| Abstract No.: | B-D2142 |
| Country: | Canada |
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| Title: | REORGANIZATION OF MOTOR REPRESENTATIONS AFTER UPPER LIMB AMPUTATION MEASURED DURING REST AND ACTIVE MUSCLE CONTRACTION |
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| Authors/Affiliations: | 2 Martin Gagné*; 2 Sébastien Hétu; 1 Karen T Reilly; 2 Joëlle Dubé; 2 Catherine Mercier;
1 Center for Cognitive Neuroscience, CNRS, Lyon, France; 2 CIRRIS, Laval University, Québec, QC, Canada
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| Content: | In the majority of cases, limb amputation is followed by the vivid sensation that the now-missing body part is still there, a phenomenon called “phantom sensation”. The perceived ability of amputees to voluntarily control movement of the phantom limb is among the most intriguing and least understood aspects of phantom sensation, and is of clinical interest as the results of previous studies suggest a relationship between the extent of motor plasticity following amputation, the perceived ability to move the phantom limb, and the severity of phantom pain. The aim of the present study was to evaluate the functional reorganization in the primary motor cortex (M1) controlling the amputated side. While previous studies have focused on the cortical reorganization observed at rest, the objective of this study was to compare motor reorganization observed at rest with that observed during two types of active contraction: stump muscle contraction or phantom-movement-related contraction. Methods: Seven above-elbow traumatic amputees were tested. Reorganization was assessed by mapping the M1 representation of a stump muscle and of the homologous muscle on the intact side using MRI-guided transcranial magnetic stimulation (TMS). Prior to mapping procedures, subjects executed various distal phantom movements (phantom hand and wrist). The mapped muscle was chosen the stump muscle with the most marked and reliable electromyographic modulation during these movements. On the hemisphere controlling the amputated side, TMS maps were performed at rest, during a voluntary contraction of the stump muscle (10% of maximal voluntary contraction, MVC) and during one phantom movement that activated the stump muscle to the same extent (10% MVC). On the intact side, maps of the homologous muscle were created at rest and during voluntary contraction. TMS intensity was adjusted to 120% of rest (rMT) and active (aMT) motor thresholds determined separately for each tested muscle. Results: In agreement with previous studies, the rMT was found to be reduced on the hemisphere contralateral to the amputation. This excitability difference disappeared, however, during active contraction. The center of gravity (CoG) of the amplitude of motor evoked potentials was calculated for each map. CoGs of maps from the same hemisphere created under different conditions had similar medio-lateral coordinates, as did the CoGs of maps from each hemisphere created under the same condition. Conclusion: Classic representations of the somatotopic organisation of M1 propose that the hand representation is lateral to the proximal limb representation. As such, we expected to observe that the CoG of the map created during phantom movement was more lateral than that of the map created during stump muscle contraction. The stability of these CoGs most likely reflects the now well-demonstrated overlap of distal and proximal arm muscle representations in M1. Nevertheless, M1 contralateral to the amputation has obviously been subject to physiological changes, as shown by the substantially lower rMT in this hemisphere. However, the absence of an inter-hemisphere difference for aMT raises questions about the functional relevance of physiological changes observed at rest in the hemisphere contralateral to the amputation. |
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