The handwriting of Parkinson’s disease (PD) patients is more impaired in terms of force amplitude than in terms of force timing when compared to handwriting of the elderly. As a result of impairments in force amplitude, PD patients experience problems with the fine tuning of forces and often report muscle weakness. The elderly, in turn, make slower and spatially more variable movements as compared to young adults, but to a lesser extent than PD patients.

It is suggested that slower and more variable movements in the elderly as compared to younger adults are due to reduced signal-to-noise ratios in the motor system of the elderly. These impaired signal-to-noise ratios are probably caused by a combination of physiological aging processes, such as loss of motor units, and reduced muscle strength as result of a reduction of the number of muscle fibers per muscle. Additionally, it is suggested that PD patients have smaller signal-to-noise ratios than the elderly. This account of movement slowness and variability is based on two assumptions about signal-to-noise ratios in the motor system. Firstly, the motor system is inherently noisy. The noise in the motor system reflects the combined result of the stochastic recruitment process of muscle units due to characteristics of muscle tissue, physiological tremor, stretch reflexes, springlike oscillations of the limbs, and correction servos from feedback processing. Secondly, noise in the motor system increases with larger force levels resulting in an increase of the variability in peak force. Therefore, rapid movements are generally more noisy than slower movements, because fast movements require higher levels of force.

Recent studies by Van Gemmert and Van Galen, 1994; Van Gemmert and Van Galen, 1996; Van Gemmert and Van Galen, 1997 and Van Gemmert et al., 1998 on the effects of different kinds of stress on point-to-point aiming and handwriting movements, suggested that both mental load and physical stress reduce signal-to-noise ratios. Furthermore, these studies showed that subjects can adapt relatively easy to moderate levels of mental load and physical stress. In these studies, it was argued that an increase of noise in the motor system as a consequence of mental load or physical stress is diminished by the filtering properties of the pen–limb system and/or the signal is enhanced by increasing processing time.

The latter view to increase signal-to-noise ratios in the motor system by increasing processing time is based on the notion that as a signal accrues over time, its noise levels off. This principle, which has a long history in detection theory, leads to prolonged movement duration (i.e., longer movement times and/or pauses). If movement duration is to be kept constant, this adaptation strategy is not optional, because longer pause intervals need to be added between movements to allow for more processing time.

The first option to increase the signal-to-noise ratio in the motor system is to use the filtering properties of the effector system to actively cope with increased levels of neuromotor noise. To this end, subjects may enhance limb stiffness which then suppresses high-frequency components of the movement, these components are thought to produce more variable movement trajectory and end-point outcomes. To some extent, the latter mechanism may increase signal-to-noise ratios without it needs to reduce movement speed. Active cocontraction of the agonist and antagonist muscle that lead to an increase of tonic limb stiffness is preferred over the detection-theory based option to increase signal-to-noise ratios, because this option requires longer movement durations. However the increase in cocontraction without loss of performance speed requires accurate modulations of muscle force. In the present experiment, the coping strategies of PD patients, young, and elderly control subjects will be investigated under various levels of mental and motor load by analyses of signal-to-noise ratios of handwriting movements.

Morris et al. (1995) suggested that movement performance of PD patients deteriorates when secondary cognitive or motor tasks are performed. Other studies failed to show cognitive deficits in mildly affected PD patients. These findings suggest that as a result of secondary cognitive tasks, neuromotor noise in the motor system of PD patients and elderly controls may be elevated to the same extent. Therefore, we argue that in mildly affected PD patients, a secondary cognitive task should deteriorate motor performance to the same extend as performance of the elderly, which will be less for a task consisting of moderate levels of cognitive load as compared to a task consists moderate levels of motor load. It is predicted that a deterioration of motor performance will be exhibited by a reduction of movement speed. Generally, a reduction in movement speed could either result from an increase in movement duration or a decrease in movement amplitude. Reduction of stroke size (i.e., movement amplitude) in handwriting is one of the characteristic clinical signs of PD often referred to as micrographia.