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Biomedical subjects

A J Thomassen

Publications and source records attributed to A J Thomassen.

6 recordsLinked to original sources
PubMed1996

Adaptation of a reaching model to handwriting: how different effectors can produce the same written output, and other results.

This report shows how a model initially developed for the control of reaching can be adapted for the control of handwriting. The main problem addressed by the model is how people can produce essentially the same written output with different effectors (e.g., the preferred or nonpreferred hand, the foot, or even the mouth). The model is based on the assumption that writers strive for invariant graphic outputs when they write with different effectors, when they write on surfaces with different orientations, or when they write large or small script; such output invariance is an essential requirement for later recognition of the written result. Given this assumption, the question is how the motor system enables the relevant effectors to generate the necessary pen strokes. The adapted model provides one possible answer to this question. It is first fully working model of multijoint activity underlying writing and related graphic tasks. We describe how the model differs from other models developed in the past, and we review the model's strengths and weaknesses.

Female↗
PubMed1993

Phonological and orthographic demands in the production of handwriting.

In an experimental handwriting task, with two parts, we varied the phonological and orthographic complexity of visually presented nonwords. Twelve adult subjects had to write these nonwords in shorthand as well as in Latin script. Phonological complexity was varied by presenting a nonword which included two identical vowel characters. These were either phonologically similar (simple condition) or phonologically different (complex condition). Orthographic complexity was varied by using nonwords which either have a graphemic format for shorthand that corresponds with the graphemic format that is applied for Latin script (simple condition) or a graphemic format for shorthand which is discrepant from the Latin script format (complex condition). It appeared that a higher degree of phonological and orthographic complexity led to a slower and less fluent performance in graphemes that preceded the actual locus of complexity of the nonword. Furthermore, complexity effects were by far the strongest under the production of shorthand. The results are interpreted from the point of view of a psychomotor theory of handwriting, which assumes that the spelling process of visually presented nonwords may follow a phonological or an orthographic (sublexical) route. The finding that orthographic complexity interferes with the production of a phonologically oriented task such as shorthand is interpreted as evidence in favour of an interactive transmission of information between these two processing routes.

Adult↗
PubMed1993

Exploitation of elasticity as a biomechanical property in the production of graphic stroke sequences.

In the present study we report several findings which indicate that subjects exploit elasticity of muscles and tendons as a biomechanical property of the motor system in the execution of graphic stroke sequences. The drawing movements of 15 right-handed subjects were analyzed, who copied a geometrical pattern consisting of four line segments. Three of these segments were connected by an acute and an obtuse angle. A first analysis concerning stroke-direction preferences shows that subjects tended to produce final strokes in preferred movement directions and obeyed an end-state stability constraint. Subsequently, we analyzed the copying movements with respect to (1) pauses at acute and obtuse angles, (2) local deviations in angle size, and (3) size variations of the strokes surrounding the angles. The results reveal a higher incidence of pauses at obtuse than at acute angles. Furthermore, a local sharpening of angles was found which was most pronounced at obtuse angles. Finally, systematic size variations of the strokes surrounding the angles were found. The results are considered to reflect the functional use of elasticity during task performance. It is concluded that biomechanical properties of the motor system significantly influence higher-order preparatory processes involved in multi-trajectory control.

Adult↗
PubMed1993

Limb-segment selection in drawing behaviour.

How do we select combinations of limb segments to carry out physical tasks? Three possible determinants of limb-segment selection are hypothesized here: (1) optimal amplitudes and frequencies of motion for the effectors; (2) preferred movement axes for the effectors; and (3) a tendency to continue using already-recruited limb-segments. We tested these factors in a graphic production task. Seven subjects produced back-and-forth drawing movements of gradually changing amplitude. The largest amplitude to be covered, trial duration, movement axis, and direction of amplitude change (from small to large or vice versa) were varied between trials. Selspot recordings were used to study the relative contributions of the fingers, hand, and arm to displacements of the pen. The temporal order of limb-segment involvement was also studied. The results confirmed the predicted effects of the three limb-segment selection factors. We conclude that limb-segment coordination is adaptively related to biomechanical features of the motor system and to the computational demands of movement selection itself.

Adult↗