ABSTRACT
Viewed as a motor task, writing involves producing sequences of graphic symbols (letters, numbers) that correspond to predefined spatial forms (production of morphokineses, to use the terminology of Jacques Paillard) while following a set of conventional rules, such as progressing from left to right in our alphabetic system (production of topokineses, again according to Paillard) (→SPACE, SYMBOL). For normal-sized writing, the morphokinetic component is ensured by distal motricity (fingers, wrist) and requires a proactive type of movement control (anticipatory) specific to automated movements (→AUTOMATISM, CONTROL). Wrist movements produce lateral oscillations along a nearly rectilinear x′ axis; finger movements generate front-back oscillations on a y′ axis more or less parallel to the axis of the hand. One can understand why certain peripheral models (John Hollerbach) consider writing to result from the coordinated coupling of two orthogonal oscillation systems, one responsible for the pen’s horizontal movements in the graphic space, and the other, for its vertical movements. Letter writing is known to be governed by certain invariant principles (Paolo Viviani, Carlo Terzuolo) such as spatial homothety and temporal homothety, which stipulate that the size and duration ratios of the strokes in a letter (a stroke being defined as the part of the trajectory that falls between two speed minima) are maintained across variations in the letter’s size and execution time, respectively (→TIME AND TENSE). Local covariation of space and time, called the isogony principle, has also been demonstrated. This principle stipulates that angular speed remains constant across variations in the trajectory’s curve radius. Greater stability in the spatial aspects of writing than in its temporal aspects has often been reported, so it is hypothesized that the spatial encoding of letters is a central process. Some authors consider allographs (specific spatial representations of a grapheme, such as lowercase and uppercase; →REPRESENTATION) to be a good candidate for the unit that gets stored centrally (Hans-Leo Teulings, Arnold Thomassen). Much less studied, the topokinetic component of writing relies on proximal motricity (elbow, shoulder). It is responsible for translation movements along the horizontal axis, and is controlled by movement feedback (mostly visual). Many arguments, especially neuropsychological ones (→NEUROPSYCHOLOGY), have been set forth to support the independence of the morpho-and topokinetic components of writing (Andrew Ellis).
