In his 1865 book, Expression of Emotions in Man and Animals, Charles Darwin first proposed an evolutionary explanation for the human fascination with faces. He argued that critical social cues are expressed through facial gestures during situations of extreme fear and excitement, strongly suggesting that the face is an important feature of our ancestral social communication. Today our ability to quickly recognize familiar faces and subtle facial gestures continues to be a source of great interest to anthropologists, psychologists, neuroscientists and doctors. Discovering whether face recognition is a specialized human ability may lead to new insights into how our brain functions.
The act of recognizing a face is actually quite complex. Like many visual stimuli, faces must be accurately recognized in any orientation or lighting condition, and even while moving. But unlike other objects, faces are intimately involved in communication, and our brains must be able to extract a tremendous amount of subtle detail from just a glance. So while some of the issues involved in face recognition are the same as for recognizing any object, other issues are unique to faces. Confronted with this dilemma, is the brain’s most efficient solution to have special mechanisms for face recognition, or to simply extend the abilities of existing object recognition mechanisms?
That question is at the heart of a deep controversy in face recognition research. Studies with monkeys suggest that unique face recognition mechanisms might exist, while brain imaging experiments, studies with babies, and studies of people who cannot recognize faces show evidence supporting both sides of the argument. Compelling questions persist within the scientific community: what exactly are the mechanisms for processing faces in the brain? What compromises must be made for the brain to recognize faces quickly and accurately? And what does that indicate about how the human brain functions in general?
Behavior as a Beginning
Faces enter the human visual experience from the very beginning of life. Studies show that as soon as nine minutes after birth, babies prefer to look at pictures that most resemble human faces. Moreover, young infants have great propensity for mimicking the facial gestures of people around them. Although infants practice many motor skills, these studies show that even at a very early age, humans devote a great deal of attention and energy to the movements required for facial expression. By adulthood, our infantile preferences translate into an ability to recognize human faces better than other visual stimuli. Psychologist Robert Yin conducted some of the first studies that compared recognition of faces with recognition of objects in healthy adults. He found that people recognized faces significantly more often than they recognized objects.
Dr. Yin also uncovered a phenomenon now known as the inversion effect. He demonstrated that his subjects were able to recognize most objects about as often upside-down as they did right-side-up; however, subjects had a much more difficult time recognizing upside-down faces than they did with right-side-up faces. Yin hypothesized that images can be processed either holistically (all at once), or feature-by-feature. Furthermore, he suggested that inversion of an image prevents holistic processing, but does not interfere with feature processing. Since Dr. Yin’s subjects showed a compromised ability to recognize inverted faces but not other inverted objects, he argued that faces must be processed in a holistic way, while other objects are processed feature-by-feature.
Yin’s theory of holistic processing became quite controversial, and critics were quick to point out that his studies did not provide any direct evidence of two distinct systems for processing faces and objects. Moreover, skeptics argued that if the brain did process faces in a distinct way, the exact method in which this occurred remained unclear.
Primate Studies
Studies with primates have also suggested that processing faces involves distinct neural mechanisms. For example, measuring the activity of individual neurons in the cortex of monkeys shows that certain patches of neurons in the superior temporal sulcus are activated most strongly when the monkeys observed pictures of faces. On the other hand, non-face stimuli provoked a significant decrease in neuronal activity.
There is also evidence of specialization within these “face cell regions”. Some neurons within the superior temporal sulcus were particularly sensitive to the orientation of the face presented. Certain cells responded strongest to front-views of faces, while others showed more activity when presented with face profiles. Other subsets of cells showed great sensitivity to the direction of the eye’s gaze.
These studies strongly suggest that at least some primate brains have specific neural machinery for processing faces.
Using Technology to Uncover the Neural Mechanisms of Human Face Perception
Technologies such as functional magnetic resonance imaging (fMRI) and electrophysiological equipment can be used to safely investigate face processing in the human brain. Studies in humans using such technologies also report neural activity uniquely related to viewing faces. These studies suggest that the right hemisphere of the brain may be more specialized to process faces than other objects in the visual field.
In 1997, Nancy Kanwisher and colleagues at Massachusetts Institute of Technology used fMRI to record the brain activity of subjects who were shown a series of faces and common objects. Their research showed that the fusiform gyrus became significantly more active when the subjects were presented with faces than when they were looking at other objects. In 1999, Kanwisher conducted a different fMRI study that looked at brain activity while subjects viewed pictures of human faces and pictures of animals. Kanwisher reported that again the fusiform gyrus became significantly more active when subjects looked at human faces than when they looked at animals. Results from these studies suggest that the fusiform gyrus is a region of the brain specialized for processing faces.
In a 1999 study conducted by Isabel Gauthier and colleagues, a different interpretation of Kanwisher’s results came to light. Gauthier argued that the role of the fusiform gyrus may be to distinguish a specific type of object from any general class of objects, not just faces. Her team used fMRI to record the brain activity of people who were shown a picture of a bird and were asked identify the type of bird. Gauthier’s data showed that the fusiform area is active during this task, a result that differed from previous studies in which the fusiform gyrus was not active when subjects viewed animals. Gauthier’s interpretation of this data is that the fusiform gyrus can distinguish a particular face within the general category of faces, in much the same way that this gyrus can distinguish a particular type of bird within the general category of birds. This interpretation implies that the brain does not have a special mechanisms for recognizing faces, and given the conflicting evidence provided in these two brain imaging studies, it is still unclear whether or not the brain handles face recognition in a unique way.
What Does Prosopagnosia Tell Us?
Clinical studies provide additional information about how the brain processes faces. Since the first documented report in 1962, clinical studies of prosopagnosic patients have contributed significantly to the study of face perception. Prosopagnosics can recognize a face as a face, however, they are not able to link the face with anything they once knew about the person such as their name, age, occupation or relationship.
Often, prosopagnosia is accompanied by impaired ability to recognize other objects such as animals and abstract signs. This fact leads some scientists to argue that prosopagnosia is evidence against a unique brain mechanism for face recognition. But scientists who do believe that the brain processes faces in a special way point out that there have been a few cases of prosopagnosia in which the symptoms were strictly limited to faces. In 1986, Dr. De Renzi reported that one of his patients, a 72-year old lawyer, could discriminate between different kinds of coins, cats and a host of other objects, but displayed a severe inability to recognize familiar faces. This case study proves that the ability to recognize faces can be functionally separated from the ability to recognize any other object, an observation which strongly suggests that face recognition is at least anatomically distinct from other types of object processing.
The Devil is in the Details
So what is it that causes humans to attach special significance to a human face? Our fascination begins at birth, and as adults we remember faces better than any other object. Facial gestures project emotion and are important to how we communicate with each other. But how does the brain recognize and interpret this expressive structure? Time, clever experimentation, and technology will provide new facts, and new facts will continue to provide clues to the mechanisms behind this intriguing neural process.
Interactive Experiment
Test your face perception skills with two interactive experiments; one shows the inversion effect, and the other demonstrates how edges, features and surface area affect the process of human face recognition.
