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The distinctiveness of a face is a crucial factor for its ability to be discriminated, memorized, and identified correctly. In this thesis, we provided a contribution to face distinctiveness by examining the impact of socially relevant factors such as attractiveness, group and emotional expression on distinctiveness. We show that attractive faces are more similar to each other than unattractive faces, using female Caucasian faces and male faces of a Caucasian and heterogeneous Non-Caucasian group. We also show, using 3D face generated faces of photo-realistic quality, that attractive faces were closer to the prototype of only their specific group, and that hence typicality of attractive faces was group specific. Lastly, we show that assigning an emotional expression to a face that is evaluatively incongruent with its race makes it more psychologically distinctive, as revealed in perceptual discrimination and memory tasks.

The distinctiveness of a face is a crucial factor for its ability to be discriminated, memorized, and identified correctly. In this thesis, we provided a contribution to face distinctiveness by examining the impact of socially relevant factors such as attractiveness, group and emotional expression on distinctiveness. We show that attractive faces are more similar to each other than unattractive faces, using female Caucasian faces and male faces of a Caucasian and heterogeneous Non-Caucasian group. We also show, using 3D face generated faces of photo-realistic quality, that attractive faces were closer to the prototype of only their specific group, and that hence typicality of attractive faces was group specific. Lastly, we show that assigning an emotional expression to a face that is evaluatively incongruent with its race makes it more psychologically distinctive, as revealed in perceptual discrimination and memory tasks.

Le propos de ce travail de thèse est d’examiner le développement de la formation de catégories de visages, par l’étude des préférences visuelles des nourrissons dans la première année de vie. Nous avons cherché à préciser les mécanismes de formation des préférences visuelles en les intégrant dans le cadre théorique développé par Valentine (1991), le face-space. Nous avons proposé de lier ces préférences à la manière dont l’expérience perceptive des nourrissons avec différentes catégories de visages va structurer l’espace de représentation des visages. De manière générale, nous avons postulé que les nourrissons présenteront des préférences pour les visages proches de la tendance centrale (i.e., prototype) du face-space. Nous avons mis en évidence une tendance des nourrissons de 0 à 6 mois à présenter un biais pour des visages d’adultes par rapport à des visages de nourrissons (Etudes 1 et 2), les premiers correspondant à une catégorie de visages prépondérante de l’environnement des nourrissons, là où les seconds correspondent à une catégorie de visages peu rencontrée. Ce biais pour la familiarité s’est avéré disparaitre à 9 et 12 mois (Etude 3). Ces préférences liées à la familiarité pourraient être liées à une forme de fausse reconnaissance du visage des proches des nourrissons, issue de la surreprésentation de ces visages dans le quotidien des nourrissons. Ce pattern de préférences n’a en revanche pas été retrouvé lorsque des nourrissons de 3 à 12 mois ont été confrontés à des visages d’enfants ou de nourrissons (Etudes 4 et 5), les résultats montrant plutôt une préférence pour les visages les moins familiers, relativement à l’expérience des nourrissons. Nous avons ensuite étudié les capacités de catégorisation de nourrissons de 9 et 12 mois pour des visages de différentes catégories d’âges, i.e., adulte, enfant, nourrisson (Etude 6). Les nourrissons de 12 mois ont formé des catégories discrètes des visages d’adulte et de nourrissons d’une part, et d’enfants et de nourrissons d’autre part. Les nourrissons de 9 mois en revanche ont montré un pattern plus asymétrique en ce qu’ils ont formé une représentation des visages d’enfants excluant un nouveau visage de nourrisson, et une représentation des visages de nourrissons incluant un nouveau visage d’enfant. Les nourrissons ayant tous une expérience de la crèche, donc des visages de nourrissons, cette asymétrie pourrait être liée à une influence de la connaissance de cette catégorie de visage. Dans une dernière étude (Etude 7) nous avons cherché à montrer plus directement le lien entre préférences visuelles et proximité par rapport au prototype, chez des nourrissons humains de 12 mois et des nourrissons macaques de 3 mois (Macaca mulatta). La mise en évidence de préférences liées à la distance par rapport au prototype chez ces deux populations suggère la présence d’un mécanisme commun aux deux espèces conduisant à la formation de préférences visuelles pour les visages. / The purpose of this work is to examine of the development of face category formation using infants’ visual preferences. We investigated the mechanisms leading to differential face preferences by integrating them in the theoretical framework developed by Valentine (1991), the face-space. We proposed that the way perceptual experience shape the structure of the face-space is a determinant of infants’ face preferences. We postulated that faces close to the central tendency of the face-space (i.e., prototype) will be preferred. We first reported a bias to look more toward adult faces than infant faces from birth to 6 month of age (Studies 1 and 2). Adult faces correspond to a frequently encountered category while infant faces represent a less frequently encountered category. We also showed a downturn of this familiarity bias as infants grow older (Study 3). The preferences showed by younger infants might be linked to a form of false recognition of the caregivers’ faces, due to the massive exposure to these faces. This pattern of preferences was not found in 3-to 12-month-olds presented with child and infant faces (Studies 4 and 5). Conversely, infants showed a tendency to prefer the less familiar faces, depending on their perceptual experience. We then studied 9- and 12-month-olds’ abilities to form categories of faces differing by age, i.e., adult, child, and infant faces, (Study 6). Twelve-month-olds formed discrete categories of adult and infant faces in one hand, and of child and infants faces on the other hand. Nine month-olds showed an asymmetric pattern of behavior, forming categories of child faces that exclude a new infant face, and categories of infant faces that include a new child face. All these infants being exposed to infant faces via nursery, the asymmetry might stem from the influence of the knowledge of this category of faces. In the last study (Study 7), we tried to provide more direct evidences of the link between face preferences and the distance from the prototype in two different populations: 12-month-old human infants, and 3-month-old macaque infants (Macaca mulatta). Preferences for faces close to the prototype in both species suggest a common mechanism leading to face preferences.

The simplicity principle posits that we interpret sense data as the simplest consistent distal cause, or that our high level perceptual representations of stimuli are optimized for simplicity. The traditional paradigm used to test this principle is coding theory, where alternate representations of stimuli are constructed, simplicity is measured as shortness of representation length, and behavioural experiments attempt to show that the shortest representations correspond best to perception. In this study we apply coding theory to marker-based human walking motion. We compare two representation schemes. The first is based on marker coordinates in a body-centred Cartesian coordinate system. The second is based on a model of 15 rigid body segments with Euler angles and a Cartesian translation for each. Both of our schemes are principal component (PC)-based implementations of a norm-based multidimensional object space – a type of model for high level perceptual schemes that has received attention in the literature over the past two decades. Representation length is quantified as number of retained PC’s, with error increasing with discarded PC’s. We generalize simplicity to efficiency measured as error across all possible lengths, where more efficient schemes admit less error across lengths. We find that the Cartesian coordinates-based scheme is more efficient than the Euler angles and translations-based scheme across a database of 100 walkers. In order to link this finding to perception we turn to the caricature effect that subjects can identify caricatures of familiar stimuli more accurately than veridicals. Our design was to compare walker caricatures generated in our two schemes in the hope of finding that one gives caricatures that benefit identification more than the other, from which we would conclude the former to be a better model of the true perceptual scheme. However, we find that analogous caricatures between the two schemes are only distinguishable at caricature levels so extreme that identification performance breaks down, so our design became infeasible and no conclusion for a simplicity principle in walker perception is reached. We also measure a curve of increasing then decreasing identification performance with caricature level and an optimal level at approximately double the distinctiveness of a typical walker. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2010-10-29 19:16:39.943