The Influence of Surface Detail on Object Identification in Alzheimer's Patients and Healthy Participants
Image format (Laws, Adlington, Gale, Moreno-Martínez, & Sartori, 2007), ceiling effects in controls (Fung et al., 2001; Laws et al., 2005; Moreno-Martínez, & Laws, 2007; 2008), and nuisance variables (Funnell & De Mornay Davis, 1996; Funnell & Sheridan, 1992; Stewart, Parkin & Hunkin, 1992) all influence the emergence of category specific deficits in Alzheimer‟s dementia (AD). Thus, the predominant use of line drawings of familiar, everyday items in category specific research is problematic. Moreover, this does not allow researchers to explore the extent to which format may influence object recognition. As such, the initial concern of this thesis was the development of a new corpus of 147 colour images of graded naming difficulty, the Hatfield Image Test (HIT; Adlington, Laws, & Gale, 2009), and the collection of relevant normative data including ratings of: age of acquisition, colour diagnosticity, familiarity, name agreement, visual complexity, and word frequency. Furthermore, greyscale and line-drawn versions of the HIT corpus were developed (and again, the associated normative data obtained), to permit research into the influence of image format on the emergence of category specific effects in patients with AD, and in healthy controls. Using the HIT, several studies were conducted including: (i) a normative investigation of the effects of category and image format on naming accuracy and latencies in healthy controls; (ii) an exploration of the effects of image format (using the HIT images presented in colour, greyscale, and line-drawn formats) and category on the naming performance of AD patients, and age-matched controls performing below ceiling; (iii) a longitudinal investigation comparing AD patient performance to that of age-matched controls, on a range of semantic tasks (naming, sorting, word-picture matching), using colour, greyscale, and line-drawn versions of the HIT; (iv) a comparison of naming in AD patients and age-matched controls on the HIT and the (colour, greyscale and line-drawn) images from the Snodgrass and Vanderwart (1980) corpus; and (v) a meta-analysis to explore category specific naming in AD using the Snodgrass and Vanderwart (1980) versus other corpora. Taken together, the results of these investigations showed first, that image format interacts with category. For both AD patients and controls, colour is more important for the recognition of living things, with a significant nonliving advantage emerging for the line-drawn images, but not the colour images. Controls benefitted more from additional surface information than AD patients, which chapter 6 shows results from low-level visual cortical impairment in AD. For controls, format was also more important for the recognition of low familiarity, low frequency items. In addition, the findings show that adequate control data affects the emergence of category specific deficits in AD. Specifically, based on within-group comparison chapters 6, 7, and 8 revealed a significant living deficit in AD patients. However, when compared to controls performing below ceiling, as demonstrated in chapters 7 and 8, this deficit was only significant for the line drawings, showing that the performance observed in AD patients is simply an exaggeration of the norm.