Each face tells us an instant story.
If you glimpse a photo of a friend, parent, or celebrity, you don't need to spend any time assessing the creases and folds of their nose before you can definitively say who they are.
Recognizing a face is instant and effortless. It is so effortless that it is forgivable to have never considered how we manage it.
If you take a moment to think how complex a face is, yet how similar each face is to each other - two eyes, a mouth, a nose, in the same order each time - you start to appreciate how incredible it is that we manage this feat with such ease.
Within a single moment, we recognize that we are indeed looking at a face; but we also recognize who they are and what kind of a mood they are in. Visible for the duration of most social interactions, the face is a pivotal part of the human experience.
Of course, nothing evolves in an animal unless it is important, and recognizing faces has been vital for humanity as a species. Our ancient cousins would have been at a significant disadvantage if they could not tell when a colleague or stranger was enraged, for instance.
As social animals, it is essential that we are able to read the tiny tweaks that go into making a facial expression - spotting the smallest fluctuation in an eyebrow's elevation that tells you to keep your distance or the trembling lip that says your partner needs help.
Always seeing faces
Within a split-second, even in a dimly lit room or while moving past on the bus, we can recognize an individual's identity, mood, sex, race, age, and direction of attention. Expressions appear to jump from people's faces directly into our brains.
Just three drops of coffee are effortlessly perceived as a face. This is an example of pareidolia.
This moon-like dial we wear on the front of our heads gives the reader an abbreviated autobiography.
Television, movies, magazines, and social media are awash with faces. They carry immediate emotive qualities unlike images of any other type of object.
Our brains are so eager to spot faces that they often see them in places where they do not even exist, a phenomenon known as pareidolia.
In a world consumed by a barrage of emoticons, we forget how amazing it is that this - :-) - is instantly recognizable as a happy face.
Our facial addiction begins at an early age, too. Infants who are only minutes old show a preference for face-like stimuli over stimuli that is similarly complex but not face-like.
Although recognizing our nearest and dearest is a synch, understanding how we manage this feat is anything but. In order to appreciate how marvelous we are at assessing faces, let's start with an experiment.
Which of these two faces is most attractive?:
Although it is easy enough to spot the difference, the true horror is not immediately clear.
Now, turn your head and look at the images the right way up. If assessing a face was a simple as it feels, surely we would have noticed the upside-down monstrosity?
This so-called Thatcher effect demonstrates that facial recognition is something separate from standard object recognition. Most objects - a chair, a hat, a telephone - can all be easily recognized whichever way up they are viewed. An upside-down face, however, hides many of the salient properties which we take for granted.
Understanding facial emotions
Conveying emotion is one of the most important roles of the human face, and perhaps why facial perception has been a topic of discussion since scientific inquiry was born.
Certain facial expressions have been shown to be unanimous across a variety of cultures, at least in part. A study, conducted in 1969, investigated responses to common facial expressions - anger, revulsion, sadness - in people from New Guinea, Borneo, Japan, Brazil, and the United States.
They found that, even in preliterate societies, many of the emotions were easily understood by the participants. This infers that the feelings painted on our faces by 43 individually controlled muscles evolved these specific patterns millions of years ago.
The look of disgust that spreads across your face after smelling sour milk would have been easily understood by your prehistoric great, great, great [...] great aunt.
Which parts of the brain are involved?
It will come as no surprise that a job as important and complicated as facial perception requires chatter between a variety of brain areas.
Face processing relies on a patchy network of regions in the temporal and frontal lobes. It also involves other parts of the brain that do not normally dabble in visual stimuli, such as the somatosensory cortex - an area mostly concerned with receiving information about touch sensation.
The stimulation of the somatosensory cortex during the perception of facial expressions backs up the "simulationist model." This model theorizes that, to understand the meaning behind a facial expression, individuals attempt to replicate the activity in their own brains.
Facial perception relies on a wide array of brain centers.
The simulationist model is backed up by a few lines of research. For instance, patients who cannot themselves produce the facial expressions related to fear, disgust, or anger also find it difficult to recognize the same emotions in others.
Additionally, neuroimaging researchers have shown that similar brain regions are active when someone watches an emotional expression as when they attempt to imitate that same expression.
The amygdala also plays a part; the almond-shaped subcortical structure is involved in memory, decision-making, and emotional reactions. If the amygdala is damaged, it can result in an inability to recognize fear in other people.
This difficulty in recognizing faces and their expressions in amygdala-damaged patients seems to be due to a problem understanding information coming from the eye region of the faces they see.
According to neuroscientists, the occipital face area (OFA) is involved at an early stage of facial perception; it is activated very rapidly after the presentation of a face (around 100 milliseconds) and recognizes the basic components of the face - the eyes, nose, and mouth. These details are then passed onto other areas to process the information in more depth.
There is still much to learn about the complex neuroscience behind facial perception, but it is already clear that it relies on the interplay between a number disparate regions and networks.
Another network that is known to be important in facial perception is the fusiform face area (FFA). As with many discoveries within the neurosciences, it was an error in the FFA region that alerted researchers to its role in facial recognition.
The FFA and prosopagnosia
The FFA has been shown to light up during MRI scans of individuals in the throes of facial perception. Although there is debate as to whether the FFA is purely devoted to facial expression, or whether it carries out other types of recognition, too, it is clearly important in facial recognition.
The FFA is thought to help our brains to draw out more detail from a face than we would any other inanimate object of equal complexity. It allows us to treat faces as a special case, to really dive deep into their details.
Prosopagnosia, also known as "face blindness," is a condition generally present from birth. As its name suggests, individuals with prosopagnosia cannot recognize faces, even of family members and friends.
Lesions in the OFA, FFA, and the anterior temporal cortex are now known to be the cause of prosopagnosia.
In 1947, Joachim Bodamer, a German neurologist, was the first to describe cases of this condition. One of his landmark case studies was a 24-year-old male who had suffered brain damage due to a bullet wound and lost the ability to recognize his family, friends, and even his own face.
It is only when face recognition fails that we understand how important it is.
The condition is fascinating but, at the same time, distressing and highly disruptive for anyone who deals with it daily; it is much more common than most people realize, affecting an estimated 2 percent of Americans.
Individuals with prosopagnosia have to figure out their own ways to recognize people. For instance, this quote comes from a father with the condition:
"When my son started school I dyed his hair so I'd know which kid was mine without having to be a detective every day."
Because there is variation in the severity of prosopagnosia's symptoms in individuals with similar brain lesions, it is assumed that the FFA and OFA are not the only sections of the brain involved in facial recognition.
Further study slowly pulls disparate parts of the brain into the frame, but the whole picture is a long way from being painted.
Interestingly, research into prosopagnosia has discovered people with a polar opposite condition; these savants have been dubbed "super recognizers." These individuals remember, for the rest of their lives, the faces of people which they pass in the street or briefly see in the grocery store for a fleeting few seconds.
This subset of super recognizers have only recently come to light, so it is unclear how common their ability is. Of course, the majority of the population lie within the normal range of facial perception. Although, with the wonderful array of expressions we can perceive and relay, and the minuscule differences we can spot in faces, "normal" seems a little modest.