Age and hemispheric differences in transcallosal inhibition between motor cortices: an ispsilateral silent period study

Travis Davidson,François Tremblay

doi:10.1186/1471-2202-14-62

Abstract

BackgroundIn 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.ResultsOur 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.ConclusionOverall, 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.

Highlights

In this study, we investigated age and hemispheric differences in transcallosal inhibition (TCI) in the context of active contraction using the ipsilateral silent period
In terms of corticomotor excitability, both age groups showed a tendency for higher resting motor threshold in the left hemisphere than in right hemisphere; a difference that reached significance only in the senior group (t15=3.01, p=0.008)
The present study examined age and hemispheric differences in mutual inhibition between motor cortices using the ipsilateral silent period (iSP) as a marker of transcallosally mediated inhibition

Summary

Introduction

We investigated age and hemispheric differences in transcallosal inhibition (TCI) in the context of active contraction using the ipsilateral silent period (iSP). Indirect evidence of changes in interhemispheric inhibition with age comes from observations of difficulties experienced by older adults in bimanual tasks along with the presence of motor overflow during performance of unimanual actions [reviewed in 10] This is paralleled by reports from functional neuroimaging studies showing that task-related activation patterns in sensorimotor areas are typically less lateralized and more widespread in older adults than in young adults [11]. As suggested by Seidler and colleagues [10], such observations point to a shift in the balance of mutual inhibition between motor cortices with age towards excitatory processes In agreement with this view, Talelli et al [14] reported that the degree of LIHI from the left MI to the right MI during right hand grip was progressively reduced with advancing age, there was even a switch from inhibition to facilitation in the very old participant. Observations from behavioral and neuroimaging studies seem compatible with an age-related shift in the balance of mutual inhibition between motor cortices, recent findings from TMS studies remains controversial in this regard

Results

Discussion

Conclusion