The visual role of the sclera and the teeth in facial interactions

Paul Bouissac (University of Toronto)

1. Introduction: visual signaling. Like its closest relatives, the human primate has evolved as a diurnal animal that relies mostly   on vision for identifying appropriate food, adequate shelter, and potential mates at a distance.  Vision also plays a crucial role in social interactions within the group through recognizing individuals and the signals they direct to each other. Visual signaling requires movements that create dynamic patterns and   chromatic contrasts within the species’s perceptual  range. One of these contrasts, which is shared by many other mammals, involves the white of the eyes and the teeth that is made more or less visible through facial muscular contractions. Some contractions such as wincing or chewing serve vital functions; others have primarily evolved as social signals. The latter are often subject to cultural morphing that may involve hand gestures or artifacts in addition to controlling facial  muscles.   


2. From natural to cultural signaling. The variations of the surface and contour of the visible white areas cannot be assessed independently from each other and from the other variations occurring in the face. These natural signaling repertories have evolved as adaptive social tools from fitness-enhancing properties  and  behavior. Cultural tinkering through visual   transformations of selected facial features is meta-semiotic in the sense that it applies to the robust semiotic system that has been fine-tuned by evolution. It therefore necessarily takes into account the relevant dynamic  patterns and contrasts of this primary semiotic basis, which is species-specific as long as this tinkering does not attempt to achieve  cross-specific transformations for hunting or ritual purposes.


3. The sclera. The visible part of the eye globe is comprised of three concentric areas: the colored iris, a circular muscle that controls the opening of the pupil in its center, and the sclera, a white fibrous tissue irrigated by tiny blood vessels, that surrounds the former. The whole is framed by the eyelids and eyelashes. Functionally, the eye globe  has evolved as a movable organ under the constraints of optimal positioning and velocity of the retinal image. Six extraocular muscles (four “rectus” and two “oblique”) move the globe so as to optimally position the pupil and compensate for head movements. The result is a relatively stable range of directions of gaze (Leigh & Zee 1991:3-12, 264-283). From an observer’s point of view the iris appears as a darker dot moving across a lighter field. These ocular gestures provide an observer with information regarding the tracking of moving targets, the holding  and  shifting of attention, and the selective focusing on aspects of an object within the visual field. Since the muscles involved are susceptible to voluntary control an observer’s attention can be thus manipulated by deceptive directing of the gaze. The source of information can also be partly occulted by the partial closing of the eyelids which are controlled by the superficial muscles that surrounds the orbit.


4. The teeth. Teeth form another striking visual contrast with their surrounding perceptual field, the peri-buccal area. They are covered by the lips, whose reddish pigmentation enhances the contrast with their whiteness when the lips are  drawn open under the action of the  muscles of the mouth. Like the eyelids, the lips’ contour are underlined, most prominently in the mature male, by natural pilosity. Lips are formed of  connective tissue which, like the outer border of the eyelids, fills up with blood (turgescence) in situation of sexual or social arousal. The richness of the musculatory system of the lower face makes possible a wide array of lip movements which reveal the dental white area  framed by various contours combined with degrees of aperture of the mouth. In natural conditions, the optimal reflective quality of the teeth depends on the age and health of the individuals. Lack of vitamin D, poor maintenance and missing teeth considerably reduce the signaling effectiveness of the contrasts by impairing the visibility of the contours.


5. Economy of semiotic resources. Eye and mouth signaling has been the object of intense scrutiny in the context of research in primate and human ethology, nonverbal communication and face-to-face interaction  (Fridlund 1994). The  importance of ocular gestures has been noted for describing emotions, group structures, and conversations; the expressive functions of the mouth have also been the focus of systematic observations and classifications notably in relation to smile, laughter and play (Redikan 1982). But focusing attention on the three white patches that all human faces offer, irrespective of their skin pigmentation and cultural upbringing, reveals a primary signaling system which allows the interactants to quickly draw conclusions regarding the attitude and intention of conspecifics at a safe distance. These white patches can be present or absent. In the latter case caution is in order because the mouth is not relaxed and potentially aggressive, and it is not possible to  assess the direction of the gaze. Total absence of visible white patches therefore indicates potential hostility.  But in the case of optimal visibility of the three white patches, it can be inferred that the approaching individual discloses unambiguously the direction of the gaze and makes possible the reading of other eye-related information, and that the uncovered teeth flash the signal of peaceful intent or submission since the jaws are not clenched. More fundamentally, the display of areas of maximal light reflection indicates that the presence of an approaching individual is clearly advertised and intends to be friendly rather than hostile.


6. Signaling dominance and subservience. The ethological evaluation of these signals cannot take into consideration the very recent (on the evolutionary time scale) emergence of new environmental features such as high density city dwelling, urban civility, electric and electronic technologies  and the digitalization of human interactions. It is reasonable to assume that the signaling repertory of humans with respect to dominance, submission, courtship, hostility and peace-making within the group, has remained stable under local and superficial variations.  Dominance is consistently associated with minimal showing of the facial white patches whereas submission and seduction are indicated by displays of the white of the eyes and teeth. There are significant variations in patterns and duration that can be measured during any face-to-face interactions since the constraints under which they evolved required them to be clearly visible at a distance and facilitate individual recognition (Bruce et al. 1992). They indeed constitute unambiguous marks that are perceptually foregrounded and that each interactant spontaneously monitors during encounters and transactions. A human ethologist’s  trained eye can assess the dynamic of group structures and the nature of social interactions through exclusively focusing on the relative quantity, shape and directionality of these flashing white patches.    


7. Cultural codifications. The study of make-up and masks shows that the signaling value of these white patches has not gone unnoticed in the art of artificial facecraft. It is particularly interesting to observe how white patterns are introduced in the make up of performers in a comparative, cross cultural perspective. For example, two types of contrastively related characters are found across several cultures in traditional performances: the authoritarian and the transgressor clowns. The former (figure 1) display thoroughly whitened faces, a transformation that relatively neutralizes the chromatic contrast between the white of the eyes and the surrounding skin area; the latter tend to add artificial white patterns in the ocular and buccal regions of the face so as to emphasize expressions of surprise, seduction, submissiveness or candor (figure 2). These additional patterns fit the general outlines of the natural signaling areas, thus freezing the facial expressions most relevant to their represented cultural functions, and making them clearly perceptible through the distance that usually separates performers from their audience, often under dim light. By opposition, the figures of authority acquire a high degree of impenetrability by whitening the face, thus blurring the most  productive chromatic contrast of its signaling potential.


8. Conclusion: experimental perspectives. While the modulation of ocular and dental  white patterns is not the only signaling system of the face and enters in various combinations with other significant muscular contractions, it nevertheless provides the ground for a theoretical model of human face-to-face interactions. The evolutionary argument proposed here is that it is, in the natural conditions of a diurnal organism endowed with a retina that processes fine-grained chromatic information in its center and a-chromatic contrasts in its periphery, the single most cost-effective device for prompt decoding of this facial signals even in reduced luminosity  and excentric positions. Advertising submission during close, potentially damaging encounters with conspecifics  is , in many mammal species, the surest way to survive. This model  could be easily quantitatively tested by recording the instant responses of subjects to photographs in which the natural white areas of faces would be varied in shape and magnitude. Phenomenological observations of interactions between individuals whose relative status is not in question should also provide relevant qualitative data, as would further analyses of  performers’ make-up and masks in the context of their cultural traditions.


Bruce, V., Cowey, A., Ellis, A. W., & Perrett, D.I. (eds.) 1992, Processing the Facial Image. Oxford, Clarendon Press.

Fridlund,A.J.1994, Human Facial Expression. New York, Academic Press.

Leigh, R.J. & Zee, D. S. 1991, The Neurology of Eye Movements. Philadelphia, F.A.Davis Co.

Redican, W. K. 1982, “An evolutionary perspective on human facial displays” in Ekman, P. (ed.), Emotion in the Human Face. Cambridge, Cambridge University Press, 212-280.