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The central aim of this PhD thesis was to study typical development of visual mechanisms underlying face processing. Despite the importance of faces for interaction with the environment, little is known about the factors influencing the development of face processing. Perception of a face, and the use of information within a face for social interaction requires multiple processing steps. Among these are perception of all the elements of a face, i.e. both its details and global outlines, binding of elements together into an object, and categorizing the object as a face. Furthermore, a crucial step is to consciously perceive the face. For interaction with the environment, one also needs to derive specific information from the face, such as age, gender, and emotion. Understanding of the ongoing development of face processing requires a clear view of developmental changes in these steps. The research in this thesis aimed to study typical development of two basic visual mechanisms, and their relation to face processing. Interest was in processing of details and global information, related to emotional face processing. The second process of interest was visual segmentation (i.e. integration of elements into an object and figure-background segregation) and its relation to face categorisation. Using EEG and behavioural measurements and multiple types of visual stimulation, the findings provide important insights into the development of factors underlying face processing. Both global/detail processing and segmentation continue to develop until early puberty. The role of global and detail information in emotional face processing changes with age: whereas previous research showed that emotional face processing is driven by global information in adults and details in school-aged children, we show that infants use both global and detailed information. We explored the role of segmentation in face processing in adults, as this is not fully established and needs to be clear before theoretical accounts can be generated on its development. Focus was on the temporal relation between segmentation and face categorization. Results showed that in cases when categorization is difficult, faces are first segmented and then categorized in the brain. A final goal of the thesis was to study the role of the inhibitory neurotransmitter GABA in visual segmentation in children, because neural mechanisms underlying visual perception function, among others, via GABA. General anaesthesia was used as a possible GABAergic modulator. We demonstrated that anaesthesia affected visual segmentation for a short time, but that effects had deteriorated at one day after surgery. Overall, results presented in the current thesis have important implications for future developmental research that often uses visual stimulation as input to study behavioural and social development. Furthermore, findings on the typical development of visual processing provide an important background for research into the atypical development as is for instance observed in children with Autism Spectrum Disorder. |
