On-line size control in handwriting demonstrates the continuous nature of motor programs

Abstract

Two views on the nature of motor programs are contrasted, one holding that motor programs are discrete and integrated sets of movement goals, of which only the total size may be scaled up or down. The alternative view is a more continuous appreciation of the motor program as a generalized but segmented sequence of goal trajectories, each of which is more or less open to the influence of local spatial constraints. For a task like handwriting the second view implies that writers, when they adapt their current writing size to the space left at the end of a writing line, upstrokes may be scaled up or down differently from downstrokes and, secondly, adaptations in the horizontal dimension may be disconnected from adaptations in the vertical dimension of script. Twelve right-handed subjects wrote nonsense task words (each of a length of nine letters and composed of letters l, h, e, n, and m). Pen-tip movements were recorded by means of a digitizer tablet and real-time visual feedback of the writing trace was presented on a computer display. When their writing had proceeded halfway the task word, the computer program presented the target writing space for that word which was either normal, shortened or extended (both by 7%). Realized trajectory length of up and down strokes, horizontal and vertical displacements and stroke durations were analyzed. Significant size adaptations occurred immediately following the disclosing of the spatial constraint. These size adaptations were more of a continuous than of a discrete nature. During a transition stage of three letters, writing size was gradually adapted to the local constraint and upstrokes were affected earlier than downstrokes. Also, adaptations of the horizontal progression were earlier and more pronounced than changes of vertical displacements. After the transition, the ratio of horizontal and vertical displacement returned to its original value. The results support the more dynamically and continuous view on motor programs.