Differential effect of Huntington's and Parkinson's diseases in programming motor sequences of varied lengths

Background Parkinson's disease (PD) and Huntington's disease (HD) patients have difficulties executing sequential movements. Attention control and short–term memory probably play an important role in programming sequential movements. To investigate the contribution of these cognitive factors to programming and executing visuomotor sequences in HD and PD patients a computerized version of the Corsi Block Tapping–Test was employed. Methods the performance of 11 patients with early stage PD, 11 HD patients with borderline to mild caudate atrophy and 20 healthy subjects was compared. The task was a reaction time task where targets were illuminated in groups of sequences increasing from 2 items to 5 items. Subjects reproduced the sequence (pressing the illuminated target) in the same order of appearance. Reaction Times and movement times were recorded. Results PD patients had increasing difficulties in programming and executing series greater than three components. HD patients did not differ significantly from the controls, although they showed a tendency to lose accuracy in the longer series. Both patient groups did not differ in their attention span. Conclusions In PD although the spatial information may be well stored, they have difficulty accessing it when their attention is overloaded, leading to poor encoding and slow information processing. This process interferes with programming and execution of movement sequences. HD patients in the early stages of the illness seem to have more attention resources than PD patients, so that they start to show more problems in executing visuomotor sequences with longer movement sequences than PD patients.