Automaticity in Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder. An intriguing observation in the HD literature is that simple psychomotor tasks with low rather than high demands on attention and executive function are the most sensitive markers oflongitudinal chance in HD, and most consistently distinguish pre-manifest carriers of the HD mutation from non-carriers. It has been suggested that this reflects in HD a breakdown in the ability to execute simple tasks automatically. This hypothesis was explored in a series of studies that examined the performance of HD patients and pre-manifest carriers of the HD mutation on a variety of psychomotor tasks using dual-task and procedural learning paradigms. On a simple 'dual-task' paradigm, in which patients were required to perform a simple paced finger-tapping task with one or both hands, patients with early HD showed increased tapping variability and reported greater subjective difficulty for the bimanual 'dual-task' condition compared with controls, suggesting that the simple finger-tapping action was not executed automatically but rather, placed greater demands on HD patients than controls. On a visually cued reaching task in which stimuli were ordered in a highly simple, repeating sequence, HD patients showed slow and minimal motor-skill improvement and, despite extensive practice, failed to reach the level of rapid, efficient performance associated with skill automisation. In a further study it was shown that a subset of patients, who had greater cognitive impairment, failed to acquire any knowledge of the repeating sequence. It was suggested that, in these patients, the performance of the reaching task itself was so demanding that patients lacked the attentional resources required to abstract the sequential pattern. Subtle indications of a breakdown in automaticity could also be detected in premanifest carriers of the HD mutation (P-HD). On a paced finger-tapping task, P-HD participants exhibited greater performance variability, the degree of which was correlated with estimated proximity to disease onset. In a subset ofP-HD participants who were estimated to be close-to-onset, variability was greater still and was disproportionately increased by the performance of a secondary cognitive task, indicating impaired ability to execute the tapping task without directed attention. Taken together, the studies suggest that a breakdown in automaticity is a fundamental feature of HD, reflecting progressive striatal degeneration. The findings have implications for the understanding of attentional impairment in HD. If simple tasks cannot be automated then they will necessarily place greater demands on attentional resources in HD.