P 160. The “dormant potential” of the non-dominant hemisphere: In teremispheric a symmetry of cortial plasticity

Abstract

Introduction Lateralization of manual dexterity is a striking example of interemispheric asymmetry in cortical organization. Transcranial magnetic stimulation (TMS) studies have documented handedness related functional asymmetries in corticomotor excitability. Particularly, TMS has been used to analyze movement related cortical plasticity. Motor evoked potentials (MEP) amplitude increases immediately after brief periods of exercise (post-exercise facilitation) and then increases again after a rest period of 15 min following a defined motor task (delayed facilitation). The post-exercise facilitation seems to be due to transient increase of motor cortex excitability. Objective The delayed facilitation seems to reflect an intra-cortical synaptic reorganization consequent to motor tasks. In the present study, we compared changes in cortical excitability from respectively dominant (DH) and non dominant hemisphere (NDH) in both right-handed and left-handed subjects in function of a bimanual non-fatiguing motor task. We evaluated handedness related asymmetries. Material and methods As primary screening, the 24 enrolled subjects were classified as right or left handed by according to their description of the hand used for writing. Then, each subject completed the Edinburgh Handedness Inventory and was reclassified as a function of the handedness score. For each subject, the experimental protocol was repeated twice so that the cortical excitability could be separately assessed in each hemisphere. By mean of TMS, MEP amplitude, motor threshold, and silent period were assessed. Then subjects performed a bimanual motor task (regular repetitive opening-closing bilateral movements of the index finger onto the thumb). MEPs of the first dorsal interosseus were recorded before exercise (baseline condition), immediately after each exercise periods of 30, 60, 90 s (exercise conditions), and after 15 min of rest (rest condition). MEP amplitude elicited in each exercise conditions, and then in rest condition was compared with baseline, to evaluate the presence of post-exercise facilitation and delayed facilitation. Hemispheric differences of MEP amplitude after the rest period were related to the handedness score. Results Mean threshold intensity of TMS was significantly lower for DH compared to NDH. MEP amplitude was significantly increased in exercise conditions, independently of the hemisphere stimulated. However, in rest condition, only the NDH presented increased MEP amplitude, showing delayed-facilitation. Handedness scores and MEP asymmetry in rest condition were positively correlated. Conclusion Delayed facilitation asymmetry was lateralized to NDH and varied as a linear function of handedness. These results suggest that cortical plasticity changes in the two hemispheres. Our data provide evidence that DH and NDH answer differently to motor requests and they differently recover after exercise. Bimanual motor task induces changes in cortical excitability differently in the two hemispheres, in favor of the non-dominant site. We speculate that NDH has a “dormant potential” that can be activated if necessary. The examination of the effects of a bimanual motor task on cortical excitability may be relevant to better understanding cortical plasticity processes and may provide new tools to study neurological disorders characterized by central fatigue and less capacity to recovery.