THE EFFECT OF IMAGE INVERSION ON THE PERCEPTION OF FACIAL EXPRESSION

The Thatcher illusion provides a compelling example of the cost of face inversion. When the eyes and the mouth are turned upside-down relative to the rest of the face - a transform now known in the research literature as 'thatcherization' - the facial expression appears grotesque. This distortion of the face is immediately perceived when the face is upright. However, when the image is inverted the grotesque appearance is no longer visible. The aim of this thesis was to explore the behavioural and neural basis of this compelling illusion. This thesis provides a significant contribution to our understanding of the Thatcher Illusion using a combination of neuroimaging and behavioural results. The key findings of this thesis are that the neural basis of the Thatcher illusion is founded on the orientation-sensitivity of face-selective regions which are involved in the processing of facial expression. Behavioural findings suggest that the perception of the Thatcher illusion is still evident in the absence of configural information. Our findings demonstrated that a key component of the Thatcher illusion is to be found in orientation-specific encoding of the expressive features (eyes and mouth) of the face. This challenges previous interpretations of the Thatcher illusion that are based on a disruption of configural processing. Further results suggest that the effect of inversion found in the Thatcher illusion is not specific to grotesque expressions, but reflects a more general orientation-specific encoding of expressive features. Finally, the selectivity of the Thatcher illusion to the processing of expression is shown by the lack of effect of thatcherization on the processing of facial identity. These results provide further support for the idea that different processes underlie the perception of identity and expression.