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Within-limb somatotopy of excitatory and inhibitory processes probed by paired pulse TMS – Preliminary results
Question:Paired pulse transcranial magnetic stimulation(ppTMS) is a common approach to probe cortical excitatory and inhi-bitory processes [a]. ppTMS paradigms can be classified by the inter-stimulus interval (ISI) to long and short interval ppTMS. It isassumed, that short interval ppTMS phenomena such as short-inter-val cortical inhibition phenomenon (SICI) are rather focal and arediscussed in terms of surround inhibition in the motor cortex [b].Longer interval ppTMS phenomena are believed to be based on morewidespread trans-synaptic mechanisms[c]. The topographic aspectsof these ppTMS phenomena are not fully understood yet. The associ-ation of the various ppTMS phenomena studied in different musclesof the same limb remains unexplored. The aim of this study was toassess the interaction of ppTMS phenomena, probed in differentupper limb muscles. We hypothesized that the correlation amongthe different hand muscles within the ppTMS phenomenon willdepend on the ppTMS ISI.Methods:19 healthy right-handed volunteers participated in thestudy (15 females, 18–30 y.o.). Four ppTMS phenomena (SICI/LICI –short-interval/long-interval intracortical inhibition, SICF – short nterval intracortical facilitation and ICF – intracortical facilitation)were probed using navigated TMS (MagPro X100, Localite TMSNavigator) applied at the APB hotspot of the left primary motor cor-tex. We registered motor evoked potentials (MEPs) from four rightupper limb muscles: abductor pollicis brevis (APB), extensor digito-rum communis (EDC), abductor digiti minimi (ADM) and biceps bra-chii (BB). Spearman’s rank correlation coefficient was calculated toevaluate an association of inter-muscle ppTMS phenomena.Multiple comparisons were controlled with the FDR. For the finalanalysis we considered only correlations between spatially sepa-rated muscles with sufficiently high amplitudes of MEPs during sin-gle pulse TMS.Results:SICI, ICF and LICI phenomena correlated significantlyamong all the considered muscles‘ pairs. SICI for APBEDC:r=0.793,p= 0.000; for ADMEDC:r= 0.905,p= 0.000. ICF for APBEDC:r= 0.639,p= 0.000; for ADMEDC:r= 0.639,p= 0.000.SICF for APBEDC:r= 0.423,p> 0.05; for ADMEDC:r= 0.779,p= 0.000. LICI for APBEDC:r= 0.575,p= 0.000; for ADMEDC:r= 0.704,p= 0.004.Conclusion:In agreement with the previous data, we did not finda clear association among different ppTMS phenomena. In contrastto our hypothesis, there is no link between ISI and the correlationof the same ppTMS phenomenon among muscles. The lack of thecorrelation between APB and EDC in SICF might be explained bySICF peculiar mechanism through the superposition of D-and I-waves [d]; and by a specialized functionality of the thenar, compar-ing to hypothenar muscles, – the idea which should be further ver-ified in the following experiments.