Alterations of graphic production under various constraints follow principled modifications in underlying pattern dynamics

Cursive handwriting is composed of several shapes (strokes, loops...) finely articulated in order to produce a legible message. A recent study (Athènes et al., submitted) identified a small number of shapes that are performed stably and precisely (i.e., four lines and four ellipsoids of various orientations). The present study investigated whether such shapes are the outcome of learning and how they deteriorate with task constraints. We assumed that their relative stability determines their deterioration with increasing speed or lack of practice. Performance in drawing a large ensemble of ellipsoids with the right and left hand (viz. practiced vs. unpracticed) was evaluated at spontaneous and maximal speed. First, results confirmed the same preferred shapes observed in the previous experiment. Second, all shapes were degraded with increasing the constraints. Third, this effect was stronger for shapes that were initially the least stable. These findings confirm the idea that handwriting bears on basic graphic shapes. Moreover, they show that the stability of such shapes determines how they degrade with increasing constraints. We argue that such a set of preferred shapes is the manifestation of underlying dynamics, which arises from the coordination between two orthogonal nonlinear oscillators.