Automaticity and attention in Huntington's disease: When two hands are not better than one

People with Huntington's disease (HD) commonly report difficulty carrying out two everyday tasks simultaneously. This difficulty, confirmed by experimental studies, is typically ascribed to impaired attention. Yet, dual-task problems extend to relatively simple tasks, such as walking and talking, which would ordinarily be considered relatively undemanding of attention. The study tests the hypothesis that in HD there is a deficit in the ability to automatise task performance. Thus, simple tasks, which place minimal demands on conscious attention in healthy controls, make disproportionately high demands on attentional resources in HD. We examined the performance of HD patients and healthy controls on a simple, paced finger-tapping task, comparing single-task (tapping with one hand) and dual-task (tapping with both hands simultaneously) performance. For HD patients, bimanual tapping increased the task demands: there was greater variability in tapping rate and patients reported that the 'dual-task' condition was more difficult. The opposite pattern was observed for controls. Variability in tapping performance in HD was highly correlated with performance on cognitive tasks that have the potential to be automatised but not with performance on tasks that are more demanding of executive control, suggesting a common substrate for cognitive and motor automaticity. The data support the hypothesis that HD patients are impaired in their capacity for automisation, and suggest that impaired automaticity may be one source of attentional deficits in HD. The findings have implications for the interpretation of 'high level' deficits in attention and executive function previously reported in HD.