Visual control of automated handwriting movements.

This study provides new insight into the subtle interaction between visual feedback and automated handwriting movements. To separate effects of visual feedback from effects of speed/accuracy trade-off, subjects were trained to perform the tasks with rapid and automated movements. To control the visual feedback the written trace was presented only on a computer screen. In the first experiment the visual feedback of script size was manipulated. Subjects had to write the character combination "ll" onto a pattern of squares. In some trials the script size was unpredictably enlarged by 133% or reduced by 66%. Results showed that the ongoing "l" was not affected by the manipulation, but subjects adapted script size in the following "l" without any change in the kinematic characteristic of the movements. In a second experiment trajectory correction strategies in perturbed writing movements were studied. Two small boxes had to be connected by rapid and single-stroke movements. In some of the trials the position of the target box was changed unpredictably to a new position. Kinematic analysis revealed that the initial movement was not aborted, but rather a second independent movement was added to correct the trajectory to the new target. Thus, this proves that a distortion of visual feedback does not directly slow down open-loop movements to allow control of the motor output in a closed-loop mode. The ballisticity and automation was maintained during movement correction. Our findings fit perfectly well with recent theories of kinematic organization where complex movements are composed by elemental movement strokes.