The Function of a Fearful Expression

Human beings use stereotyped facial expressions to identify the feelings of others. We can tell what another person is feeling in part because of how their face looks. However, this says very little about why the particular changes in facial musculature are associated with particular feelings. Why do the eyebrows go up when we are afraid instead of down?

To address this issue, Susskind et al., publishing in Nature Neuroscience, looked at the visual and physiological effects of fearful expressions as opposed to expressions of disgust and neutral expressions. They found that when someone looks afraid, their visual field enlarges, their eyes move more rapidly, and their inspiratory rate increases. This suggests that the particular expression of fearfulness is an adaptive.

To identify what constituted a fearful or a disgusted expression, the authors first generated a set of computer images of faces. By varying the degree of eye-lid opening and brow raising, they identified those expressions that a group of a subjects was most likely to label as fearful or disgusted. Unsurprisingly, the subjects identified fearful expressions as characterized by eye-opening and brow-raising and disgusted expressions as characterized by eye-closing and brow-lowering.

Figure 2 in the paper (below) shows the prototypical expressions for fear (a) and disgust (b). The green arrows indicate the motions in the expression that occur from a face least likely to be identified as fearful or disgusted to a face most likely to be identified as such. The authors also noted that these expressions are in a sense opposite one another. A face least likely to be identified as fearful is likely to be identified as disgusted.

The authors then performed various psychometric evaluations on subjects performing posed fearful or disgusted expressions. Here is a summary of what they found:

• Using a test for peripheral vision called perimetry, they found that fearful expressions enhanced the size of the upper visual field as opposed to both disgusted and neutral expressions. Disgusted expressions decreased the size of both the lower and the upper visual fields.
• Using eye movement tracking, they found that horizontal saccade -- orienting eye movements -- velocity increased in the fearful expressions.
• Using respirometry, they found that the inspiratory capacity and rate increased with the fearful expressions.
• Finally, using an MRI, they showed that the nasal cavities increase in size during fearful expressions.

All of these data support the notion that the function of the fearful expression is to help the body react to whatever might be triggering it -- that it facilitates a flight reaction. The eyes open wider and move more quickly to identify the axe-murderer trying to kill you, and the nasal cavities open to facilitate rapid flight from said murderer. These findings suggest that facial expressions may serve a communication function, but the actual movements of facial anatomy also serve an evolutionary function.

The authors summarize the significance of their work:

In contrast with suggestions that humans are endowed with an intricate facial musculature for the purposes of social communication, here we examined the idea that emotional expressions not only signal emotions, but may have originated to modify preparedness for perception and action. Specifically, we sought evidence for Darwin's principles of expression opposition in form and function. Fear and disgust were shown to be near opposites in form, supported by fundamentally opposing action tendencies. Evidence of enhanced visual-field size, saccadic velocity and nasal inspiratory capacity in fear and the direct inverse in disgust indicate that there is a parallel opposition in function between the two expressions, serving the functions of sensory vigilance to enhance detection of the source of potential threat and sensory rejection to reduce sensory exposure. As our experiments focused on a subset of expressions, our results do not preclude the possibility that expressions other than fear and disgust are shaped by sensory function or whether fear and disgust are the prototypical manifestations of this selection pressure. Nevertheless, our findings provide evidence that is consistent with the possibility that facial expressions originate in egocentric sensory function rather than allocentric communicative value, indicating that the varieties of facial expression form may be shaped by a common underlying sensory regulatory function. (Emphasis mine. Citations removed.)

Importantly, the authors note that an evolutionary function for a fearful expression is not mutually exclusive with a social function. This is particularly true if we consider how disgusted and fearful expressions appear to be opposites. Being opposites on numerous characteristics may facilitate other people identifying and differentiating these expressions. Thus, the expressive function of faces and their physiological function may have co-evolved.

Any interesting question for me is: why do simulated faces have the same physiological effects as faces actually generated in fear or disgust? It is sort of odd that asking someone to act afraid has the same effects as being afraid. I could understand how nasal cavities would expand when you try to look afraid, but why would saccade velocity increase? Does the willful simulation of an emotion simulate the physiology of that emotion?

The idea the looking like you feel something makes you actually feel it is not new. It is from a theory called the facial feedback hypothesis. For a more scholarly treatment, William James discussed this idea in his essay What is an Emotion?:

Our natural way of thinking about these standard emotions is that the mental perception of some fact excites the mental affection called the emotion, and that this latter state of mind gives rise to the bodily expression. My thesis on the contrary is that the bodily changes follow directly the PERCEPTION of the exciting fact, and that our feeling of the same changes as they occur is the emotion. Common sense says, we lose our fortune, are sorry and weep; we meet a bear, are frightened and run; we are insulted by a rival, are angry and strike. The hypothesis here to be defended says that this order of sequence is incorrect, that the one mental state is not immediately induced by the other, that the bodily manifestations must first be interposed between, and that the more rational statement is that we feel sorry because we cry, angry because we strike, afraid because we tremble, and not that we cry, strike, or tremble, because we are sorry, angry, or fearful, as the case may be. Without the bodily states following on the perception, the latter would be purely cognitive in form, pale, colourless, destitute of emotional warmth. We might then see the bear, and judge it best to run, receive the insult and deem it right to strike, but we could not actually feel, afraid or angry.

The last assertion -- that in absence of the response you wouldn't actually feel the emotion -- is not tripe. We know that some patients with quadriplegia feel affective blunting. Mimicking an emotion does in degrees facilitate feeling it, and the inability to do so does blunt your emotions.

Anyway, I think this is a cool paper that tries to get at the importance of faces from a functional point-of-view. Faces have partially evolved to communicate, but they have also evolved to serve physiological functions.

Hat-tip: Faculty of 1000

Susskind, J.M., Lee, D.H., Cusi, A., Feiman, R., Grabski, W., Anderson, A.K. (2008). Expressing fear enhances sensory acquisition. Nature Neuroscience, 11(7), 843-850. DOI: 10.1038/nn.2138