[Truncated abstract] Recent studies have provided evidence that face-coding mechanisms reference a norm or average face (Leopold, O`Toole, Vetter & Blanz, 2001; Rhodes & Jeffery, 2006). The central aim of this thesis was to establish whether distinct norms, and dissociable neural mechanisms code faces of different race and sex categories. Chapter 1 provides a brief introduction to norm based coding of faces, and reviews evidence for the existence of distinct norms for different races and sexes. Chapter 1 then introduces adaptation as a tool for investigating these ideas. Chapter 2 presents two adaptation studies that examined how faces of different races are coded. The aim of these studies was to determine whether dissociable neural mechanisms (or distinct face norms) code faces of different races. Chinese and Caucasian participants rated the normality of Caucasian and Chinese test faces, before and after adaptation to distorted faces of one race (e.g., 'contracted' Chinese faces; Experiment 1) or distorted faces of both races (e.g., 'contracted' Chinese faces and 'expanded' Caucasian faces; Experiment 2). Following adaptation to faces of one race, there were changes in perceived normality for faces of both races (i.e., perceptual aftereffects), indicating that common neural mechanisms code Chinese and Caucasian faces. However, aftereffects were significantly smaller in faces of the unadapted race suggesting some sensitivity to the race of faces. This sensitivity was also evident in Experiment 2. ... Some dissociability was also found in the coding of faces of different iv sexes. In Experiments 2 and 3, participants adapted to oppositely distorted faces of both sexes. Weak sex-selective aftereffects were found. Taken together, the findings suggest that male and female faces are coded by dissociable but not completely distinct neural populations. Chapter 4 examined whether the aftereffects reported for faces of different races or sexes reflected the adaptation of high-level neural mechanisms tuned to the social category information in faces, or earlier coding mechanisms tuned to simple physical differences between face groups. Chinese and Caucasian participants adapted to oppositely distorted face sets that were the same distance apart on a morph continua. The face sets were either from different race categories (e.g., contracted Chinese faces and expanded Caucasian faces), or from the same race category, (e.g., contracted Chinese faces and expanded caricatured Chinese faces). Larger opposite aftereffects were found when face sets were from different race categories, than when they were from the same race category suggesting that oppositely adapted neural mechanisms are tuned to social category differences rather than simple physical differences in faces. Together, these studies shed new light on how we code faces from different face categories. Specifically, the findings indicate that faces of different races and sexes are coded by both common and race- or sex-selective neural mechanisms. In addition, the findings are consistent with the possibility that race- and sex-selective norms and dimensions are used to code faces in face space. The implications of these findings and possible avenues for future research are discussed.
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