Ageing affects interhemispheric interaction during movement preparation

Introduction: In a continuously senescent population, insight into neural correlates of healthy ageing has gained major interest and has individual, societal and economic implications. Based on structural imaging, it is well known that healthy ageing is associated with grey and white matter loss throughout lifespan predominantly affecting the frontal lobes. Functional imaging and EEG studies suggest that normal ageing leads to less lateralised motor task-related activation and recruitment of additional sensorimotor areas in both hemispheres. However, understanding of non-clinical ageing effects on motor functions is still scarse, especially with regard to the underlying connectivity patterns.

Methods: We tested the interhemispheric interaction between secondary and primary motor areas in healthy young (n=10; 7 female; age 22–27) and elderly (n=10; 7 female; age 58–71), right-handed individuals with event-related double-pulse transcranial magnetic stimulation (er-dpTMS). Subjects were instructed to execute brisk right index finger abduction after a Go-signal. Er-dpTMS was delivered during movement preparation at four time points adjusted to the individual reaction time (20, 50, 80, 95% of RT). The test pulse was applied over the left primary motor cortex (M1) and the conditioning pulse either over right M1 (M1-M1) or right premotor cortex (PMC-M1).

Results: RTs were comparable in elderly and young subjects (195±20ms vs. 185±9ms; p=0,2). Repeated measures analysis of variance revealed a significant interaction of time by condition (M1-M1, PMC-M1) for both young (p=0,014; Fig.1a) and elderly subjects (p=0,015; Fig.1b) explained by early interhemispheric facilitation during PMC-M1, whereas M1-M1 showed early inhibition with continuous release of inhibition towards movement onset. The significant interaction of time by group (young, elderly) yielded significance for PMC-M1 (p=0,013; Fig.2a), but not for M1-M1 (p=0,412; Fig.2b). Young subjects exhibited PMC-M1 facilitation in a very early time frame during movement preparation, whereas the elderly showed early inhibition for PMC-M1 (20% RT) and a delay of interhemispheric facilitation with its maximum after 80% of RT.

Discussion: The results are in line with previous EEG, MEG, TMS and combined fMRI-TMS studies showing early ipsilateral involvement of premotor cortex in motor control of the contralateral limb. The new and central finding is that ageing appears to affect interhemispheric interaction in higher-order motor regions, such as the PMC. It is noteworthy that interhemispheric interactions between primary motor areas did not differ between age groups. It can be speculated that in the motor system higher-order regions are more and earlier involved in ageing processes than primary areas as demonstrated in other cognitive domains.

Fig. 1

Fig. 2