The role of shape and texture within the neural representation of faces.

The proposed PhD project will join a dynamic and rapidly growing body of literature concerning the unraveling of how the human brain computes and codes for visual stimuli. It has been long established that within the brain there are areas that are more sensitive for certain types of stimuli (reference) for example an area located in the lateral occipital cortex (the extrastriate body area) has been shown to respond to bodies and their constituent components but minimally for objects (Downing et al, 2001). Likewise neuroimaging studies have revealed a network of regions in the occipital and temporal lobes, which form a core system for the visual analysis of faces (Haxby, Hoffman & Gobbini, 2000; Kanwisher, McDermott & Chun, 1997). These studies have consistently found regions within this network that show stronger responses to faces than other visual stimuli: the fusiform face area (FFA), the occipital face area (OFA) and the posterior superior temporal sulcus (pSTS). These constituent areas have been found to process different aspects of faces, for example; the FFA is linked to the processing of identity and the pSTS is linked to the processing of changeable aspects of faces such as expression or viewpoint (Haxby, Hoffman & Gobbini, 2000).
Although there is evidence in support of this general dissociation, the neural code in these regions is not clear. For example, is the neural code based on high-level properties such as identity and expression or can a more basic and general neural code underpin the representation in these face regions.

A number of studies have shown that the texture (which can be thought of as abrupt changes in reflectance due to the shapes and positions of facial features and a more broader pattern of reflectance within the face) plays a critical role in the invariant representation that is used for the recognition of familiar faces (Burton, Jenkins, Hancock & White, 2005; Russell & Sinha, 2007). For example, familiar face recognition is not substantially affected if the texture is presented on a standardized shape (Burton, Jenkins, Hancock & White, 2005) or when the shape is distorted by stretching the image (Hole, George, Eaves & Rasek, 2002). In contrast, line drawings of faces, which lack any surface properties, are not usually sufficient for recognition Davies, Ellis, & Shepherd, 1978). Together, these studies suggest that texture is the dominant cue in face recognition (Burton, 2013).

In contrast judgements of expression are often thought to be based primarily on the shapes and positions of critical expressive features such as the eyebrows, eyes, nose and mouth. Evidence for the importance of shape cues in facial expression recognition comes from contrast reversal (as in a photo negative). In a contrast-reversed image, the edges that define feature shape properties remain in the same positions, despite the huge change in overall surface properties (texture). Although contrast negation is well-known to be very disruptive of facial identity recognition (Bruce & Young, 1998) it turns out that judgements based on facial expression are still possible in contrast-reversed images (Bruce & Young, 1998; Harris, Young & Andrews, 2014; White, 2001).

The current proposal will determine the role of shape and texture in human face regions. The aim is to better understand how the brain represents information about faces. The main hypothesis of this study is that the neural representation in face regions, rather than being directly linked to high-level concepts such as identity or expression, is based on a more basic underlying representation, such as shape or texture. This type of representation would provide a more general code that could be used to decode information from faces in a more flexible way.