J . COMMUN . DISORD . 23 (1990) . 273-286
NEUROBIOLOGICAL AND CULTURAL ASPECTS OF FACIAL ASYMMETRY RONALD S . van GELDER Psychologie, Rokin 58. Amsterdam
JOAN C . BOROD Departments of Psychology, Queens College, and Neurology . Mount Sinai Af edit at .School, The City (Jni'ersity of New York
Facial asymmetry refers to the fact that the left and the right sides of the face during movement or rest are not identical . This asymmetry can be produced by a range of factors . such as ones that are anatomical, physiological . neurological, psychological, pathological, or sociocultural . Interaction among these various factors is not uncommon and has the potential to confound scientific investigations . When focusing on asymmetrical facial movements and expressions-which is the issue of this article-one has to make sure to control for other types of asymmetries that could confound the observations . To ensure such control, adequate judgment and measurement techniques are necessary . Some of these techniques are briefly described in this article . Furthermore, some explanations for facial asymmetrical movements are considered . From a neuropsychological perspective, it is suggested that asymmetrical facial expressions have some relationship to the functional asymmetry of the brain . But social-psychological and cultural factors, it is argued here . may also play an important role in the phenomenon of asymmetrical facial expressions, although no definitive statements regarding the nature of their role are made at this point .
INTRODUCTION During real-life conversations, one generally looks at the other person's face, one of the most informative sources of the human body . During a conversation, when the face of the other person is not inspected, it is considered to be impolite . Most frequently, and probably unconsciously, one looks at the eye and mouth regions of the face . Something out of the ordinary observed in the face of another person, however, draws our attention to a different region of the face for a short time, sometimes longer . Unusual "dynamic" features in the face may also be the cause of distraction . leading to a more conscious or better look for longer periods of time . The human face is usually asymmetrical, i .e ., there are differences beAddress correspondence to R . S . van Gelder, Psychologie . Rosin 58 Ii, 1012 KV Amsterdam, The Netherlands . CO 1990 by Elsevicr Science Publishing Co ., Inc . 655 Avenue of the Americas, New York, NY 10010
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Figure 1 . (A) Patient with a right-sided facial paralysis, when requested to smile . (B) Healthy subject with facial asymmetry, when requested to smile .
tween the left and right facial halves (or hemifaces) . This is apparent especially when one meets someone with some type of facial deformation, such as a complete unilateral facial paralysis . In such a paralysis, the muscles of the face have lost their functions on one side, making the difference between the left and the right sides easily visible (Figure la) . Also, in the healthy person, small hut sometimes even conspicuous asymmetries are visible (Figure lb) . Consider the picture of this healthy subject's face . If your take a pair of scissors and cut along the vertical midline of a normally oriented and a mirror-reversed print of the picture, it is easy to create symmetrical pictures of the face . If you take the left half of the normal and the right half of the mirrored picture and join the two, a symmetrical "left-chimeric" emerges (Figure 2a) . A combination of the right half of the normal and the left half of the mirrored pictures will produce a "right-chimeric" (Figure 2b) . Undoubtedly, these chimeric pictures do not look very much alike, and they each differ from the original picture! Of course, this is even more striking for the chimerics of the patient (Figure 2c and d) . In scientific studies, research has been conducted to determine whether asymmetrical facial expressions are consistent within and across subjects, and what the "origins" of these asymmetries might be . In this article . some of these research studies will be discussed and some of the more compelling explanations will be provided . The first question to be raised concerns the actual nature of facial asymmetry .
THE NATURE OF FACIAL ASYMMETRY Facial asymmetry refers to the fact that the left and the right sides of the face during movement or rest are not identical . This asymmetry can be produced by a range of factors, such as ones that are anatomical, phys-
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B
C
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Figure 2 . (A-D) Left- and right-chimerics (symmetrical composite pictures) .
iological, neurological, psychological, pathological, or sociocultural . Interaction among these various factors is not uncommon and has the potential to confound scientific investigations . For example, facial asymmetry can be caused by development . The basis of facial asymmetry seems to be laid out prenatally or perinatally . In the case of normal development, the growth of hones, muscles, and nerves should not then lead to new asymmetries . However, once an asymmetry emerges, it can become more pronounced . Although one cannot be sure about the cause or causes of these left-right differences from conception on, the following tentative explanation has been offered by Burke (1971) : Both sides of the body develop relatively independently, suggesting that the absence of coordinating structures may be more likely to stimulate asymmetry than symmetry . Lines of age, i .e ., wrinkles, and deposits of subcutaneous fat are changes that typically develop over many years . These changes . however, can be accelerated by psychological and social factors, e .g ., the accumulation of fat deposits due to overeating . This type of change can lead to or accentuate left-right differences in the face . However, the reasons for such changes occurring more on one side of the face than the other are not known, though many factors have been postulated . The most apparent difference between the hemifaces is caused by an extensive unilateral movement, i .e ., muscle contraction on one side only .
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When the contraction is stronger on one side than the other, an asymmetry will occur, although it will appear less pronounced than a unilateral movement . In this article, we will deal mainly with the latter type of asymmetrical movement . Facial asymmetry also can be created by the use of cosmetics or plastic surgery . In such cases, the individual's personal preference becomes the actual cause of the asymmetry . On the other hand, the left-right differences caused by illness or surgical treatment of pathological states (e .g ., head trauma) typically are not subject to an individual's own control . Thus, it is clear that different factors determine facial asymmetry . When focusing on asymmetrical facial movements, one has to make sure to control for other types of asymmetries that could confound observations of facial movements . To ensure such control, adequate judgment and measurement techniques also are necessary . In the next section, attention is directed to the issue of experimental control . DETERMINATION OF ASYMMETRICAL FACIAL MOVEMENTS When the face is relaxed, left-right differences arc easily detected by an alert observer . The results of these observations can tell us whether or not the resting face is asymmetrical . Also, asymmetrical movements are detectable by simple observation, although the multitude of the cooperating facial muscles and the complexity of facial movement require more precise measuring techniques, e.g ., via photographic equipment . However, the individual subject who will be photographed has to carry out the facial movements in such a way that the photographer can "shoot" at the right moment . For the sake of ensuring that the asymmetry is only a consequence of movement, the subject also should he photographed in the resting phase . It should be noted that it is very difficult, if not impossible, to capture spontaneous emotional expression through still photography . The use of film or video appears more appropriate for capturing more naturally occurring emotional expression . Although these techniques require more work and time in preparation and analysis, they are more ecologically valid and methodologically appropriate for research involving facial expression . especially for capturing those subtle asymmetrical facial movements . The use of film or video can he supplemented with a very sophisticated measurement technique called electromyography (EMG) . This method is similar to electrocardiography (ECG) . used for testing the functioning of the heart . In EMG . the electrical activity from the muscles (in this case, facial) is recorded by using electrical receptors (electrodes) that allow signals to be plotted and measured . Figure 3a and 3b show
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Figure 3 . (A) Signal trace of the zygomatic muscles of the patient, with a rightsided facial paralysis, 6 months after its onset . The patient was requested to smile three times during 10 seconds (solid line = intact side ; dotted line = damaged side) . Plots were made taking, every 20 milliseconds, a sample of the registered EMG-signal . Vertical axis : amplitude Integrated EMG (arbitrary units) ; horizontal axis : sample numbers (X 10) .
graphical plots of the electrical muscle activity taken from the zygomatic muscles of the individuals posing a smile in Figure In and b, respectively . Upon examining the pictures and graphs, we see an asymmetry for both subjects . In the pictures, the left corner of the poser's mouth is raised more than the right corner ; similarly, in the graphs, the "smile" muscle on the left side (solid line) shows a potential that is greater than that on the right side (dotted line) . From a superficial analysis of these data, the pictures and the plots appear consistent . A combination of recording methods is optimal, providing the most complete information on asymmetrical facial movements . At the end of the next section of this article, research findings of different investigators in the field of facial asymmetry will be presented . It appears that the different results obtained in these studies very well may be attributed to methodological differences (e .g ., photographs versus vidto versus electromyography, observer judgment versus "objective" measurement) .
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Figure 3 . (R) EMG signals of the zygomatic muscles of the healthy subject with facial asymmetry during smiling . The subject was requested to smile three times (solid line = left side ; dotted line = right side) .
ASYMMETRICAL FACIAL MOVEMENTS Facial Paresis We begin with a discussion of facial paresis (incomplete paralysis) as it occurs after brain damage . Patients with facial paresis usually are characterized by an asymmetry in the lower part of the face (i .e ., mouth region) (the contralateral side being paretic) and by symmetry in the upper part (i .e ., brow and eye region) . Furthermore, this paresis typically is observable during posed (or voluntary) facial expression and absent during spontaneous (or involuntary) facial expression . Apparently, the brain "discriminates" between impulses for voluntary and those for involuntary execution of movements and uses different pathways during these two types of movement . Although this dichotomy is not absolute, we will assume it is for the sake of clarity in this section . Now for a closer look at what happens to the impulses running from the brain to the facial muscles . The facial muscles are stimulated by the brain via the facial nerve . The
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THALAMONUCLEAR TRACT (VARIATION)
Figure 4 . The supranuclear neural pathways : (1) The precentral gyros of the cerebral cortex . (2) Thalamonuclear tract- Note : The thalamic pathway for the affective mimic movements is anatomically not yet ascertained . (3) Facial nucleus . (From Miehlke, A . (1973) . Surgery of the Facial Nerve . 2nd enlarged edition of Die Chirurgie des nervus facialis . Munchen : Urban and Schwarzenberg . Reprinted with permission .) impulses travel from the brain roughly via two different pathways (see Figure 4) . From the "higher" structures (the precentral gyrus of the cerebral cortex), impulses stimulate voluntary facial expression, while from the "lower" structures (the thalamus in the subcortex), impulses induce spontaneous facial expression . It is assumed that the impulses for voluntary movements travel from the cortex bilaterally to the muscles of the upper face and mainly contralaterally to the muscles of the lower face . The impulses for involuntary movements are thought to travel bilaterally to both upper and lower face muscles . In conclusion, with a central facial paresis in the upper face, there is no functional loss observable in posed facial expression because the innervation stems from both sides of the cortex, suggesting that the loss of impulses from the lesioned side of the cortex is compensated for by impulses from the nonlesioned side . In the lower face, asymmetry is observable contralateral to the site of the lesion, because the lower face receives impulses predominantly from the contralateral brain side . Thus, in posed expressions, the innervation appears to originate in cortical regions of the brain . In spontaneous emotional expression, facial asymmetry typically is not observed in brain-damaged patients ; impulses, in this case,
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are presumably of subcortical origin, and these lower parts of the brain are not always involved with brain damage at the cortical level . Emotional Expression In the past decade, ncuropsychological researchers have explored the relationship between asymmetrical facial movements and the "neuroanatomy" in the healthy subject . Neuropsychologists generally study the relationship between brain and behavior . The brain consists of two halves (hemispheres) which are functionally asymmetrical . The left hemisphere, for example, is specialized for language and analytical processing, while the right hemisphere is specialized for nonverbal functions and holistic/ integrative processing (Ley and Bryden . 1979) . In neuropsychological research . investigators have tested the hypothesis that asymmetrical facial expressions are controlled by one hemisphere or the other . In so doing, these researchers rely on neurological and neuropathological mechanisms underlying facial emotional expression . Three types of studies can be discerned . What follow are a global description of each of these types, general results, and interpretations offered by investigators in each area . (i) The first type of study is concerned with the relationship between asymmetrical facial expressions and handedness . The majority of these researchers have found a left hemiface superiority during posed facial expression in right-handed subjects . The muscles of the left hemiface show stronger muscular contractions than those of the right hemiface ; it is assumed that a stronger muscle contraction reflects greater expressiveness . Contradictory results have been found in research with lefthanders by Moscovitch and Olds (1982), but not by Borod et al . (1981) or Heller and Levy (1981) . Left hemiface superiority in right-handers is interpreted as supporting a special role for the right hemisphere in the execution of facial emotional expressions . This interpretation is based on the assumption that the left hemiface receives innervation predominantly from the contralateral right hemisphere . (ii) In other studies, a relationship is hypothesized between asymmetrical facial expressions and elicitation condition . Some authors have found that the left hemiface dominates during posed facial expression, while asymmetries are equally distributed over both sides of the face and/ or asymmetry is completely absent during spontaneous facial expression . Explanations for these findings emanate from the clinical neurological observations and from the literature discussed above, which suggests that innervation for facial expression in disturbances such as central facial paresis stems from different brain levels . It is assumed that at higher brain levels (e .g. . cortical) there is functional asymmetry, whereas at lower levels (e.g., subcortical) there is symmetry . Left hemiface dominance
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during posed expression would be reflecting activation of the contralateral right hemisphere . (iii) Research has been performed on the relationship between facial asymmetry and emotional valence, i .e ., pleasantness level . During emotional facial expressions with a positive valence (e .g ., happiness), the right hemiface is more expressive than the left hemiface . whereas during expressions with a negative valence (e .g ., sadness), the left hemiface is more expressive . Thus, one could speculate that the hemispheres are differentially involved in the processing of emotional valence : the left hemisphere for positive emotion, and the right hemisphere for negative emotion . To draw one conclusion from these various findings and theories is difficult . Methodological differences between the studies may account for the wide range and often contradictory results regarding facial asymmetry during emotional expression (see Hager and Ekman, 1985) . However, throughout this literature (as reviewed by Borod and Koff, 1984 ; Campbell, 1986 ; Rinn, 1984 ; Sackeim and Gar, 1982 ; Thompson, 1985), the most striking theme is the association between left-sided facial asymmetry and right hemisphere dominance for emotion . CULTURAL AND SOCIAL-PSYCHOLOGICAL INFLUENCES ON THE ASYMMETRY OF FACIAL EXPRESSION Not only neuropsychological but also cultural and social-psychological factors appear to influence the movement of facial muscles . For example, sex differences appear to play a role in the studies of facial asymmetry concerned with emotional valence . In reviewing this literature, Borod et al . (1986) noted interactions between sex of poser and valence of expression, whereby patterns of facial asymmetry for females were more affected by emotional valence than were those for males . In speculating about what accounts for these sex differences, several explanations were posited . One is that such differences reflect structural differences between male and female brains . A second is that interpersonal variables may induce different modes of responding for males and females . Clearly, subjects' tendencies to behave in a particular way in social situations could influence their performances in studies involving emotional behavior . A third explanation for these sex-by-valence findings derives from social-psychological theories about display rules . Buck (1982, 1984) and Tucker (1981) have suggested that the right hemisphere is associated with general affective or emotional processes (both positive and negative) and the left hemisphere with the control (i .e ., facilitation and inhibition) of these processes according to social display rules . A final explanation could be that factors intrinsic to the experimental conditions,
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D
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F
Figure 5 . Movie and "pop" stars displaying asymmetrical facial movements : (A) Walter Matthau ; (B) Spencer Tracy ; (C) Clark Gable ; (D) Elvis Presley ; (E) Marilyn Monroe ; (F) Marlon Brando . (From : Moscovitch and Olds . (1982) . Neuropspchologia 20 :73 . Reprinted with permission of Pergamon Press .)
e .g ., sex of experimenter, may affect the subjects' behavior in a significant way . There have been few attempts to determine the influence of cultural or social factors on facial asymmetry . Asymmetrical facial movements sometimes appear to increase an individual's attractiveness . Hollywood movie stars in the 1930s and 1940s often would raise one eyebrow when playing the role of attractive and/or intelligent women (Ekman, 1979) . When imitating these actresses, their fans would typically reproduce these onesided facial movements . In recent times, movie actors and "pop" stars also display asymmetrical facial-especially mouth-movements, as illustrated in Figures 5a-5f . In one experiment, we found that one-sided eyebrow movements were fairly easy to learn with training (Van Gelder et al ., 1985) . In fact, we have noticed that one-sided facial movements appear frequently in advertisements displayed in magazines and newspapers . It is our hypothesis that these particular photographic shots are chosen to make the product more appealing by making the model more attractive .
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Figure 6 . A slight facial asymmetry in Mona Lisa's face . (From : Moscovitch and Olds . (1982) Neuropsychologia 20 :73 . Reprinted with permission of Pergamon Press .)
We conclude with two examples . Those who have seen the Mona Lisa in the Louvre Museum in Paris may have noticed-next to the beauty of the painting-the hidden attractiveness in "La Giaconda's" face . It has been suggested that this attractiveness is caused, among other things, by the slight facial asymmetry . An acceptable, though little known explanation of that asymmetry has been offered by the American facial surgeon K . K . Adour (1977) . He suggested that Mona Lisa may have had a facial paralysis before Leonardo da Vinci began his painting . This paralysis led to dysfunction of the muscles on one side of her face . Although Mona Lisa spontaneously recovered, Adour has speculated that a slight asymmetry (a sequela) remained . When Da Vinci painted the Mona Lisa's portrait, as she was posing a smile (Figure 6), this facial asymmetry became more obvious . Maybe any kind of asymmetry in the face enhances the attractiveness . In Figure 7, Humphrey Bogart is shown with a cigarette in the corner of his mouth . This "gesture" might be interpreted likewise as an asymmetry in the face augmenting his appeal . We think that this assumption can be supported, since in 22 of 24 randomly selected advertisements for cigarette smoking, the cigarette was placed in one of the mouth corners (Van Gelder, 1986) . Although people most frequently put their cigarette in the corner of their mouth, a proportion this extreme was not anticipated . An explanation for all these cultural and social-psychological phenomena is not readily available . Perhaps, a certain degree of facial asymmetry is needed to give ourselves a particular attitude or "posture ." This explanation is supported by the main finding from the research on facial asymmetry-that asymmetries are observed typically in posed facial expressions . The question then arises, "what else is posing than taking a posture?" It appears that social-psychological factors do play a role in facial asymmetries ; maybe they even are linked to some neurological basis . This again might be a fruitful area for neuropsychological research .
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Figure 7 . Humphrey Bogart with a cigarette in the corner of his mouth (Photo : KIPPA) .
GENERAL CONCLUSIONS In order to adequately record, measure, and evaluate asymmetrical facial expressions, it is necessary to be very careful and attentive when doing research in this area . The fact that the results found in the literature on this topic are not unequivocal seems most likely to he a consequence of methodological differences between studies . From a neuropsychological perspective, we have suggested that asymmetrical facial expressions have some relationship to the functional asymmetry of the brain . Social-psychological and cultural factors may also play an important role in the phenomenon of asymmetrical facial expressions, although no definitive statements regarding the nature of their role can be made at this point . The examination of facial asymmetry provides another example pertaining to the nature/nurture controversy in the analysis of human social behavior . Further, the research pertaining to facial asymmetry may he useful to sociobiologists by indicating new avenues and methods for disentangling neurobiological and cultural determinants of human behavior . The authors wish to thank Dr . Jan Wind for his helpful criticisms on an earlier version of this article . This work was supported, in part, by USPHS grant No . NS06029 to the Aphasia Research Center, USPHS grant No . MH37592 to New York University Medical School,
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PSC-CUNY Research Award No . 667440 to Queens College, and NIH Biomedical Research Support grant No . RR07064 to Queens College .
REFERENCES Adour, K . K . (1977) . Personal communication . Borod, J . C ., Caron, H . S ., and Koff, E . (1981) . Asymmetry of facial expression related to handedness . footedness, and eyedness : A quantitative study . Cortex 17 :381-390. Borod, J . C ., and Koff, E . (1984) . Asymmetries in affective facial expression . In N . Fox and R . Davidson (eds .), The Psychobiology of Affective Development . Hillsdale, NJ : Erlbaum and Associates . Borod, J. C ., Koff, E ., and Buck . R . (1986) . The neuropsychology of facial expression : Data from normal and brain-damaged adults . In P . Blanck, R . Buck, and R . Rosenthal (eds .), Nonverbal Communication in the Clinical Context . Philadelphia : Pennsylvania State University Press . Buck . R . (1982) . A theory of spontaneous and symbolic expression ; Implications for facial lateralization . Paper presented at the International Neuropsychological Society Meeting, Pittsburgh . Buck . R . (1984) . The Communication of Emotion . New York : Guilford Press . Burke, P . H . (1971) . Stereophotograrnmetric measurement of normal facial asymmetry in children . Human Biol. 43 :936-548 . Campbell, R . (1986) . Asymmetries of facial action : Some facts and fancies of normal face movement . In R . Bruyer (ed .), The Neurop .sychology of Face Perception and Facial Expression . Hillsdale, NJ : Erlhaum and Associates . Ekman, P . (1979) . About brows : Emotional and conversational signals . In M . von Cranach, K . Foppa, W . Lepenies, and D . Ploog (eds .), Human Ethology . Cambridge : University Press . Hager, J . C ., and Ekman, P . (1985) . The asymmetry of facial actions is inconsistent with models of hemispheric specialization . Psychophysiology 22 :307318 . Heller, W., and Levy, J . (1981) . Perception and expression of emotion in righthanders and left-handers . Neuropsychologia 19 :263-272 . Ley, R . G . . and Bryden, M . P . (1979) . Hemispheric differences in recognizing faces and emotions . Brain Lang . 7 :127-138 . Miehlke, A . (1973) . Die Chirurgie des Nervus Facialis . [Surgery of the Facial Nerve .] 2nd enlarged edition . Munich : Urban and Schwarzenberg . Moscovitch, M ., and Olds, J . (1982) . Asymmetries in spontaneous facial expression and their possible relation to hemispheric specialization . Neuropsychologia 20 :71-81 . Rinn, W . E . (1984) . The neuropsychology of facial expression : A review of the neurological and psychological mechanisms for producing facial expression . Psychol . Bull . 95 :52-77 .
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Sackeim, H . A ., and Gur, R . C . (1983) . Facial asymmetry and the communication of emotion . In J . T . Caccioppo and R . E . Petty (eds .), Social Psychophysiology . New York : Guilford Press . Thompson . K . J . (1985) . Right brain, left brain : left face, right face : Hemisphericity and the expression of facial emotion . Cortex 21 :281-299 . 'pucker, D . M . (1981) . Lateral brain function, emotion and conceptualization . Psychol . Bull . 89 :19-46 . Van Gelder, R . S ., Zegers, J . G ., and Kobus, M . H . (1985) . Training van eenzijdige wenkbrauwbeweging . [Training of one-sided eyebrow movement .) Proceedings of the Dutch Neuropsychological Society, Nijmegen, The Netherlands . Van Gelder, R . S . (1986) . Roken doe je nit je mondhoek! [Smoking out of the corner of your mouth!] Psychologie 9 :38-39 .
NEUROBIOLOGICAL AND CULTURAL ASPECTS OF FACIAL ASYMMETRY RONALD S . van GELDER Psychologie, Rokin 58. Amsterdam
JOAN C . BOROD Departments of Psychology, Queens College, and Neurology . Mount Sinai Af edit at .School, The City (Jni'ersity of New York
Facial asymmetry refers to the fact that the left and the right sides of the face during movement or rest are not identical . This asymmetry can be produced by a range of factors . such as ones that are anatomical, physiological . neurological, psychological, pathological, or sociocultural . Interaction among these various factors is not uncommon and has the potential to confound scientific investigations . When focusing on asymmetrical facial movements and expressions-which is the issue of this article-one has to make sure to control for other types of asymmetries that could confound the observations . To ensure such control, adequate judgment and measurement techniques are necessary . Some of these techniques are briefly described in this article . Furthermore, some explanations for facial asymmetrical movements are considered . From a neuropsychological perspective, it is suggested that asymmetrical facial expressions have some relationship to the functional asymmetry of the brain . But social-psychological and cultural factors, it is argued here . may also play an important role in the phenomenon of asymmetrical facial expressions, although no definitive statements regarding the nature of their role are made at this point .
INTRODUCTION During real-life conversations, one generally looks at the other person's face, one of the most informative sources of the human body . During a conversation, when the face of the other person is not inspected, it is considered to be impolite . Most frequently, and probably unconsciously, one looks at the eye and mouth regions of the face . Something out of the ordinary observed in the face of another person, however, draws our attention to a different region of the face for a short time, sometimes longer . Unusual "dynamic" features in the face may also be the cause of distraction . leading to a more conscious or better look for longer periods of time . The human face is usually asymmetrical, i .e ., there are differences beAddress correspondence to R . S . van Gelder, Psychologie . Rosin 58 Ii, 1012 KV Amsterdam, The Netherlands . CO 1990 by Elsevicr Science Publishing Co ., Inc . 655 Avenue of the Americas, New York, NY 10010
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Figure 1 . (A) Patient with a right-sided facial paralysis, when requested to smile . (B) Healthy subject with facial asymmetry, when requested to smile .
tween the left and right facial halves (or hemifaces) . This is apparent especially when one meets someone with some type of facial deformation, such as a complete unilateral facial paralysis . In such a paralysis, the muscles of the face have lost their functions on one side, making the difference between the left and the right sides easily visible (Figure la) . Also, in the healthy person, small hut sometimes even conspicuous asymmetries are visible (Figure lb) . Consider the picture of this healthy subject's face . If your take a pair of scissors and cut along the vertical midline of a normally oriented and a mirror-reversed print of the picture, it is easy to create symmetrical pictures of the face . If you take the left half of the normal and the right half of the mirrored picture and join the two, a symmetrical "left-chimeric" emerges (Figure 2a) . A combination of the right half of the normal and the left half of the mirrored pictures will produce a "right-chimeric" (Figure 2b) . Undoubtedly, these chimeric pictures do not look very much alike, and they each differ from the original picture! Of course, this is even more striking for the chimerics of the patient (Figure 2c and d) . In scientific studies, research has been conducted to determine whether asymmetrical facial expressions are consistent within and across subjects, and what the "origins" of these asymmetries might be . In this article . some of these research studies will be discussed and some of the more compelling explanations will be provided . The first question to be raised concerns the actual nature of facial asymmetry .
THE NATURE OF FACIAL ASYMMETRY Facial asymmetry refers to the fact that the left and the right sides of the face during movement or rest are not identical . This asymmetry can be produced by a range of factors, such as ones that are anatomical, phys-
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B
C
D
Figure 2 . (A-D) Left- and right-chimerics (symmetrical composite pictures) .
iological, neurological, psychological, pathological, or sociocultural . Interaction among these various factors is not uncommon and has the potential to confound scientific investigations . For example, facial asymmetry can be caused by development . The basis of facial asymmetry seems to be laid out prenatally or perinatally . In the case of normal development, the growth of hones, muscles, and nerves should not then lead to new asymmetries . However, once an asymmetry emerges, it can become more pronounced . Although one cannot be sure about the cause or causes of these left-right differences from conception on, the following tentative explanation has been offered by Burke (1971) : Both sides of the body develop relatively independently, suggesting that the absence of coordinating structures may be more likely to stimulate asymmetry than symmetry . Lines of age, i .e ., wrinkles, and deposits of subcutaneous fat are changes that typically develop over many years . These changes . however, can be accelerated by psychological and social factors, e .g ., the accumulation of fat deposits due to overeating . This type of change can lead to or accentuate left-right differences in the face . However, the reasons for such changes occurring more on one side of the face than the other are not known, though many factors have been postulated . The most apparent difference between the hemifaces is caused by an extensive unilateral movement, i .e ., muscle contraction on one side only .
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When the contraction is stronger on one side than the other, an asymmetry will occur, although it will appear less pronounced than a unilateral movement . In this article, we will deal mainly with the latter type of asymmetrical movement . Facial asymmetry also can be created by the use of cosmetics or plastic surgery . In such cases, the individual's personal preference becomes the actual cause of the asymmetry . On the other hand, the left-right differences caused by illness or surgical treatment of pathological states (e .g ., head trauma) typically are not subject to an individual's own control . Thus, it is clear that different factors determine facial asymmetry . When focusing on asymmetrical facial movements, one has to make sure to control for other types of asymmetries that could confound observations of facial movements . To ensure such control, adequate judgment and measurement techniques also are necessary . In the next section, attention is directed to the issue of experimental control . DETERMINATION OF ASYMMETRICAL FACIAL MOVEMENTS When the face is relaxed, left-right differences arc easily detected by an alert observer . The results of these observations can tell us whether or not the resting face is asymmetrical . Also, asymmetrical movements are detectable by simple observation, although the multitude of the cooperating facial muscles and the complexity of facial movement require more precise measuring techniques, e.g ., via photographic equipment . However, the individual subject who will be photographed has to carry out the facial movements in such a way that the photographer can "shoot" at the right moment . For the sake of ensuring that the asymmetry is only a consequence of movement, the subject also should he photographed in the resting phase . It should be noted that it is very difficult, if not impossible, to capture spontaneous emotional expression through still photography . The use of film or video appears more appropriate for capturing more naturally occurring emotional expression . Although these techniques require more work and time in preparation and analysis, they are more ecologically valid and methodologically appropriate for research involving facial expression . especially for capturing those subtle asymmetrical facial movements . The use of film or video can he supplemented with a very sophisticated measurement technique called electromyography (EMG) . This method is similar to electrocardiography (ECG) . used for testing the functioning of the heart . In EMG . the electrical activity from the muscles (in this case, facial) is recorded by using electrical receptors (electrodes) that allow signals to be plotted and measured . Figure 3a and 3b show
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Figure 3 . (A) Signal trace of the zygomatic muscles of the patient, with a rightsided facial paralysis, 6 months after its onset . The patient was requested to smile three times during 10 seconds (solid line = intact side ; dotted line = damaged side) . Plots were made taking, every 20 milliseconds, a sample of the registered EMG-signal . Vertical axis : amplitude Integrated EMG (arbitrary units) ; horizontal axis : sample numbers (X 10) .
graphical plots of the electrical muscle activity taken from the zygomatic muscles of the individuals posing a smile in Figure In and b, respectively . Upon examining the pictures and graphs, we see an asymmetry for both subjects . In the pictures, the left corner of the poser's mouth is raised more than the right corner ; similarly, in the graphs, the "smile" muscle on the left side (solid line) shows a potential that is greater than that on the right side (dotted line) . From a superficial analysis of these data, the pictures and the plots appear consistent . A combination of recording methods is optimal, providing the most complete information on asymmetrical facial movements . At the end of the next section of this article, research findings of different investigators in the field of facial asymmetry will be presented . It appears that the different results obtained in these studies very well may be attributed to methodological differences (e .g ., photographs versus vidto versus electromyography, observer judgment versus "objective" measurement) .
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Figure 3 . (R) EMG signals of the zygomatic muscles of the healthy subject with facial asymmetry during smiling . The subject was requested to smile three times (solid line = left side ; dotted line = right side) .
ASYMMETRICAL FACIAL MOVEMENTS Facial Paresis We begin with a discussion of facial paresis (incomplete paralysis) as it occurs after brain damage . Patients with facial paresis usually are characterized by an asymmetry in the lower part of the face (i .e ., mouth region) (the contralateral side being paretic) and by symmetry in the upper part (i .e ., brow and eye region) . Furthermore, this paresis typically is observable during posed (or voluntary) facial expression and absent during spontaneous (or involuntary) facial expression . Apparently, the brain "discriminates" between impulses for voluntary and those for involuntary execution of movements and uses different pathways during these two types of movement . Although this dichotomy is not absolute, we will assume it is for the sake of clarity in this section . Now for a closer look at what happens to the impulses running from the brain to the facial muscles . The facial muscles are stimulated by the brain via the facial nerve . The
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THALAMONUCLEAR TRACT (VARIATION)
Figure 4 . The supranuclear neural pathways : (1) The precentral gyros of the cerebral cortex . (2) Thalamonuclear tract- Note : The thalamic pathway for the affective mimic movements is anatomically not yet ascertained . (3) Facial nucleus . (From Miehlke, A . (1973) . Surgery of the Facial Nerve . 2nd enlarged edition of Die Chirurgie des nervus facialis . Munchen : Urban and Schwarzenberg . Reprinted with permission .) impulses travel from the brain roughly via two different pathways (see Figure 4) . From the "higher" structures (the precentral gyrus of the cerebral cortex), impulses stimulate voluntary facial expression, while from the "lower" structures (the thalamus in the subcortex), impulses induce spontaneous facial expression . It is assumed that the impulses for voluntary movements travel from the cortex bilaterally to the muscles of the upper face and mainly contralaterally to the muscles of the lower face . The impulses for involuntary movements are thought to travel bilaterally to both upper and lower face muscles . In conclusion, with a central facial paresis in the upper face, there is no functional loss observable in posed facial expression because the innervation stems from both sides of the cortex, suggesting that the loss of impulses from the lesioned side of the cortex is compensated for by impulses from the nonlesioned side . In the lower face, asymmetry is observable contralateral to the site of the lesion, because the lower face receives impulses predominantly from the contralateral brain side . Thus, in posed expressions, the innervation appears to originate in cortical regions of the brain . In spontaneous emotional expression, facial asymmetry typically is not observed in brain-damaged patients ; impulses, in this case,
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are presumably of subcortical origin, and these lower parts of the brain are not always involved with brain damage at the cortical level . Emotional Expression In the past decade, ncuropsychological researchers have explored the relationship between asymmetrical facial movements and the "neuroanatomy" in the healthy subject . Neuropsychologists generally study the relationship between brain and behavior . The brain consists of two halves (hemispheres) which are functionally asymmetrical . The left hemisphere, for example, is specialized for language and analytical processing, while the right hemisphere is specialized for nonverbal functions and holistic/ integrative processing (Ley and Bryden . 1979) . In neuropsychological research . investigators have tested the hypothesis that asymmetrical facial expressions are controlled by one hemisphere or the other . In so doing, these researchers rely on neurological and neuropathological mechanisms underlying facial emotional expression . Three types of studies can be discerned . What follow are a global description of each of these types, general results, and interpretations offered by investigators in each area . (i) The first type of study is concerned with the relationship between asymmetrical facial expressions and handedness . The majority of these researchers have found a left hemiface superiority during posed facial expression in right-handed subjects . The muscles of the left hemiface show stronger muscular contractions than those of the right hemiface ; it is assumed that a stronger muscle contraction reflects greater expressiveness . Contradictory results have been found in research with lefthanders by Moscovitch and Olds (1982), but not by Borod et al . (1981) or Heller and Levy (1981) . Left hemiface superiority in right-handers is interpreted as supporting a special role for the right hemisphere in the execution of facial emotional expressions . This interpretation is based on the assumption that the left hemiface receives innervation predominantly from the contralateral right hemisphere . (ii) In other studies, a relationship is hypothesized between asymmetrical facial expressions and elicitation condition . Some authors have found that the left hemiface dominates during posed facial expression, while asymmetries are equally distributed over both sides of the face and/ or asymmetry is completely absent during spontaneous facial expression . Explanations for these findings emanate from the clinical neurological observations and from the literature discussed above, which suggests that innervation for facial expression in disturbances such as central facial paresis stems from different brain levels . It is assumed that at higher brain levels (e .g. . cortical) there is functional asymmetry, whereas at lower levels (e.g., subcortical) there is symmetry . Left hemiface dominance
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during posed expression would be reflecting activation of the contralateral right hemisphere . (iii) Research has been performed on the relationship between facial asymmetry and emotional valence, i .e ., pleasantness level . During emotional facial expressions with a positive valence (e .g ., happiness), the right hemiface is more expressive than the left hemiface . whereas during expressions with a negative valence (e .g ., sadness), the left hemiface is more expressive . Thus, one could speculate that the hemispheres are differentially involved in the processing of emotional valence : the left hemisphere for positive emotion, and the right hemisphere for negative emotion . To draw one conclusion from these various findings and theories is difficult . Methodological differences between the studies may account for the wide range and often contradictory results regarding facial asymmetry during emotional expression (see Hager and Ekman, 1985) . However, throughout this literature (as reviewed by Borod and Koff, 1984 ; Campbell, 1986 ; Rinn, 1984 ; Sackeim and Gar, 1982 ; Thompson, 1985), the most striking theme is the association between left-sided facial asymmetry and right hemisphere dominance for emotion . CULTURAL AND SOCIAL-PSYCHOLOGICAL INFLUENCES ON THE ASYMMETRY OF FACIAL EXPRESSION Not only neuropsychological but also cultural and social-psychological factors appear to influence the movement of facial muscles . For example, sex differences appear to play a role in the studies of facial asymmetry concerned with emotional valence . In reviewing this literature, Borod et al . (1986) noted interactions between sex of poser and valence of expression, whereby patterns of facial asymmetry for females were more affected by emotional valence than were those for males . In speculating about what accounts for these sex differences, several explanations were posited . One is that such differences reflect structural differences between male and female brains . A second is that interpersonal variables may induce different modes of responding for males and females . Clearly, subjects' tendencies to behave in a particular way in social situations could influence their performances in studies involving emotional behavior . A third explanation for these sex-by-valence findings derives from social-psychological theories about display rules . Buck (1982, 1984) and Tucker (1981) have suggested that the right hemisphere is associated with general affective or emotional processes (both positive and negative) and the left hemisphere with the control (i .e ., facilitation and inhibition) of these processes according to social display rules . A final explanation could be that factors intrinsic to the experimental conditions,
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D
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Figure 5 . Movie and "pop" stars displaying asymmetrical facial movements : (A) Walter Matthau ; (B) Spencer Tracy ; (C) Clark Gable ; (D) Elvis Presley ; (E) Marilyn Monroe ; (F) Marlon Brando . (From : Moscovitch and Olds . (1982) . Neuropspchologia 20 :73 . Reprinted with permission of Pergamon Press .)
e .g ., sex of experimenter, may affect the subjects' behavior in a significant way . There have been few attempts to determine the influence of cultural or social factors on facial asymmetry . Asymmetrical facial movements sometimes appear to increase an individual's attractiveness . Hollywood movie stars in the 1930s and 1940s often would raise one eyebrow when playing the role of attractive and/or intelligent women (Ekman, 1979) . When imitating these actresses, their fans would typically reproduce these onesided facial movements . In recent times, movie actors and "pop" stars also display asymmetrical facial-especially mouth-movements, as illustrated in Figures 5a-5f . In one experiment, we found that one-sided eyebrow movements were fairly easy to learn with training (Van Gelder et al ., 1985) . In fact, we have noticed that one-sided facial movements appear frequently in advertisements displayed in magazines and newspapers . It is our hypothesis that these particular photographic shots are chosen to make the product more appealing by making the model more attractive .
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Figure 6 . A slight facial asymmetry in Mona Lisa's face . (From : Moscovitch and Olds . (1982) Neuropsychologia 20 :73 . Reprinted with permission of Pergamon Press .)
We conclude with two examples . Those who have seen the Mona Lisa in the Louvre Museum in Paris may have noticed-next to the beauty of the painting-the hidden attractiveness in "La Giaconda's" face . It has been suggested that this attractiveness is caused, among other things, by the slight facial asymmetry . An acceptable, though little known explanation of that asymmetry has been offered by the American facial surgeon K . K . Adour (1977) . He suggested that Mona Lisa may have had a facial paralysis before Leonardo da Vinci began his painting . This paralysis led to dysfunction of the muscles on one side of her face . Although Mona Lisa spontaneously recovered, Adour has speculated that a slight asymmetry (a sequela) remained . When Da Vinci painted the Mona Lisa's portrait, as she was posing a smile (Figure 6), this facial asymmetry became more obvious . Maybe any kind of asymmetry in the face enhances the attractiveness . In Figure 7, Humphrey Bogart is shown with a cigarette in the corner of his mouth . This "gesture" might be interpreted likewise as an asymmetry in the face augmenting his appeal . We think that this assumption can be supported, since in 22 of 24 randomly selected advertisements for cigarette smoking, the cigarette was placed in one of the mouth corners (Van Gelder, 1986) . Although people most frequently put their cigarette in the corner of their mouth, a proportion this extreme was not anticipated . An explanation for all these cultural and social-psychological phenomena is not readily available . Perhaps, a certain degree of facial asymmetry is needed to give ourselves a particular attitude or "posture ." This explanation is supported by the main finding from the research on facial asymmetry-that asymmetries are observed typically in posed facial expressions . The question then arises, "what else is posing than taking a posture?" It appears that social-psychological factors do play a role in facial asymmetries ; maybe they even are linked to some neurological basis . This again might be a fruitful area for neuropsychological research .
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Figure 7 . Humphrey Bogart with a cigarette in the corner of his mouth (Photo : KIPPA) .
GENERAL CONCLUSIONS In order to adequately record, measure, and evaluate asymmetrical facial expressions, it is necessary to be very careful and attentive when doing research in this area . The fact that the results found in the literature on this topic are not unequivocal seems most likely to he a consequence of methodological differences between studies . From a neuropsychological perspective, we have suggested that asymmetrical facial expressions have some relationship to the functional asymmetry of the brain . Social-psychological and cultural factors may also play an important role in the phenomenon of asymmetrical facial expressions, although no definitive statements regarding the nature of their role can be made at this point . The examination of facial asymmetry provides another example pertaining to the nature/nurture controversy in the analysis of human social behavior . Further, the research pertaining to facial asymmetry may he useful to sociobiologists by indicating new avenues and methods for disentangling neurobiological and cultural determinants of human behavior . The authors wish to thank Dr . Jan Wind for his helpful criticisms on an earlier version of this article . This work was supported, in part, by USPHS grant No . NS06029 to the Aphasia Research Center, USPHS grant No . MH37592 to New York University Medical School,
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PSC-CUNY Research Award No . 667440 to Queens College, and NIH Biomedical Research Support grant No . RR07064 to Queens College .
REFERENCES Adour, K . K . (1977) . Personal communication . Borod, J . C ., Caron, H . S ., and Koff, E . (1981) . Asymmetry of facial expression related to handedness . footedness, and eyedness : A quantitative study . Cortex 17 :381-390. Borod, J . C ., and Koff, E . (1984) . Asymmetries in affective facial expression . In N . Fox and R . Davidson (eds .), The Psychobiology of Affective Development . Hillsdale, NJ : Erlbaum and Associates . Borod, J. C ., Koff, E ., and Buck . R . (1986) . The neuropsychology of facial expression : Data from normal and brain-damaged adults . In P . Blanck, R . Buck, and R . Rosenthal (eds .), Nonverbal Communication in the Clinical Context . Philadelphia : Pennsylvania State University Press . Buck . R . (1982) . A theory of spontaneous and symbolic expression ; Implications for facial lateralization . Paper presented at the International Neuropsychological Society Meeting, Pittsburgh . Buck . R . (1984) . The Communication of Emotion . New York : Guilford Press . Burke, P . H . (1971) . Stereophotograrnmetric measurement of normal facial asymmetry in children . Human Biol. 43 :936-548 . Campbell, R . (1986) . Asymmetries of facial action : Some facts and fancies of normal face movement . In R . Bruyer (ed .), The Neurop .sychology of Face Perception and Facial Expression . Hillsdale, NJ : Erlhaum and Associates . Ekman, P . (1979) . About brows : Emotional and conversational signals . In M . von Cranach, K . Foppa, W . Lepenies, and D . Ploog (eds .), Human Ethology . Cambridge : University Press . Hager, J . C ., and Ekman, P . (1985) . The asymmetry of facial actions is inconsistent with models of hemispheric specialization . Psychophysiology 22 :307318 . Heller, W., and Levy, J . (1981) . Perception and expression of emotion in righthanders and left-handers . Neuropsychologia 19 :263-272 . Ley, R . G . . and Bryden, M . P . (1979) . Hemispheric differences in recognizing faces and emotions . Brain Lang . 7 :127-138 . Miehlke, A . (1973) . Die Chirurgie des Nervus Facialis . [Surgery of the Facial Nerve .] 2nd enlarged edition . Munich : Urban and Schwarzenberg . Moscovitch, M ., and Olds, J . (1982) . Asymmetries in spontaneous facial expression and their possible relation to hemispheric specialization . Neuropsychologia 20 :71-81 . Rinn, W . E . (1984) . The neuropsychology of facial expression : A review of the neurological and psychological mechanisms for producing facial expression . Psychol . Bull . 95 :52-77 .
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Sackeim, H . A ., and Gur, R . C . (1983) . Facial asymmetry and the communication of emotion . In J . T . Caccioppo and R . E . Petty (eds .), Social Psychophysiology . New York : Guilford Press . Thompson . K . J . (1985) . Right brain, left brain : left face, right face : Hemisphericity and the expression of facial emotion . Cortex 21 :281-299 . 'pucker, D . M . (1981) . Lateral brain function, emotion and conceptualization . Psychol . Bull . 89 :19-46 . Van Gelder, R . S ., Zegers, J . G ., and Kobus, M . H . (1985) . Training van eenzijdige wenkbrauwbeweging . [Training of one-sided eyebrow movement .) Proceedings of the Dutch Neuropsychological Society, Nijmegen, The Netherlands . Van Gelder, R . S . (1986) . Roken doe je nit je mondhoek! [Smoking out of the corner of your mouth!] Psychologie 9 :38-39 .
