Neurophysiology of Face Perception and Social Information Processing
Face perception of brain involves a series of neural processes that enable and facilitate the perception, recognition, and interpretation of the face, particularly the human face. While face perception allows humans to readily recognize faces from other non-face objects and to distinguish individual identities based on face, it also facilitates processing social information by enabling us to read emotions and make impression from faces. This wiki page summarizes brain areas involved in general face processing and neural basis of face perception that entails social information processing.
Neuroanatomy and physiology of face perception
Face perception involves diverse and extensive areas in the brain. After an initial stage of visual analysis in occipital lobe, the visual information of facial features travels through two parallel neural streams. One is a ventral pathway, which includes portions of fusiform gyrus and inferior temporal cortex, dedicated to discriminating faces from non-face objects or processing invariant aspects of faces like an individual identity. Another pathway is more dorsal sectors of the temporal lobe, including the superior temporal sulcus (STS), and they are found to process variant facial features such as facial expressions and movements. The ventral stream is linked to anterior temporal lobe and hippocampal area, which takes charge in semantic and episodic knowledge, and the dorsal stream is linked to amygdala and other limbic systems for analyzing emotions, multisensory areas for integrating facial movements and sensory inputs, and parietal lobe for directing spatial attention to facial movements. While these neural pathways work in parallel, they interact with each other as well to give feedback to preceded processes and thus to facilitate face perception process (see Figure 1) [1].
Occipital face area (OFA)
Occipital face area (OFA) is an area located in the inferior occipital gyrus. OFA is active especially during an early stage of face detection, and its major role is to recognize individual pieces of facial parts like eyes, nose, and mouth as separate constructs, rather than as a whole [3]. A study using BOLD fMRI mapping that observed the brain activation in response to the visual perception of facial parts showed that OFA is activated when a single part of face (e.g., a nose) is presented separately, and that when facial parts are presented in combination, OFA activation is greater for a combination of related facial features like two eyes than other groupings of parts[4].
A finding from a study using a TMS simulation further supports the notion that OFA plays a critical role in the recognition of individual face features. Pitcher et al.[5] administered TMS stimulation to the right OFA immediately after participants saw two faces to be compare to each other, and they showed impairment in comparing the appearance of face parts (“are those the same eyes that I just saw? ”), rather than comparing their arrangements (“are those eyes closer together than they were?”), and no impairment was found for non-face objects (e.g., houses).
Fusiform face area (FFA)
While OFA is activated in response to individual facial features, fusiform face area (FFA), located in the lateral middle fusiform gyrus, responds to holistic identities of a human face and how facial features are configured [4]]. FFA was found to show greater activation in response to a whole than a scrambled face, full front view of a face rather than a house, and a partial presentation of a human face than a human hand [6], suggesting that FFA is involved in the selective perception of human face, with preference to an intact rather than a disorganized face (see Figure 2). Also, FFA activation was correlated with within-category identification performance regarding human faces, but not with the one for non-face objects[7]. Stronger FFA activation was observed for detecting the presence of faces and distinguishing individual identities of specific faces, but not for viewing or identifying identities of non-face objects (e.g., cars seen by car experts).
Further causal link between FFA and face perception can be found from a study using electrical brain stimulation. When the FFA of a patient was stimulated by electrical charge, the patient reported a “metamorphosed” appearance of the face, but not to a non-face object, which suggests that FFA stimulation cause face-specific perceptual distortion (see related video clip)[8].
Superior temporal sulcus (fSTS)
Both OFA and FFA are involved in recognizing faces from non-face objects (see Figure 3) and processing invariant aspects of faces like facial features or individual identities of faces. Conversely, superior temporal sulcus (fSTS), separating the superior temporal gyrus from the middle temporal gyrus, is involved in perceiving and analyzing changeable aspects of faces such as eye gaze or facial movements [9]. In an fMRI study, viewing moving eyes or mouths activated fSTS area, while moving radial background activated brain regions other than fSTS[10], suggesting that fSTS is preferentially attuned to movements of facial features. A surgical removal of fSTS in rhesus monkeys resulted in impaired accuracy at perceiving frontal eye gaze[11], providing further evidence of the causal link between fSTS and facial movement perception.