Perceptual learning occurs because observers become more sensitive to informative aspects of the stimuli. Learning the informative aspects of one stimulus set does not transfer to another stimulus set of the same class. In this dissertation, the argument will be made that if observers learn how to discover informative aspects, learning will be more generalizable. However, discovery requires that the informative aspects are not easily apparent. To this end, stimulus orientation structure can be manipulated to contain informative structure in one orientation band, and non-informative structure in the other orientation band. Such a manipulation was inspired by research on face perception: Faces are best identified when decisions are based more on the horizontal relative to the vertical facial structure. Hence, the first three chapters focus on understanding the horizontal bias during face identification, and the final two chapters introduce a novel stimulus set for which horizontal bias may be learned. Chapter 2 identifies a neural marker of horizontal bias that is correlated with face identification accuracy, suggesting that we can predict how well observers identify faces based on their neural sensitivity to horizontal relative to vertical structure. Chapter 3 shows that when face identification accuracy declines due to healthy ageing, so too do behavioural and neural horizontal bias, but Chapter 4 shows that perceptual learning can increase horizontal bias in healthy older adults. Chapter 5 uses texture stimuli and shows that observers can learn to discover informative horizontal structure embedded in uninformative vertical structure. Chapter 6 extends these findings to show that adequate practice results in learning that generalizes to novel textures for which the orientation-selective processing is relevant. The results presented inform our understanding of the neural representations associated with orientation-selective processing, and suggest that observers can learn to discover informative structure conveyed by a particular orientation band. / Thesis / Doctor of Philosophy (PhD)

Face perception is impaired in a variety of ways in older adults, but the mechanisms underlying these changes remain unclear. A central theme of this dissertation is that task performance is constrained by two factors intrinsic to the observer: sources of random variability -- internal noise -- and the efficiency with which task-relevant stimulus information is utilized. This thesis uses several behavioural, psychophysical methods to examine how age-related changes in one or both of these factors affect face processing. Chapter 2 used the classification image (CI) method to characterize the spatial sampling patterns of younger and older observers performing a face discrimination task. Compared to younger adults, older adults used information in the eye/brow region less consistently and instead relied on relatively less informative regions such as the forehead. The differences in CIs accounted for the lower absolute efficiency that was found in older observers. Chapter 3 estimated internal noise and calculation efficiency by measuring threshold-vs.-noise (TvN) curves and response consistency in a face discrimination task. Compared to younger observers, older observers had higher additive internal noise and lower calculation efficiency, but the magnitude of multiplicative internal noise did not differ between age groups. Previous studies have shown that younger adults have a bias to rely on horizontal structure to discriminate and identify faces, and the magnitude of this so-called horizontal bias is correlated with identification accuracy. The experiments in Chapter 4 measured horizontal bias in younger and older adults, and found that age differences in horizontal bias account for some, but not all, of the age difference in face identification accuracy. In summary, my work demonstrates that additive (but not multiplicative) internal noise is greater in older adults, and that they are less efficient at sampling information that is conveyed by structure at different locations and orientations in a face. / Dissertation / Doctor of Science (PhD) / Our experience of the visual environment results from perceptual processes in the brain. Many of these processes change with age, such as our ability to identify someone from a photograph of their face. Performance is influenced by both random variability, or "noise", within the observer and how efficiently we use task-relevant information in the visual environment. By systematically manipulating the amount of available stimulus information I assessed the contribution of these factors to older adults' judgements of facial identity, and characterized the information on which these decisions are based. These experiments are the first to consider how face perception in older adults is constrained by the combined effects of internal noise and the efficiency with which the visual system utilizes various sources of information. The results provide a number of directions for future research in the fields of face perception and age-related changes in complex pattern vision.