Facial perception is an individual's understanding and interpretation of the face. Here, perception implies the presence of consciousness and hence excludes automated facial recognition systems. Although facial recognition is found in other species, this article focuses on facial perception in humans.
The perception of facial features is an important part of social cognition. Information gathered from the face helps people understand each other's identity, what they are thinking and feeling, anticipate their actions, recognize their emotions, build connections, and communicate through body language. Developing facial recognition is a necessary building block for complex societal constructs. Being able to perceive identity, mood, age, sex, and race lets people mold the way we interact with one another, and understand our immediate surroundings.
Though facial perception is mainly considered to stem from visual intake, studies have shown that even people born blind can learn face perception without vision. Studies have supported the notion of a specialized mechanism for perceiving faces.
Overview
Theories about the processes involved in adult face perception have largely come from two sources; research on normal adult face perception and the study of impairments in face perception that are caused by brain injury or neurological illness.
Bruce & Young model
One of the most widely accepted theories of face perception argues that understanding faces involves several stages: from basic perceptual manipulations on the sensory information to derive details about the person (such as age, gender or attractiveness), to being able to recall meaningful details such as their name and any relevant past experiences of the individual.
This model, developed by Vicki Bruce and Andrew Young in 1986, argues that face perception involves independent sub-processes working in unison.
A "view centered description" is derived from the perceptual input. Simple physical aspects of the face are used to work out age, gender or basic facial expressions. Most analysis at this stage is on feature-by-feature basis. This initial information is used to create a structural model of the face, which allows it to be compared to other faces in memory. This explains why the same person from a novel angle can still be recognized (see Thatcher effect). The structurally encoded representation is transferred to theoretical "face recognition units" that are used with "personal identity nodes" to identify a person through information from semantic memory. Interestingly, the ability to produce someone's name when presented with their face has been shown to be selectively damaged in some cases of brain injury, suggesting that naming may be a separate process from being able to produce other information about a person.
Traumatic brain injury and neurological illness
Following brain damage, faces can appear severely distorted. A wide variety of distortions can occur – features can droop, enlarge, become discolored, or the entire face can appear to shift relative to the head. This condition is known as prosopometamorphopsia (PMO). In half of the reported cases, distortions are restricted to either the left or the right side of the face, and this form of PMO is called hemi-prosopometamorphopsia (hemi-PMO). Hemi-PMO often results from lesions to the splenium, which connects the right and left hemisphere. In the other half of reported cases, features on both sides of the face appear distorted.
Perceiving facial expressions can involve many areas of the brain, and damaging certain parts of the brain can cause specific impairments in one's ability to perceive a face. As stated earlier, research on the impairments caused by brain injury or neurological illness has helped develop our understanding of cognitive processes. The study of prosopagnosia (an impairment in recognizing faces that is usually caused by brain injury) has been particularly helpful in understanding how normal face perception might work. Individuals with prosopagnosia may differ in their abilities to understand faces, and it has been the investigation of these differences which has suggested that several stage theories might be correct.
Brain imaging studies typically show a great deal of activity in an area of the temporal lobe known as the fusiform gyrus, an area also known to cause prosopagnosia when damaged (particularly when damage occurs on both sides). This evidence has led to a particular interest in this area and it is sometimes referred to as the fusiform face area (FFA) for that reason.