The neural representation of facial expression

Faces provide information critical for effective social interactions. A face can be used to determine who someone is, where they are looking and how they are feeling. How these different aspects of a face are processed has proved a popular topic of research over the last 25 years. However, much of this research has focused on the perception of facial identity and as a result less is known about how facial expression is represented in the brain. For this reason, the primary aim of this thesis was to explore the neural representation of facial expression.
First, this thesis investigated which regions of the brain are sensitive to expression and how these regions represent facial expression. Two regions of the brain, the posterior superior temporal sulcus (pSTS) and the amygdala, were more sensitive to changes in facial expression than identity. There was, however, a dissociation between how these regions represented information about facial expression. The pSTS was sensitive to any change in facial expression, consistent with a continuous representation of expression. In comparison, the amygdala was only sensitive to changes in expression that resulted in a change in the emotion category. This reflects a more categorical response in which expressions are assigned into discrete categories of emotion.
Next, the representation of expression was further explored by asking what information from a face is used in the perception of expression. Photographic negation was used to disrupt the surface-based facial cues (i.e. pattern of light and dark across the face) while preserving the shape-based information carried by the features of the face. This manipulation had a minimal effect on judgements of expression, highlighting the important role of the shape-based information in judgements of expression. Furthermore, combining the photo negation technique with fMRI demonstrated that the representation of faces in the pSTS was predominately based on feature shape information.
Finally, the influence of facial identity on the neural representation of facial expression was measured. The pSTS, but not the amygdala, was most responsive to changes in facial expression when the identity of the face remained the same. It was found that this sensitivity to facial identity in the pSTS was a result of interactions with regions thought to be involved in the processing of facial identity. In this way identity information can be used to process expression in a socially meaningful way.