Parkinson’s disease (PD) involves many movement disorders due to basal ganglia abnormalities involving both temporal and spatial parameters. A general slowness of movement is frequently mentioned, and this also appears in drawing or writing behavior. PD patients exhibit longer movement preparation times than controls do. They are also reported to present deficits in using advance information to improve movement performance, and to rely more on external visual feedback during movement execution, this increased reliance upon vision being possibly due to the deficit in the production of appropriate forces. In a drawing task, PD patients were shown to use a much lower axial pressure than controls, without modulating the pressure level as a function of task progression from the first to the last drawn segment, thus demonstrating difficulties in modulating forces in the course of movement. As far as spatial features are concerned, movements performed by PD patients present shorter amplitudes when compared to control subjects, possibly because of a strong impairment of the regulation of force amplitude. A recent study demonstrated that many years before the disease is diagnosed, the handwriting of premorbid PD patients already presents some specific spatial features, such as less round strokes and more abrupt changes of direction.

One task variable which provides much insight into the deficits of fine motor abilities in PD patients is related to the production of sequences of movements. Impairments in performance emerge as soon as the patients are required to link together several movement units. When performing a reaching movement for instance, PD subjects display a greater delay between the limb transport phase and the hand opening phase than is usually recorded in controls. Drawing behavior also reveals a difficulty in the execution of multi-segment (or stroke) movements. Longer pauses at the vertices of geometric figures are observed in PD patients. Similar results have been reported for the execution of other sequential skills. Impairments in the coordination of multi-joint movements are also reported. As pointed out by Teulings and Stelmach (1992), a crucial deficit in PD motor functioning may be seen in the production of movements that imply a coactivation of several effector joints. Teulings et al. (1997) recently revealed that PD patients display a reduced capability to coordinate wrist and finger movements when trying to produce handwriting-like patterns, as attested by a high degree of dysfluency of movements produced in certain directions.

The present study aims at investigating whether such a deficit in movement coordination may also cause alterations of the spatial characteristics of movements, particularly with regard to the accuracy with which movement directions are reproduced. We know that the other deficit in Parkinsonian motor functioning, the production and regulation of force, is probably responsible, at the spatial level, for the reduction of movement amplitude. It is thus likely that an analysis restricted to spatial features of movements only demonstrates specific alterations in PD patients. Three spatial variables are therefore investigated: segment length, segment direction and angle. We wonder to what extent is the reduction in movement amplitude dependent on the model’s size. PD subjects were reported to be able to modify stroke size in an handwriting task by increasing the average stroke size from 0.42 to 0.58 cm, but to display less modulation in their handwriting between small and large strokes, as if they tended to restrict their range of movement amplitude around some central value. It is thus likely that the patients experience the same kind of difficulties in a drawing task even if sizes slightly larger than those usually present in handwriting are requested.

A second spatial variable investigated in the present study concerns segment direction. Several authors agree upon the fact that a lack of coordination in movement appears as soon as PD patients are asked to perform movements which necessitate the control of a large number of muscles and joints.With regard to handwriting, two different systems of mapping between effectors and movement directions or axes have been suggested in the literature. In line with Hollerbach’s model, some authors have proposed that finger movements are responsible for the production of verticals, hand movements for that of horizontals, whereas oblique directions are obtained through a contribution of both effectors. In this perspective, oblique directions should be the most difficult to produce and should be severely impaired in PD patients, a prediction which finds support in evidence presented by Teulings et al. (1997). Meulenbroek and Thomassen (1991) provide a different anatomical description of the production of movement directions where it appears that oblique directions (oriented along two main axes, 45–225°, 135–315° when 0° corresponds to the rightward oriented horizontal, with direction increasing in a counterclockwise rotation) are the simplest directions to perform. They involve only one effector, either the finger (top-left/bottom-right direction) or the hand (top-right/bottom-left direction). Interposed oblique directions between these two main axes and the horizontal–vertical axes would elicit the control of both hand and finger movements. Vertical and horizontal directions would require the participation of both effectors also, moving either congruently (verticals) or incongruently (horizontals). However, according to the authors, these last directions benefit largely from visual control.