**8. Conclusions**

**7. General discussion**

80 Neuroplasticity - Insights of Neural Reorganization

It is generally accepted that, in both *left- and right-handed* subjects, the preferred hand is controlled mainly by the hemisphere *contralateral* to that hand, whereas the *non-preferred* hand is controlled by *both hemispheres*. In relation to *left-to-right switches* in handedness, the switched individuals have been found to share features of both left-handers and right-handers regard-

Before training, the results for both the *preferred/*left and the *non-preferred/*right hand conformed to these preexisting models: the *preferred*/left hand produced predominantly *contralateral* activation, whereas the *non-preferred*/right hand produced a more balanced *bilateral*

*After* training, however, the bilateral pattern expected in switchers was not observed any more. Instead, the non-preferred/right hand underwent a strong training-based reorganization of its motor control architecture, so as its bilateral activation pattern radically changed post-training into a contralateral one. Remarkably, this *contralaterality* (left hemisphere > right hemisphere) was caused not by increased contralateral (left) activation but by a massive suppression in the ipsilateral (right) hemisphere; it is particularly surprising that this happened

These findings show for the first time that the dominance of the preferred hemisphere can be rapidly overturned, and that this can happen even in late adulthood after decades of established dominance. Note that, until now, despite long-standing efforts across many disciplines to achieve a fully-fledged hand switching in left-handers, the best that has been achieved has been to engage the contralateral left hemisphere without being able to overturn the ipsilateral right hemisphere control [1, 12]. The fact that the Cognitive-Kinesthetic Drawing Training was able to transform the bilateral into a definitive contralateral pattern, and to do so effectively and efficiently, implies a serious deficiency in the current knowledge on motor control plasticity, and the need for enhanced investigation into this process. Moreover, the power of this memory-driven motor training to rapidly drive motor-control plasticity, in addition to the previously shown effects on memory and spatial cognition, for example [2–5, 39–41], suggests strong involvement of cognitive mechanisms in this process, as codified earlier by the

The resulting neural reorganization in this congenitally left-handed individual was correlated with similarly remarkable enhancement in the memory-drawing performance of the non-preferred hand, which post-training resembled the original much better than pre-training, and moreover, significantly better than the experienced preferred hand. This was particularly unexpected because the left hand had several additional advantages. First, the *haptic exploration* of the originals was *always* done with the *left* hand, thus providing a direct perception and encoding of this hand's movements along the lines of that image. In contrast, the non-preferred right hand *never* received any direct encoding of the trajectory but for planning and execution of the drawing trajectory it had to use the memory image developed through the other hand. Second, the nature of the left-hand exploration phase represents a strong form of *dual* memory

despite the fact that the right hemisphere has been the dominant one since birth.

introduction of the "perception-cognition-action loop" concept.

ing their motor control architectures. ([1], see Introduction).

activation, indicating control by both hemispheres.

This study is the first to show results that contradict the models of the nondominant hand always being controlled by both hemispheres, as had been previously thought. It is particularly remarkable that this brief memory-guided drawing training was able to switch lifelong handedness, overturning almost six decades of dominance of the right hemisphere by inducing profound suppression in the previously dominant hemisphere. In terms of handedness research as a whole, the study suggests a critical role for functional mechanisms, such as inter-hemispheric competition, as opposed to an inherent structural predetermination in hand dominance. The results are consequently of high relevance to the long-standing debate about the sources of hemispheric asymmetry. The unprecedented effect on handedness of the rapid Cognitive-Kinesthetic Drawing Training also implies the powerful potential of this training for further rehabilitation domains, such as the rehabilitation of stroke or trauma affecting hand control.
