Jean-Jacques Orban de Xivry

1er février 2013

Université catholique de Louvain, Louvain-la-Neuve, Belgium

Monitoring motor cortex reorganization elicited by learning with transcranial magnetic stimulation

invité par Laurent Madelain et Guillaume Masson

abstract : Motor learning in general, and force-field adaptation in particular, entails changes in the primary motor cortex (M1). In the force-field task, subjects reach to targets displayed on a screen while holding the manipulandum of a robotic arm. The robotic arm can push the hand away from its trajectory. The large initial deviation observed at the introduction of the perturbation is then reduced over the course of trials.

To explore M1 reorganization, we used a force field paradigm and delivered a single pulse of TMS around 100ms before movement onset in one third of the trials. We compared 3 groups of subjects. In group 1 (ABR, n=20), the force field was introduced at full strength on the first trial for 240 trials. In group 2 (GRA, n=20), the field was introduced gradually over 185 trials and maintained for an additional 55 trials. We measured the MEP size before movement onset (pre-movement onset) in the biceps, triceps and deltoid muscles throughout the experiment.

By the end of the learning, the ABR and GRA groups exhibited comparable performance. The force-field elicited an increase in muscle activity for the biceps during movements towards SE and for the triceps and deltoid during movement towards the NW. The size of the MEPs also changed during force-field adaptation. An index measuring the imbalance between the sizes of MEPs before movements in either direction demonstrated a significant change in MEP index for the ABR group but not for the GRA group. For the ABR group, the change in MEP index was correlated with the strength of the motor memory.

In conclusion, M1 reorganization can be monitored during force-field learning through changes in pre-movement MEPs. These MEP changes are not a mere reflection of upcoming muscle activity but are linked to changes in M1 due to repetition-dependent processes.

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