Does the production of letter strokes in handwriting benefit from vision?

In earlier studies the involvement of vision in handwriting was suggested by the finding that the production of a letter sequence in a condition without vision took more time and resulted in larger letter trajectories. The present study raises the question whether vision has an impact on the production of the individual up- and downstrokes constituting the letters. More specifically, the aim is to examine whether vision is employed either during the completion of a movement, or concurrent with the entire course of a stroke movement. Adult writers produced the letter sequence lelele under a no-vision and a vision condition, the latter serving as a base-line condition. It was found that movement time and trajectory size of acceleration and deceleration phases of a stroke movement increased under no vision, the magnitude of which depended on letter type. Letter e, with a smaller size and more frequently used in Dutch writing than letter l, was less affected by the no-vision condition. Although, close examination of downstrokes produced in later letter positions of the sequence revealed that the acceleration as compared to the deceleration phase took proportionally less time, the general finding was that increments in time and size proved to be equally distributed across entire stroke movements.

The role of vision in the temporal and spatial control of handwriting.

The general observation that handwriting is not noticeably impaired by the withdrawal of vision can be explained in two ways. One might argue that vision is not needed during the act of writing. Micro-analyses should then reveal that spatial as well as temporal writing features are identical in conditions of vision and no vision. Alternatively, it is possible that vision is needed during the act of writing, but that without vision possible errors and inaccuracies have to be prevented. Assuming that the latter would place an extra demand on movement control, this should be revealed by an increase in processing time. We have found evidence for the latter view in the present study in which 12 subjects wrote a nonsense letter sequence with and without vision. Close examination showed that writing shapes remained equally invariant under both vision conditions, suggesting that spatial control was unaffected by withdrawing vision. The prediction that invariance of shapes is preserved in the absence of vision at the expense of processing time increments was confirmed. The increase of reaction time observed when visual guidance was withdrawn suggests that more processing time was needed prior to the movement start. Moreover, the RT increment was larger when a short writing duration was instructed. The present findings will be discussed in light of the remarkable flexibility of writing as a motor skill in which writers appear to be able to employ specific strategies to preserve shape in the absence of visual guidance.

Effects of motor programming on the power spectral density function of finger and wrist movements.

Power spectral density analysis was applied to the frequency content of the acceleration signal of pen movements in line drawing. The relative power in frequency bands between 1 and 32 Hz was measured as a function of motoric and anatomic task demands. Results showed a decrease of power at the lower frequencies (1-4 Hz) of the spectrum and an increase in the middle (9-12 Hz), with increasing motor demands. These findings evidence the inhibition of visual control and the disinhibition of physiological tremor under conditions of increased programming demands. Adductive movements displayed less power than abductive movements in the lower end of the spectrum, with a simultaneous increase at the higher frequencies. The relevance of the method for the measurement of neuromotor noise as a possible origin of delays in motor behavior is discussed.