| 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.
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-process
working in unison.
- 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.
- 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 that 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.
- 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 reason.
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.
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. 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,
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, rather than it being a
face-specific process.
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 cars3 and even when participants become expert at distinguishing
computer generated nonsense shapes known as greebles4. 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 areas deal with non-face objects5. 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. For example, one study tested modern car
experts with antique cars and failed to find an expertise effect.
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 the 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.
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