Abstract
Interhemispheric interactions demonstrate a crucial role for directing bimanual movement control. In humans, a well-established paired-pulse transcranial magnetic stimulation paradigm enables to assess these interactions by means of interhemispheric inhibition (IHI). Previous studies have examined changes in IHI from the active to the resting primary motor cortex during unilateral muscle contractions; however, behavioral relevance of such changes is still inconclusive. In the present study, we evaluated two bimanual tasks, i.e., mirror activity and bimanual anti-phase tapping, to examine behavioral relevance of IHI for bimanual movement control within this behavioral framework. Two age groups (young and older) were evaluated as bimanual movement control demonstrates evident behavioral decline in older adults. Two types of IHI with differential underlying mechanisms were measured; IHI was tested at rest and during a motor task from the active to the resting primary motor cortex. Results demonstrate an association between behavior and short-latency IHI in the young group: larger short-latency IHI correlated with better bimanual movement control (i.e., less mirror activity and better bimanual anti-phase tapping). These results support the view that short-latency IHI represents a neurophysiological marker for the ability to suppress activity of the contralateral side, likely contributing to efficient bimanual movement control. This association was not observed in the older group, suggesting age-related functional changes of IHI. To determine underlying mechanisms of impaired bimanual movement control due to neurological disorders, it is crucial to have an in-depth understanding of age-related mechanisms to disentangle disorder-related mechanisms of impaired bimanual movement control from age-related ones.
Highlights
Bimanual movement control is an important form of control of dexterous movements for crucial daily life activities; loss of bimanual movement control leads to relevant functional impairment of dexterity impacting on daily living
Values below 100% indicate inhibition and values above 100% indicate facilitation
The results indicate that the degree of IHIactive in older participants could be predicted by the degree of IHIrest
Summary
Bimanual movement control is an important form of control of dexterous movements for crucial daily life activities; loss of bimanual movement control leads to relevant functional impairment of dexterity impacting on daily living. It has been considered and discussed that IHI between the primary motor cortices may play an important role in suppressing activity of the contralateral side (mirror activity) contributing critically to bimanual movement control (e.g., Duque et al 2007). Based on this view, the previous studies have investigated the effects of unilateral muscle contractions on IHI from the active to the resting primary motor cortex in a detailed neurophysiological manner (e.g., Ferbert et al 1992; Perez and Cohen 2008). In case of L-IHI, no modulation (Sattler et al 2012; Morishita et al 2014; Uehara et al 2014; Turco et al 2019) and decreased L-IHI (Nelson et al 2009) were reported
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