The Ipsilateral Silent Period as a Measure of Transcallosal Inhibition: An Investigation of Individual and Methodological Factors Influencing Interhemispheric Inhibition between Motor Cortices

Background: In this study, we investigated age and hemispheric differences in transcallosal inhibition (TCI) in the context of active contraction using the ipsilateral silent period (iSP). We also examined whether age-related changes in TCI would be related to corresponding changes in manual performance with age. Participants consisted of right-handed individuals from two age groups (young adults, n=13; seniors, n=17). The iSP was measured for each hemisphere using suprathreshold TMS pulses delivered over the primary motor cortex ipsilateral to the maximally contracting hand while the homologue muscles of the opposite hand were lightly contracting (~15% of the maximum). Manual performance was assessed bilaterally for both grip strength and fine dexterity. Results: Our results yielded two main findings. First, TCI measures derived from iSP were strongly influenced by age, whereas differences between hemispheres were only minor. Second, correlation analyses revealed that age-related variations in TCI measures were related to changes in manual performance, so that left-to-right TCI correlated with right hand performance and vice-versa for the opposite hand/hemisphere. Conclusion: Overall, these results concur with other recent reports indicating that mutual inhibition between motor cortices tends to decline with age. In this respect, our observations are in line with the notion that the balance of normally predominantly inhibitory interactions between motor cortices is shifted toward excitatory processes with age. Background Coordination of movements relies on both the activation of each independent hemisphere and the communication between hemispheres which is mediated via the corpus callosum [1]. This transcallosal pathway is essential for the interhemispheric transfer of perceptual, sensory and motor information underlying complex and integrated behaviors [2]. Although transcallosal connections can be facilitatory, mutual inhibition appears to be the primary mode of action between the two primary motor cortices [3]. This mutual inhibition has been shown to be finely modulated depending on task demands, unilateral actions leading to increased inhibitory drive from the active hemisphere, whereas bilateral actions lead to more balanced inhibition between hemispheres allowing for coordinated actions of the two extremities [4,5]. In the primary motor cortex (MI), interhemispheric inhibition can be assessed non-invasively with transcranial magnetic stimulation (TMS) using either paired-pulse paradigms or via the ipsilateral silent period (iSP) [6]. With paired-pulse paradigms, the target hemisphere is first conditioned by applying suprathreshold TMS on the opposite hemisphere at short (8–12 ms) or long (e.g., 40 ms) inter-stimulus intervals leading to two corresponding periods of inhibition of test motor evoked potentials (MEPs), i.e. short-latency (SIHI) and longlatency (LIHI) interhemispheric inhibition. The iSP assesses transcallosal inhibition by applying focal TMS to the motor cortex ispsilateral to the test hand while the target muscle is activated voluntarily, leading to a brief interruption of the ongoing muscle activity [7]. This interruption of voluntary muscle activity is thought to reflect transcallosal inhibition (TCI) mediated by the MI * Correspondence: francois.tremblay@uottawa.ca School of Human Kinetics, University of Ottawa, Montpetit Hall, 125 University Private, Ottawa, ON K1N 6N5, Canada School of Rehabilitation Sciences, University of Ottawa, Guindon Hall, 451Smyth Rd., Ottawa, Ontario K1H 8M5, Canada Full list of author information is available at the end of the article © 2013 Davidson and Tremblay; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Davidson and Tremblay BMC Neuroscience 2013, 14:62 http://www.biomedcentral. com/1471-2202/14/62