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

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.

Reliability of Transcallosal Inhibition in Healthy Adults

“…Davidson and Tremblay (2013) used a stimulation intensity of 120% resting motor threshold, which is likely to be lower than the 80% MSO used in the present study and therefore could have influenced the iSP measure (Meyer et al, 1995; Chen et al, 2003). We chose a higher intensity, which is consistent with several other studies (Chen et al, 2003; Trompetto et al, 2004; Stinear et al, 2008, 2015; Spagnolo et al, 2013), in an attempt to ensure maximal activation of transcallosal neurons.…”

“…Previous studies have indicated a change in TCI with aging, with reduced duration and area of the silent period for older adults (McGregor et al, 2011; Davidson and Tremblay, 2013; Coppi et al, 2014), which physical activity may ameliorate (McGregor et al, 2011). The present study showed no clear difference in reliability between younger and older groups suggesting that reproducibility of these measures is not dependent on age.…”

“…Additionally, there are different methods for determining the criterion EMG level at which the onset of the silent period is detected. We chose to use 75% of the baseline RMS EMG, but others have used the baseline level itself (Davidson and Tremblay, 2013; Spagnolo et al, 2013; Stinear et al, 2015), a reduction of one-third (Stinear et al, 2008), 50% (Shimizu et al, 2002) or 1 or 2 standard deviation (Chen et al, 2003; Fling et al, 2011). It is possible that the reproducibility may be affected by the criterion used.…”

“…However, to our knowledge the reliability of the iSP has not been assessed, despite its use in longitudinal studies. Older adults typically show delayed iSP onset latency and a lower magnitude of inhibition than younger adults (Davidson and Tremblay, 2013) as well as reduced bimanual coordination (Fling et al, 2011). It is therefore necessary to assess reliability in both of these populations.…”

Age-Related Changes in Motor Cortical Representation and Interhemispheric Interactions: A Transcranial Magnetic Stimulation Study

“…These studies demonstrated a reduction in ISP in terms of duration (Petitjean and Ko, 2013) and area (Davidson and Tremblay, 2013) in older subjects compared to younger participants. The ISP consists in stimulating one motor cortex during maximal voluntary contraction of the ipsilateral hand (Ferbert et al, 1992).…”