Implicit Processes in People with Huntington's Disease

Abstract

Although there has been a proliferation of studies examining both implicit memory and learning in neurologically impaired populations, most of this research has been based on Multiple Memory Systems theories rather than Transfer Appropriate Processing methodology and theory. These theoretical frameworks and an introduction to the area of implicit memory and implicit learning are discussed. The neurodegenerative disease known as Huntington's Disease (HD) is a particularly appropriate population of people to recruit to investigate implicit memory and learning, because of the specificity of the neuropathology and the certainty regarding diagnosis of the disease. As such, the investigation of implicit processes in HD became the focus of this thesis. Relevant aspects of HD are outlined, and implicit memory and learning research conducted with this population is discussed. Experiment 1 was designed to compare HD patients and healthy control participants on an implicit learning task. A verbal version of the classic Serial Reaction Time (SRT) task was employed to ensure the motor component of the task was minimised, because of the motor impairment inherent in HD. The results of this study indicated that HD patients in the mid-latter stages of the disease demonstrate sequence learning deficits even when the motor component of the task was minimised. This indicated a more general impairment in sequence learning and provides further evidence for the involvement of the caudate nucleus in this type of learning. Experiment 2 was designed to compare the performance of people with HD and healthy control participants on conceptual and perceptual implicit and explicit tests. The aim was to investigate the perceptual versus conceptual processing distinction and priming (implicit memory) performance in HD, as well as explicit memory. Overall, the results of Experiment 2 indicated that the dissociations appear to be predominantly driven by the explicit-implicit distinction rather than the processing dimension, although both dimensions appear to be important. A difference between the groups in generativity was found on the conceptually-driven explicit memory task. However, although the control group appeared to have higher rates of generativity compared to the HD group on the perceptually-driven explicit memory task, a statistically significant difference between the groups was not found, which may be due to low statistical power. The results of Experiment 2 also indicated intact conceptual and perceptual priming for people with HD. Furthermore, the performance of all participants on all tests (except the Graphemic Cued Recall test) showed the expected read or generate encoding condition advantage. Theoretical and practical implications, as well as thelimitations and strengths of the studies are discussed.