The functional relevance of interhemispheric premotor-motor connectivity for bimanual movements in young and elder healthy subjects

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Introduction: With healthy aging, the motor cortical network subserving the execution of voluntary movements is less lateralized. However, aging effects on the temporal dynamics of interhemispheric connectivity patterns and the impact of connectivity changes on bimanual skills are not completely understood. In the present study we evaluated interhemispheric interactions between premotor and primary motor cortex and their functional relevance for bimanual movements in elderly compared to young subjects. Methods: We studied healthy young (n=8; age 23±0.6 years) and elderly (n=9; age 65.2±1.4 years) right-handed individuals with event-related double-pulse transcranial magnetic stimulation (er-dpTMS). er-dpTMS was delivered during movement preparation at four time points adjusted to the individual reaction time (20, 50, 80, 95% of RT) between homologue primary motor areas (M1-M1) and between the right premotor cortex and left M1 (PMC-M1). In a separate session, bimanual performance was determined by the maximum frequency at which cyclical bimanual anti-phase movements could be maintained. Results: Repeated measures analysis of variance revealed a significant three-way interaction of time (20, 50, 80, 95% of RT) by group (young, elderly) by location (M1-M1, PMC-M1) (p=0,03). At the level of each location, the interaction of time by group (young, elderly) yielded significance for PMC-M1 (p=0,013), but not for M1-M1 (p=0,412). Young subjects exhibited PMC-M1 facilitation early during movement preparation, whereas the elderly showed less persistence and a delay of interhemispheric facilitation in the PMC-M1 interaction with its maximum around the mid-phase of RT. In the young, the maximum frequency of bimanual anti-phase movements correlated with the amount of interhemispheric PMC-M1 facilitation at 20% of RT in the young and at 50% of RT in the elderly subjects. Conclusions: We here show that the amount of facilitation in PMC-M1 interaction early during movement preparation correlated positively with better performance during the bimanual movement task (higher frequency in the anti-phase mode). In the elderly, facilitatory PMC-M1 interaction was decelerated and less prominent. This finding was underscored by a shift in the correlation of PMC-M1 with bimanual skill towards the mid-phase of movement preparation.

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