EPV 9. The role of ipsilateral motor areas in hand motor function in healthy ageing – Insights from online TMS

Abstract

Background Event-related (“online”) transcranial magnetic stimulation (TMS) allows to interfere with neural processing of distinct cortical areas. While we have a rather good knowledge on the role of frontoparietal areas for motor performance of the contralateral hand, the contribution of areas in the hemisphere ipsilateral to the moving hand is far less understood ( [Davare et al., 2007] , [Vollmer et al., 2015] ). For example, neuroimaging studies found an increasing ipsilateral recruitment of motor regions in healthy eldery subjects, implying a supportive influence during normative ageing ( [Hutchinson et al., 2002] , [Riecker et al., 2006] ). To test this hypothesis, we used online-TMS to investigate the role of ipsilateral primary motor cortex (M1), premotor cortex (dPMC) and anterior intraparietal sulcus (aIPS) in motor tasks of different complexity in young and elderly subjects. Methods Young (range: 20–30 years) and elderly (range: 50–70 years) healthy, right-handed subjects conducted four motor tasks of different complexity with their right hand: (i) simple reaction time, (ii) maximum finger and (iii) hand tapping frequencies and (iv) rapid pointing between two defined targets. Tapping and pointing tasks were measured with kinematic, three-dimensional recordings using an ultrasound motion analyzer system (Zebris). TMS pulses were applied as 10 Hz-trains time-locked to task execution. Stimulation was applied at 90% of resting motor threshold (rMT) to ipsilateral M1, dPMC and aIPS. Sham-stimulation at 90% of rMT to parieto-occipital vertex served as control condition. Results Online-TMS applied to ipsilateral M1 led to a significant response slowing and to a significantly increased coeffcient of variation in the simple reaction time task in elderly subjects. In contrast, performance of more complex motor tasks was not affected by ipsilateral M1-stimulation. TMS applied to the ipsilateral dPMC or aIPS affected the more complex motor tasks. DPMC-stimulation caused a reduction of tapping amplitude and a significant reduction of overall performance in the finger tapping task. TMS applied to the aIPS led to a significant reduction of tapping amplitude and tended to reduce overall performance in the hand tapping task. Furthermore, subjects showed a significant decrease of aiming accuracy in the pointing task, upon TMS interference with ipsilateral dPMC as well as aIPS in elderly subjects. Finally, within the group of elderly subjects, age strongly correlated with a frequency reduction of hand tapping induced by both M1- and aIPS-stimulation. Conclusion Consistent with previous work ( Vollmer et al., 2015 ), our findings underline the impact of ipsilateral M1 on simple motor task. We also show that in eldery subjects ipsilateral dPMC as well as ipsilateral aIPS seem to be causally engaged in maintaining performance in more complex motor tasks. These findings agree with neuroimaging studies indicating the IPS’ particular involvement in visuomotor tasks ( Grefkes et al., 2006 ). Of note, our results support the notion of age-dependent recruitment of ipsilateral M1 in motor tasks ( [Hutchinson et al., 2002] , [Riecker et al., 2006] ) and suggest a similar relevance of aIPS for motor performance in healthy ageing individuals.