Neurosensory and Cognitive Correlates of Cursive Writing.

Fr?d?ric ALBERT
Department of Biobehavioral Sciences

Edith Ribot-Ciscar
Universit? de Provence

Mikael Bergenheim
University of Gavle and Central Hospital Karlstad

Jean-Pierre Roll
Universit? de Provence

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     Last modified: February 28, 2007

Abstract
Writing a graphic symbol deformed in precise and time-ordered manner each muscle of a joint. This pattern of mechanical events evoked specific pattern of lengthening and shortening of the various muscles generating particular pattern of activity of the muscle spindle populations lying in these muscles. We assume that the proprioceptive feedback, as a whole, encodes the spatial and temporal characteristics of writing trajectories and that it will be sufficient to allow symbols
recogition.
The unitary activity of sixty muscle spindle afferents of the main ankle muscles was recorded using the microneurographic method during imposed writing movements. These activities were analysed using the ?vector population model?. Muscle spindle afferents responded according to the tuning properties of the parent muscle, i.e. increasing their discharge rate when the direction of movement was within the preferred sensory sector of the parent muscle. The whole trajectory of the writing movements was coded by the activity of afferents arising from all the muscles of the joint. Both single and population afferent responses were found to be highly specific and reproducible with each graphic sign. The complex multi-muscle afferent pattern involved seems to constitute a ?proprioceptive signature? for each graphic symbol. The ensemble of muscle spindle afferents were found to encode the instantaneous direction and velocity of writing movements remarkably accurately. Furthermore, the neuronal trajectories attained from populations of muscle spindles clearly depict the path and kinematic parameters and express the movement invariants, i.e. the trajectory segmentation and the relationship between velocity and curvature of a trajectory, i.e. the two-thirds power law. Finally, we investigate the contribution of muscle spindle feedback to proprioception and higher brain functions like movement trajectory recognition. For this purpose, writing illusory movements were evoked in subjects by applying, to each muscle group, patterns of tendon vibration mimicking the natural afferent pattern previously recorded. The results show that the afferent feedback of a given movement evokes the illusion of the same movement when applied to the subject via tendon vibration. The subjects were able to recognize and name symbols evoked by vibration in 83% of the trials. These findings suggest that the ??proprioceptive signature?? of a given movement is associated with the corresponding ??perceptual signature??. The fact that the only ??proprioceptive signature?? of a graphic movement allows not only the perception of its trajectory but also its symbolic identification gives a character of cognitive nature to the proprioceptive messages coming from the writing hand.