23 Mar 2015
Face perception is suggested to be one of the human's most developed visual skills. It has been found by Frank, Vul and Johnson (2009) that "infants as young as newborns prefer faces and face-like stimuli over any other distractors". Our faces are complicated system that carry broadcasting source where it could provide the others with information about our thoughts, our interests and feelings, also our characteristics like age, race, gender, physical health, emotional state, and focus of attention, giving observers a clue about people's mental states. In this essay we are going to discuss the human neural systems that mediate face perception and try to show how cognitively distinct aspects of face perception are mediated by distinct neural representations. We will prove mainly by functional brain imaging studies, that face perception is mediated by a distributed neural system in the human brain, which is made up of multiple bilateral regions, and hence to investigate whether we process faces in some kind of different way from other objects, making faces special for human to perceive.
The face perception system works by stimulating different parts of distributed cortical network in the human brain, that it is "mediated by a distributed neural system in humans that is comprised of multiple, bilateral regions" according to Haxby, Hoffman & Gobbini (2000). In regard to their proposed model of identity recognition, face perception is separated and based on 2 different aspects which is the "representation of invariant and changeable aspects". The representation of invariant aspects of faces indicates the recognition of individuals by facial structures, whereas the representation of changeable aspects of faces, such as eye gaze, emotion, and lip movement, indicates the perception of information that facilitates social communication. The face perception system must included both the invariant aspects of a face which represents identity, as well as the changeable aspects of a face that involved in social communication. The representation of identity must be relatively independent of the representation of the changeable aspects of a face, otherwise a change in expression or a speech-related movement of the mouth could be misinterpreted as a change of identity.
The network includes visual (core) regions, and the limbic and prefrontal (extended) regions, which are responsible in processing invariant facial features and changeable aspects of faces respectively. The core of the human neural system for face perception consists of three bilateral regions in occipitotemporal visual extrastriate cortex that mediates the visual analysis of faces (Haxby, 1999). These regions are in the inferior occipital gyri, the lateral fusiform gyrus, and the superior temporal sulcus. The representation of invariant aspects is mediated more by the face-responsive region in the fusiform gyrus, whereas the the face-responsive region in the superior temporal sulcus plays important roles in the perception of expressions that are detected the face.
Hoffman and Haxby (2000) had conducted a study to support the above theory by finding that "the perception of face identity was mediated more by regions in the inferior occipital and fusiform gyri, and perception of eye gaze was mediated more by regions in the superior temporal sulci". A study by Kanwisher, McDermott and Chun (1997) using functional magnetic imaging (fMRI), have found the area in the fusiform gyrus was significantly more active in 12 out of 15 subjects tested, when the observants viewed faces than when they viewed various common objects stimuli.
On the other hand, the extended system consists of regions from neural systems for other cognitive functions that can be recruited to combined working with the core system to detect meaning from faces.
However, it is currently lacking of reliable evidences to prove whether the core and the extended systems actually comprise a cortical network and the way of how these regions are functionally connected is still unknown.
McNeil and Warrington (1993) produced the first study to investigate the existence of a specialized neural system for face perception in the human they done it by observing patients with prosopagnosia syndrome. These patients suffer from focal brain damage who had a selectively impaired ability to recognize familiar faces, but are relatively still able to recognize other objects. Prosopagnosia is associated with ventral temporal lesions that are usually bilateral. Furthermore, many reports have described patients with damage in the occipitotemporal region of the right hemisphere, have selectively lost the ability to recognize faces (De Renzi, 1997).
More evidence of a specialized neural system for face perception came from studies of non-human primates. Perrett et al (1982) studied monkeys by using single unit recording method, and it has discovered neurons in the superior temporal sulcus and the inferior temporal cortex that respond selectively to. These results suggested that similar type of face-selective neurons might exist in homologous regions in the human brain.
With the development of technologies over the years, functional brain imaging method became more and more popular, the brain regions that are involved in face perception could be studied non-invasively in the intact human brain more precisely than is possible in patients with naturally occurring brain lesions. Further evidence for specialization in the fusiform gyrus for human face perception comes from both neuroimaging and neuropsychological studies. The functional brain imaging studies have identified the face-responsive region in the lateral fusiform gyrus have used either passive viewing tasks or tasks that requires participants focusing on invariant aspects of the facial configuration. These tasks have included simultaneous and delayed matching of identical or different pictures of the same individual, for example studies by Hoffman and Haxby (2000) and Clark et al (1996), and also by identifying the gender or profession (which requires recognition of identity) of pictured individuals (Sergent et al, 1992).
The perception of faces has been frequently found to stimulate a region in the lateral fusiform gyrus that is usually bilateral, but more consistently found on the right (Sergent et al, 1992). In this region, the activity in response to faces is stronger than when viewing nonsense (control) stimuli or non-face objects. One of the investigators, Kanwisher (1997) has proposed that this region is a module that is specialized for face perception and it has been termed with the name 'fusiform face area'. The location of this region has been supported by a lot of studies.
However, there are also studies which do not support the above theories, and some of the investigators have also proposed that there are other factors that could affect the human ability of recognizing faces.
Result of research from Hoffman and Haxby (2000) shows that attending to a changeable aspect of the face, specifically eye gaze direction, the level of response to faces in the fusiform face-responsive region could be reduced. This suggests that the face responsive region may involved more in the perception of invariant aspects of faces, rather than playing a central role of perceiving faces alone. Other functional imaging studies suggested that face-responsive regions have been found consistently located in the lateral inferior occipital gyri and the posterior superior temporal sulcus (Haxby, 1999). The suggestion that more than one of regions participating together in face perception is approved by studies of evoked potentials recorded with electrodes placed on the cortical surfaces, responsiveness of N200 and related ERPs to the perceptual features of faces and other images was measured (Puce, MaCarthy and Allison, 1999). Different recordings were found at some of the electrode sites, including an "additional face-responsive region in right anterior ventral temporal cortex, which may correspond to sites of activation in studies of the retrieval of biographical information associated with faces" (Leveroni, 2000).
It has also been suggested by Diamond and Carey (1986) that the effect of expertise demonstrates that faces are not unique because the human's vulnerability to inversion. According to the investigators, perception of human faces is strongly influenced by their orientation. Although we might be still able to recognize inverted photographs of faces, it becomes harder or even impossible to categorize as the revealing of characteristics would became less obvious. It is a well-known phenomenon that participants failed to recognize familiar individual when the photographs are placed up-side down. ( e.g., Arnheim, 1954; Attneave, 1967; Brooks & Goldstein, 1963; Kohler, 1940; Rock, 1974; Yarmey, 1971). Diamond and Carey explored the question "whether faces are unique in the sense of being represented in memory in terms of distinguishing features that are especially sensitive to inversion". After studies were conducted, they clearly provided it with an answer no, faces are not special for recognition. In regard to their findings, they have concluded that "experts represent items in memory in terms of distinguishing features of a different kind than do novices".
To conclude, by looking at different fMRI studies, the neural response indiced by looking at faces was measured and activation was found in the face responsive regions, which includes the lateral inferior occipital gyri, fusiform gyrus, superior temporal sulcus. All stimuli that are used in different studies have produced stronger activation in the right hemisphere within all these regions. Furthermore, the response to famous and emotional faces was stronger than the response to unfamiliar faces.
Besides, we have also found that viewing faces stimulates multiple, bilateral regions in the distributed cortical network for face perception. We found activation in the "core" system, the lateral inferior occipital gyri and the fusiform gyrus are the extrastriate regions that is mainly responsible for face detection and identification, whereas the superior temporal sulcus is where information about social communication is processed. Interestingly, a fMRI study of monkeys has also proved the existence of similar face-selective patches in superior temporal sulcus and the inferior temporal cortex, suggesting that the core system, possible because of the biological significance of faces, is evolutionary preserved in primates . We also found activation in the "extended" system, is where information about facial expression is processed . Although viewing faces evoked bilateral activation in all face-responsive regions, the right hemisphere relatively showed more obvious responses. This hemispheric differnce can be made obvious by stronger and more significant clusters of activation, and larger numbers of subjects will showthe effect.
However, on the other hand, some of the opposing ideas are that the fusiform face-responsive region may not play a central role in all aspects of face perception but, may be involved more in the perception of invariant aspects of faces instead. Other face-responsive regions have also been found in functional imaging studies. Moreover, Diamond and Carey's expertise effect have suggested that we might recognize faces only by distinguishing the features with our memory instead of really perceiving them. When pictures of faces are inversed, we are no longer able to distinguish faces, as the characteristics are different from what we remember. It has also a lack of evidence to prove how the extended system works in order to act together with the core system.
By summing up all the points, we can say that faces are unique and special for human to perceive, we process faces in some kind of different way from other objects, but only to a certain extent due to all the unknowns and drawbacks.
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