Face perception

Face perception is the process by which the brain and mind understand and interpret the face, particularly the human face.

The face is an important site for the identification of others and conveys significant social information. Probably because of the importance of its role in social interaction, psychological processes involved in face perception are known to be present from birth, complex, involve large and widely distributed areas in the brain and can be selectively damaged to cause a specific impairment in understanding faces known as prosopagnosia.

Development of face perception
Whilst there is no question that the majority of face perception skills developed by adults are not present in babies, there is evidence of an innate tendency to pay attention to faces from birth. It is known that early perceptual experience is crucial to the development of visual perception and this orienting response undoubtedly encourages the rapid development of face specific skills such as the ability to identify friendly others and relatively complex pre-verbal communication. By two months of age face perception has developed so specific areas of the brain are known to be activated by viewing faces.1

Adult face perception
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.

One of the most widely accepted theories of face perception2 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 psychologists Vicki Bruce and Andrew Young) argues that face perception might involve several independent sub-processes working in unison.


 * 1) A 'view centred description' is derived from the perceptual input. Simple physical aspects of the face are used to work out age, gender or simple facial expressions. Most analysis at this stage is on feature-by-feature basis.
 * 2) This initial information is used to create a structural model of the face, which allows it to be compared to other faces in memory, and across views. This explains why the same person seen from a novel angle can still be recognised. This structural encoding can be seen to be specific for upright faces as demonstrated by the Thatcher effect.
 * 3) The structurally encoded representation is transferred to notional 'face recognition units' which in conjunction with 'person identity nodes' allow the person to be identified by 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.

The study of prosopagnosia (an impairment in recognising faces which 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.

Face perception is an ability which involves a great deal of the brain, however some areas have been shown to be particularly important. 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 for that reason3.

Controversies
Whilst a great deal of resources seem to be used by the mind and brain to understand the face, opinion is divided whether we genuinely develop specific skills for understanding faces, or whether face perception is just part of a general skill for making within-category discriminations, such as recognising and differentiating between similar animals or plants. Recognising a face involves a process of analogy.

Proponents of this view argue that the differences seen between faces and non-face objects in experimental studies are due to faces being particularly difficult to distinguish and observers having acquired expertise at making these discriminations. Although we often assume that faces are relatively unique, statistically they are quite similar, so a great deal of cognitive effort is needed to differentiate them. According to this view, faces are nothing more than a particularly difficult class of perceptual object which we have learned to distinguish at the expert level, much as we would learn to distinguish between other similar objects if much of our communication and survival depended on it.

Cognitive Neuroscientists Isabel Gauthier and Michael Tarr are two of the major proponents of the view that face recognition involves expert discrimination of similar objects (See the Perceptual Expertise Network). Other scientists, in particular Nancy Kanwisher and her colleagues, argue that face recognition involves processes that are face-specific and that are not recruited by expert discriminations in other object classes (See the domain specificity).

Studies by Gauthier have shown that an area of the brain known as the fusiform gyrus (sometimes called the 'fusiform face area' because it is active during face recognition) is also active when study participants are asked to discriminate between different types of birds and cars4 and even when participants become expert at distinguishing computer generated nonsense shapes known as Greebles5. This suggests that the fusiform gyrus may have a general role in the recognition of similar visual objects.

However, the activity found by Gauthier when participants viewed non-face objects was not as strong as when participants were viewing faces. Furthermore, not all of findings of this research have been successfully replicated, for example, other research groups using different study designs have found that the fusiform gyrus is specific to faces and other nearby regions deal with non-face objects6. However, these failures to replicate are often based upon different designs and often fail to use objects from the specific domain of expertises for the expert subjects. Gauthier and colleagues have argued that one study that failed to find an expertise effect used "mostly antique cars", however in fact the target stimuli in this study were jeeps, and only some of the distractors were antique cars (because of their smilarity in shape to jeeps). More to the point, failures to replicate are null effects and can occur for many different reasons. In contrast, each replication adds a great deal of weight to a particular argument. With regard to "face specific" effects in neuroimaging, there are now multiple replications with Greebles, with birds and cars7, and a new study with chess experts.

Therefore, it is still not clear in exactly which situations the fusiform gyrus becomes active, although it is certain that face recognition relies heavily on this area and damage to it can lead to severe face recognition impairment.

Artificial face perception
A great deal of effort has been put into developing software that can recognise human faces; see facial recognition system. Much of the work has been done by a branch of artificial intelligence known as computer vision which uses findings from the psychology of face perception to inform software design.