstrated that the amygdala and anterior cingulate gyrus are differentially responsive to happy and sad faces presented subliminally. Because the ability to perceive subtle facial signals communicating sadness is an important aspect of prosocial development, and is critical for empathic behavior, we examined this phenomenon from a developmental perspective using a backward masking paradigm. While undergoing functional magnetic resonance imaging (fMRI), 10 healthy adolescent children were presented with a series of happy and sad facial expressions, each lasting 20 ms and masked immediately by a neutral face to prevent conscious awareness of the affective expression. Relative to fixation baseline, masked sad faces activated the right amygdala, whereas masked happy faces failed to activate any of the regions of interest. Direct comparison between masked happy and sad faces revealed valence specific differences in the anterior cingulate gyrus. When the data were compared statistically to our previous sample of adults, the adolescent group showed significantly greater activity in the right amygdala relative to the adults during the masked sad condition. Groups also differed in several non-hypothesized regions. Development of unconscious perception from adolescence into adulthood appears to be accompanied by reduced activity within limbic affect processing systems, and perhaps increased involvement of other cortical and cerebellar systems.

Left hemisphere specialization for oro-facial movements of learned vocal signals by captive chimpanzees

BACKGROUND

The left hemisphere of the human brain is dominant in the production of speech and signed language. Whether similar lateralization of function for communicative signal production is present in other primates remains a topic of considerable debate. In the current study, we examined whether oro-facial movements associated with the production of learned attention-getting sounds are differentially lateralized compared to facial expressions associated with the production of species-typical emotional vocalizations in chimpanzees.

METHODOLOGY/PRINCIPAL FINDINGS

Still images captured from digital video were used to quantify oro-facial asymmetries in the production of two attention-getting sounds and two species-typical vocalizations in a sample of captive chimpanzees. Comparisons of mouth asymmetries during production of these sounds revealed significant rightward biased asymmetries for the attention-getting sounds and significant leftward biased asymmetries for the species-typical sounds.

CONCLUSIONS/SIGNIFICANCE

These results suggest that the motor control of oro-facial movements associated with the production of learned sounds is lateralized to the left hemisphere in chimpanzees. Furthermore, the findings suggest that the antecedents for lateralization of human speech may have been present in the common ancestor of chimpanzees and humans approximately 5 mya and are not unique to the human lineage.

Hemispheric laterality and dissociative tendencies: Differences in emotional processing in a dichotic listening task

The present work investigates whether the hemispheric processing of both verbal and emotional stimuli, studied by means of a dichotic listening task, differs between normal high and low dissociators as assessed by the Dissociative Experiences Scale (DES; Bernstein & Putnam (1986). Development, reliability and validity of a dissociation scale. Journal of Nervous and Mental Disease, 174(2), 727-735). Two groups of subjects (50 high and 50 low dissociators), participated in the experiment. The task consisted in identifying both verbal and emotional stimulus-targets, respectively, on successive sessions. Reaction time and response accuracy were registered and analysed using ANOVA (Analysis of Variance). The interaction between stimuli (verbal, emotional), channel (right ear, left ear), and dissociation level (high, low) reached statistical significance in terms of accuracy measures (d': F(1,98)=4.75; p<.05). Both high and low dissociators exhibited the expected right ear advantage (REA effect) on verbal targets. On the other hand, whereas low dissociators exhibited the expected left ear advantage (LEA effect) on emotional targets, high dissociators failed to follow this typical pattern of hemispheric asymmetry: both hemispheres exhibited similar performances. These results confirm the hypothesis that dissociation is related to changes in hemispheric processing, specifically of emotional information.

Semantic, perceptual and number space: Relations between category width and spatial processing

Coarse semantic encoding and broad categorization behavior are the hallmarks of the right cerebral hemisphere's contribution to language processing. We correlated 40 healthy subjects' breadth of categorization as assessed with Pettigrew's category width scale with lateral asymmetries in perceptual and representational space. Specifically, we hypothesized broader category width to be associated with larger leftward spatial biases. For the 20 men, but not the 20 women, this hypothesis was confirmed both in a lateralized tachistoscopic task with chimeric faces and a random digit generation task; the higher a male participant's score on category width, the more pronounced were his left-visual field bias in the judgement of chimeric faces and his small-number preference in digit generation ("small" is to the left of "large" in number space). Subjects' category width was unrelated to lateral displacements in a blindfolded tactile-motor rod centering task. These findings indicate that visual-spatial functions of the right hemisphere should not be considered independent of the same hemisphere's contribution to language. Linguistic and spatial cognition may be more tightly interwoven than is currently assumed.

Behavioural and neurophysiological evidence for face identity and face emotion processing in animals

Visual cues from faces provide important social information relating to individual identity, sexual attraction and emotional state. Behavioural and neurophysiological studies on both monkeys and sheep have shown that specialized skills and neural systems for processing these complex cues to guide behaviour have evolved in a number of mammals and are not present exclusively in humans. Indeed, there are remarkable similarities in the ways that faces are processed by the brain in humans and other mammalian species. While human studies with brain imaging and gross neurophysiological recording approaches have revealed global aspects of the face-processing network, they cannot investigate how information is encoded by specific neural networks. Single neuron electrophysiological recording approaches in both monkeys and sheep have, however, provided some insights into the neural encoding principles involved and, particularly, the presence of a remarkable degree of high-level encoding even at the level of a specific face. Recent developments that allow simultaneous recordings to be made from many hundreds of individual neurons are also beginning to reveal evidence for global aspects of a population-based code. This review will summarize what we have learned so far from these animal-based studies about the way the mammalian brain processes the faces and the emotions they can communicate, as well as associated capacities such as how identity and emotion cues are dissociated and how face imagery might be generated. It will also try to highlight what questions and advances in knowledge still challenge us in order to provide a complete understanding of just how brain networks perform this complex and important social recognition task.

Exploring number space by random digit generation

There is some evidence that human subjects preferentially select small numbers when asked to sample numbers from large intervals "at random". A retrospective analysis of single digit frequencies in 16 independent experiments with the Mental Dice Task (generation of digits 1-6 during 1 min) confirmed the occurrence of small-number biases (SNBs) in 488 healthy subjects. A subset of these experiments suggested a spatial nature of this bias in the sense of a "leftward" shift along the number line. First, individual SNBs were correlated with leftward deviations in a number line bisection task (but unrelated to the bisection of physical lines). Second, in 20 men, the magnitude of SNBs significantly correlated with leftward attentional biases in the judgment of chimeric faces. Finally, cognitive activation of the right hemisphere enhanced SNBs in 20 different men, while left hemisphere activation reduced them. Together, these findings provide support for a spatial component in random number generation. Specifically, they allow an interpretation of SNBs in terms of "pseudoneglect in number space." We recommend the use of random digit generation for future explorations of spatial-attentional asymmetries in numerical processing and discuss methodological issues relevant to prospective designs.

Processing of Facial Blends of Emotion: Support for Right Hemisphere Cognitive Aging

Clinical research on facial emotions has focused primarily on differences between right and left hemiface. Social psychology, however, has suggested that differences between upper versus lower facial displays may be more important, especially during social interactions. We demonstrated previously that upper facial displays are perceived preferentially by the right hemisphere, while lower facial displays are perceived preferentially by the left hemisphere. A marginal age-related effect was observed. The current research expands our original cohort to include 26 elderly individuals over age 62. Fifty-six, strongly right-handed, healthy, adult volunteers were tested tachistoscopically by flashing randomized facial displays of emotion to the right and left visual fields. The stimuli consisted of line drawings displaying various combinations of emotions on the upper and lower face. The subjects were tested under two conditions: without attend instruction and with instructions to attend to the upper face. Based on linear regression and discriminant analyses modeling age, subject performance could be divided into two distinct groups: Young (< 62 years) and Old (> 62 years). Without attend instructions, both groups robustly identified the emotion displayed on the lower face, regardless of visual field presentation. With instructions to attend to the upper face, the Old group demonstrated a markedly decreased ability to identify upper facial displays, compared to the Young group. The most significant difference was noted in the left visual field/right hemisphere. Our results demonstrate a significant decline in the processing of upper facial emotions by the right hemisphere in older individuals, thus providing partial support for the right hemisphere hypothesis of cognitive aging. The decreased ability to perceive upper facial displays coupled with age-related deficits in processing affective prosody may well cause impaired psychosocial competency in the elderly.

Lateralized scratching in chimpanzees (Pan troglodytes): Evidence of a functional asymmetry during arousal

This study evaluated laterality in scratching by chimpanzees (n = 89) during socially arousing circumstances. Hand use and the side of the body scratched was recorded during a baseline and experimental condition. In the experimental condition, chimpanzees were shown a video of other conspecifics sharing, fighting over, and consuming a watermelon. Self-touches were categorized as either rubs or scratches. The chimpanzees showed a significant right hand bias for rubbing and also significantly directed the rubs to the right side of the body. For scratching, the chimpanzees showed no hand preference but a significant bias for scratching on the left side of the body. These results support the view that the right hemisphere regulates the autonomic nervous system during arousal.

Emotion and Pain: A Functional Cerebral Systems Integration

Emotion and pain are psychological constructs that have received extensive attention in neuropsychological research. However, neuropsychological models of emotional processing have made more progress in describing how brain regions interact to process emotion. Theories of emotional processing can describe inter-hemispheric and intra-hemispheric interactions during emotional processing. Due to similarities between emotion and pain, it is thought that emotional models can be applied to pain. The following review examines the neuropsychology of emotion and pain using a functional cerebral systems approach. Specific comparisons are made between pain and anger. Attention is given to differences in cerebral function and physiology that may contribute to the processing of emotion and pain. Suggestions for future research in emotion and pain are given.

Brain lesions and emotional disorders

Many authors think that emotional disorders of brain-damaged patients result directly (as in the case of language, memory and other cognitive disorders) from the disruption of specific cortico–subcortical circuits. This claim, however, is only in part correct, as the homology between emotional and cognitive systems is partial and only some emotional disorders of brain-damaged patients are due to the disruption of specific brain structures. Other emotional disorders result from a more general mechanism, namely from the appraisal of the personal implications that physical and cognitive consequences of brain injury will have for the quality of life of the patient. The aim of this review is to stress the distinction between emotional and cognitive systems, the componential nature of emotions, the brain structures subtending the different components of emotion and the nature of the process of ‘emotional appraisal’. Starting from these theoretical premises, the author will attempt to distinguish the emotional disorders of brain-damaged patients that result from the disruption of specific brain structures from those that are due to more general appraisal and coping mechanisms.

Left-right asymmetry of the facial microvascular control

Facial blood flow and temperature were significantly higher on the right side of the forehead compared to the left. This asymmetry implies that the hemispheric autonomic control of the face differs and could influence the expression of emotion.

Lateralized asymmetry of facial motor evoked potentials

BACKGROUND

There is considerable evidence that emotions are expressed more intensely on the left side of the face. This asymmetry could reflect a right hemisphere advantage in processing emotional material or an asymmetry in corticobulbar motor systems. Transcranial magnetic stimulation (TMS) was used to test for lateralized asymmetry in the cortical control of muscles of facial expression in the lower face.

METHODS

We administered TMS to the motor cortex of 50 subjects during contraction of bilateral orbicularis oris muscles. We analyzed motor evoked potentials (MEPs) with a repeated measures analysis of variance (ANOVA) using hemisphere stimulated and orbicularis oris side recorded as within subject factors.

RESULTS

TMS elicited contralateral MEPs in 42 of 50 subjects. Forty of these 42 subjects showed bilateral MEPs. The ANOVA showed a significant main effect of face side, such that MEPs elicited in left face were larger than in right face (p < 0.0001). The analysis also showed a significant interaction between the hemisphere stimulated and face side, such that the difference between contralateral and ipsilateral MEPs with right brain TMS was greater than with left brain TMS (p < 0.0001).

CONCLUSIONS

The results provide evidence of lateralized asymmetry of corticobulbar projections to muscles of facial expression in the lower face.

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.

Resting Brain Asymmetry and Affective Reactivity

Abstract. Neuropsychological evidence has given rise to alternative models on brain asymmetry in emotion, each with different implications concerning the biological basis of individual differences in affective responses. The present study tested these implications. Resting EEG and self-reported emotions after the presentation of film clips were collected on four occasions of measurement. Subjects with greater right-sided and smaller left-sided cortical activity reported greater intensities of felt emotions after the presentation of films irrespective of valence. This finding is in line with a recent formulation of the right-hemisphere hypothesis, which proposes that the right hemisphere may play an important role in the automatic generation of emotional responses, whereas the left hemisphere may be involved in the control and modulation of emotional reactions.

Changes in posed facial expression of emotion across the adult life span

The purpose of this study was to examine changes in the facial expression of emotion across the adult life span. Two positive and two negative emotional expressions were posed by 30 young (21 to 39 years), 30 middle-aged (40 to 59 years), and 30 older (60 to 81 years) healthy, right-handed women. Photographs of the four emotional expressions were rated by independent judges for intensity, accuracy, and confidence. Special features of this study were the use of a neutral face as a nonemotional control, as well as careful cognitive and affective screening procedures for posers and judges. Overall, the expressions of older posers were rated as significantly less accurate and with significantly less confidence than those of younger posers. Although the neutral faces of older posers were rated as significantly more intense than those of younger posers, there were no significant age-related intensity differences for positive and negative emotions. The results are discussed in terms of theoretical models of aging.

Individual differences in anterior EEG asymmetry: methodological problems and solutions

In the last two decades, a substantive body of empirical work investigated the association between individual differences of electroencephalogram (EEG) alpha asymmetry and affective/motivational dispositions. Recent work indicated that several methodological problems persist in this field. The present paper reviews these problems with a focus on the reliability and validity of measures of anterior resting EEG asymmetry, which serve as a proxy for trait-like asymmetries of cortical activity. These issues include the treatment of ocular and muscle artifacts, the choice of the EEG reference, the use of current source density (CSD) measures, the state-trait nature of resting asymmetry, and the treatment of state-like fluctuations of the measures. In addition, the statistical problem of inflated type-I error due to multiple testing is also considered and different approaches to counteract this problem are suggested.

Issues and assumptions on the road from raw signals to metrics of frontal EEG asymmetry in emotion

There exists a substantial literature examining frontal electroencephalographic asymmetries in emotion, motivation, and psychopathology. Research in this area uses a specialized set of approaches for reducing raw EEG signals to metrics that provide the basis for making inferences about the role of frontal brain activity in emotion. The present review details some of the common data processing routines used in this field of research, with a focus on statistical and methodological issues that have captured, and should capture, the attention of researchers in this field.

Detecting hemifacial asymmetries in emotional expression with three-dimensional computerized image analysis

Emotions are expressed more clearly on the left side of the face than the right: an asymmetry that probably stems from right hemisphere dominance for emotional expression (right hemisphere model). More controversially, it has been suggested that the left hemiface bias is stronger for negative emotions and weaker or reversed for positive emotions (valence model). We examined the veracity of the right hemisphere and valence models by measuring asymmetries in: (i) movement of the face; and (ii) observer's rating of emotionality. The study uses a precise three-dimensional (3D) imaging technique to measure facial movement and to provide images that simultaneously capture the left or right hemifaces. Models (n = 16) with happy, sad and neutral expressions were digitally captured and manipulated. Comparison of the neutral and happy or sad images revealed greater movement of the left hemiface, regardless of the valence of the emotion, supporting the right hemisphere model. There was a trend, however, for left-sided movement to be more pronounced for negative than positive emotions. Participants (n = 357) reported that portraits rotated so that the left hemiface was featured, were more expressive of negative emotions whereas right hemiface portraits were more expressive for positive emotions, supporting the valence model. The effect of valence was moderated when the images were mirror-reversed. The data demonstrate that relatively small rotations of the head have a dramatic effect on the expression of positive and negative emotions. The fact that the effect of valence was not captured by the movement analysis demonstrates that subtle movements can have a strong effect on the expression of emotion.

Activation of the amygdala and anterior cingulate during nonconscious processing of sad versus happy faces

Previous functional neuroimaging studies have demonstrated that the amygdala activates in response to fearful faces presented below the threshold of conscious visual perception. Using a backward masking procedure similar to that of previous studies, we used functional magnetic resonance imaging (fMRI) to study the amygdala and anterior cingulate gyrus during preattentive presentations of sad and happy facial affect. Twelve healthy adult females underwent blood oxygen level dependent (BOLD) fMRI while viewing sad and happy faces, each presented for 20 ms and "masked" immediately by a neutral face for 100 ms. Masked happy faces were associated with significant bilateral activation within the anterior cingulate gyrus and amygdala, whereas masked sadness yielded only limited activation within the left anterior cingulate gyrus. In a direct comparison, masked happy faces yielded significantly greater activation in the anterior cingulate and amygdala relative to identically masked sad faces. Conjunction analysis showed that masked affect perception, regardless of emotional valence, was associated with greater activation within the left amygdala and left anterior cingulate. Findings suggest that the amygdala and anterior cingulate are important components of a network involved in detecting and discriminating affective information presented below the normal threshold of conscious visual perception.