Facial Representation and Processing in the Brain

facial recog pic

One of the more interesting aspects of the human cognitive system is ability for facial recognition, given the social nature of the human race. The process of facial recognition and perception is distinct from that of other visual stimuli. When facial stimuli (visual or other) are processed within the brain, a very specific portion of the temporal lobe known as the fusiform face area (FFA) is activated. This activation occurs predominantly in the right hemisphere although activation in the left hemisphere also occurs. One unique aspect of facial perception is its production of the N170 response. This is the large negative response observed through electrophysiological methods that occurs 170ms after a facial stimulus is perceived. Although similar negative responses are observed with other visual stimuli, the negative response produced by facial stimuli is much greater in magnitude. The only exception to this anomaly is when visual stimuli is perceived by an expert, an expert being a person who can readily distinguish a stimuli based on specific details, in which case the stimulus generates a N170 response [1].

Traditionally, the FFA has been conceptualized as a specialized section of the brain that only processes facial information. This system has been challenged by some scientists who believe the FFA to be more broadly involved in the perception of any highly familiar and recognizable stimulus [1]. They claim a familiar stimulus would be classified as an image, which an ‘expert’could distinguish from other comparable images. A study by Rossin et al. compared the event-related potentials of participants trained in recognizing novel objects (Greebles) to that of recognizing faces (both upright and inverted for both cases) [2]. Their study utilized the N170 inversion effect, which is a trend observed in facial processing where the N170 is delayed when a facial stimulus is inverted; this trend is not observed in other types of object recognition [2]. The study found that before the participants were trained in recognizing Greebles, they did not exhibit the inversion effect.  However, upon becoming ‘experts’ the inversion effect was observed. Interestingly, noticeable discrepancy between the Greeble and facial processing was discovered. The latter predominantly occurred in the right hemisphere whereas the former primarily occurred in the left hemisphere [2]. This depicts the specificity of the FFA, and its distinction from recognition of any other visual stimuli.

Despite the hemispheric discrepancies, this study demonstrated that visual expertise is somewhat comparable in terms of processes to facial. This could signify that the FFA is not in fact specialized for facial processing but rather, for processing familiar visual stimuli recognizable an ‘expert’. It implies this region is used in facial recognition because the perception of faces is one of the key elements of socializing and it is something we become ‘experts’ in at a very early age.

Other studies have shown that although the FFA may play a major role in facial perception, the pre-frontal cortex is also involved. Scalaidhe et at. studied the neuronal activity of neurons in the prefrontal cortex of monkeys while they were presented with visual stimuli [3]. Neurons in this region only responded to faces despite being shown a wide variety of images. To determine whether these responses were a result of viewing an image and not the emotion these images elicited, the neurons were also tested when presented with non-facial, emotionally significant images. No increased neuronal activity in this region in the monkeys in this portion of the experiment. The neurons were then tested to see if they responded to stimuli that were either whole or partial faces, which showed that the neurons only responded to stimuli of the face as a whole. Similar experiments were also conducted with scrambled versus non-scrambled faces. These also showed the face-recognizing neurons only firing when the face was anatomically correct [3]. Researchers also conducted tests on the response of these neurons based on foveal stimulation. These revealed that the face-receptive neurons in the pre-frontal cortex had a higher responsiveness to intense foveal stimulation. Ten of the neurons were then selected and tested with manipulated images of a face such color change or inversion. In this case, the neurons proved to comply with the inversion effect whereas color changes had no influence upon response magnitude [3].

Upon mapping the face-processing neurons discovered in this experiment it was observed that 95% of them were located in the regions of the prefrontal cortex that are connected to the visual areas of the temporal lobe [3]. The link between the prefrontal cortex and neurons that respond solely to facial stimuli may be linked to social evolution. The prefrontal cortex is generally involved with planning and executing behavior, and perhaps because of the extreme importance of social interactions, the processing of facial stimuli requires its own special section of the prefrontal cortex to aid in interacting with others. Perhaps through evolution facial processing has become a more integrative process involving many parts of the brain so as to aid in necessary social interactions.

The ability to perceive and respond to facial stimuli is an integral part of communicating in a society. This may explain why facial perception occurs in such specific, specialized regions of the cortex. Given the unique ability of our cortex to respond to an integrated image of a face as opposed to its elements, which is generally how other visual stimuli are processed, the unique abilities of facial recognition allow us to process faces at a much faster rate. This processing may also allow for easier detection of subtle differences between the parts of a face so as to more easily perceive the emotions.

The true function of the FFA is still under debate, with the two main theories postulating that it is either solely involved with facial perception or that it is involved with perception of objects by an ‘expert’. Although it was shown in Rossin et al. that the inverse effect commonly seen in only in facial perception can also be present with the perception of other objects, it does not definitively prove that any object can instill that type of reaction a face does when processed by an ‘expert’ [2]. Interestingly, the perception of ‘expert’ objects in comparison to faces has been documented to occur in opposing hemispheres. Without further research on the reason for this discrepancy it is difficult to know with certainty what the underlying correlation between expertly observed object perception and facial perception.

Electrophysiological research has also proven that facial perception does not only occur in the FFA.  In the study conducted by Scalaidhe et al., it was shown that the pre-frontal cortex also contains neurons involved with facial perception [3]. The majority of these neurons also proved to be connected to other facial perceiving neurons in the temporal lobe. This may indicate that facial perception is such an intricate process that it requires the use of several cortical regions. This again may be linked to the evolutionary importance of interacting socially with the environment, a feat in which understanding and properly reacting to facial information is crucial.

References

[1] Gazzaniga, M. et al. (2009). Object Recognition. Cognitive Neuroscience the Biology of the Mind. 239-242.

[2] Rossion, B. et al. (2002, May 3). Expertise Training With Novel Objects Leads to Left-Laterized Facelike Electrophysiological Responses. Psychological Science,13,  250-257.

[3] Scalaidhe, S. et.al. (1997, November 7). Areal Segregation of Face-Processing Neurons in the Prefrontal Cortex. Science, 278, 1135-1138.

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[1] Retrieved April 10, 2014, from: http://www.neuralconnections.net/2011/10/face-processing-connecting-nodes-across.html

Maria Cimporescu recently graduated with a B.A. in Psychology and is currently conducting research at The George Washington University. Follow The Triple Helix Online on Twitter and join us on Facebook.