Parkinson's disease and the control of size and speed in handwriting

This experiment investigated whether Parkinson's disease (PD) patients expe rience problems in producing stroke size, stroke duration or both, in a han dwriting task. Thirteen PD patients and 15 elderly controls wrote four patt erns of varying complexity on a digitizer tablet. The participants were ins tructed to execute the writing movements. at a normal size and speed; as fa st as possible; two times larger than normal; and two times larger and as f ast as possible. PD patients had no difficulty increasing speed while maint aining size and had no difficulty increasing size while maintaining speed. However, they showed significantly smaller size increases in the two times larger condition as compared to the elderly controls. The conditions were a lso simulated by a neural network model of normal and PD movement control t hat produced a stroke pattern that approximated the experimental data. For the instructions used, the results suggest that when patients scale speed, they have no difficulty controlling force amplitude, but when they scale st roke size, they have a problem controlling force amplitude. Thus, PD patien ts may have reduced capability to maintain a given force level for the stro ke time periods tested with the instructions. (C) 1999 Elsevier Science Ltd . All rights reserved.