Evaluating the special role of time in the control of handwriting.

Claims that time has a special role in the control of writing and the specific claim that writing time is absolutely invariant across changes in writing size are evaluated in two experiments. The first examined writing time for 24 undergraduate subjects who produced the string eyleyl with the dominant hand or arm in blocked repetitions having different vertical size targets. These variations produced small but systematic changes in writing time. The second experiment explored whether the small range of writing-time variation observed in experiment 1 was due to structural or strategic limitations. This experiment showed, for four undergraduate subjects, that writing time can be varied precisely across a wide range (0.6 to 1.66 of 'normal') while maintaining shape and vertical size constant. Taken together, these experiments suggest that, although relative stroke timing is approximately maintained, absolute timing is not critical to writing. The limited range of writing times typically observed should, rather, be ascribed to a strategic gradient that, along with other influences, broadly defines preferred writing times. This paper also describes a new application of Generalized Procrustes Analysis of shape, and this procedure is applied to the trajectories generated in both experiments. Although several small failures are noted, these analyses generally confirmed previous claims that shape is invariant across changes in writing time, size, and writing with the hand versus the arm. This result is a necessary buttress to the conclusions just described. Shape variability was also assessed in these analyses. This variability soared as writing time was reduced from normal, but showed only a small, insignificant increase as writing time was increased from normal. There were also small, predictable changes in spatial variability across changes in size and effector.