Adaptive threshold hunting reveals differences in interhemispheric inhibition between young and older adults

Abstract

Interhemispheric inhibition between bilateral motor cortices is important for the performance of unimanual activities and may be compromised with advancing age. Conventionally, interhemispheric inhibition is assessed using paired-pulse transcranial magnetic stimulation (TMS) with constant conditioning and test stimulation parameters. Adaptive threshold hunting TMS, whereby a target motor-evoked potential amplitude is maintained in the presence of the conditioning, may provide an alternative means of assessment. Furthermore, interhemispheric inhibition may suppress late indirect-waves more so than early indirect-waves which can be preferentially elicited using anterior-posterior (AP) and posterior-anterior (PA) induced currents, respectively. The aim of this study was to assess age-related effects on interhemispheric inhibition using both conventional and threshold hunting techniques with PA- and AP-induced current. In 15 young and 15 older adults, short (10 ms) and long (40 ms) interval interhemispheric inhibition was examined in the nondominant extensor carpi radialis muscle at rest and during voluntary extension of the contralateral wrist. With the conventional technique, there were no age-related differences in short-interval interhemispheric inhibition. With threshold hunting and AP-induced current, young adults exhibited greater short-interval interhemispheric inhibition during contralateral activation compared with rest and compared with older adults. Furthermore, long-interval interhemispheric inhibition was greater in older adults compared with young for both conventional and threshold hunting techniques. Age-related differences in interhemispheric inhibition are evident with threshold hunting using PA- and AP-induced current.

Impact of interhemispheric inhibition on bimanual movement control in young and old

“…It should be noted that the behavioral associations of S-IHI active were only found in the young group. The present results showed no apparent difference of IHI between the two groups—at least partly in line with the previous studies for S-IHI (Hinder et al 2010 ; Hermans et al 2018 ) but for L-IHI (Mooney et al 2018 )—despite the behavioral differences. The first possible explanation for the lack of behavioral associations in the older group could be that the amount of mirror activity and the performance of anti-phase tapping in older adults are rather associated with neural circuits other than the one represented by S-IHI active .…”

“…The CS was delivered to the left primary motor cortex and the TS was delivered to the right primary motor cortex to measure IHI from the left to the right primary motor cortex. This direction was chosen based on the previous studies examining IHI from the active to the resting primary cortex in older adults (Talelli et al 2008 ; Hinder et al 2010 ; Mooney et al 2018 ; Ermer et al 2020 ). Two ISIs were adopted to assess IHI: 10 ms for S-IHI and 40 ms for L-IHI (Chen et al 2003 ; Ni et al 2009 ).…”

“…In line with this, the previous studies have also examined motor inhibition and its decline in older adults (for review, Levin et al 2014 ); the methodology to study motor inhibition varies among studies such as different inhibitory circuits assessed by TMS (Fujiyama et al 2012 ; Heise et al 2013 ; Opie et al 2015 ), concurrent TMS-electroencephalography (EEG) (Opie et al 2018 ; Casula et al 2020 ), and magnetic resonance imaging techniques (Hermans et al 2018 ; Cuypers et al 2020 ). To this date, with regard to age, only a few studies have examined IHI from the active to the resting primary motor cortex (Talelli et al 2008 ; Hinder et al 2010 ; Mooney et al 2018 ; Ermer et al 2020 ) using paired-pulse TMS during unilateral muscle contractions and the results are inconclusive in terms of bimanual movement control. In case of older adults, we tested the hypotheses if less IHI from the active to the resting primary motor cortex compared with young adults would be associated with poorer bimanual performance.…”

Anodal Transcranial Direct Current Stimulation Over S1 Differentially Modulates Proprioceptive Accuracy in Young and Old Adults

“…For instance, although generally believed to have a dampening effect on cortical excitability (Nitsche and Paulus, 2000), cathodal tDCS has been shown to enhance motion perception in a visual tracking task (Antal et al, 2004), possibly interacting with a reduced signal-to-noise ratio in neural processing in the OA group (Hämmerer et al, 2013). Future studies should also investigate the effect of a bilateral S1 electrode setup (Fujimoto et al, 2014) specifically in OA, since it might interfere stronger with age-related interhemispheric activation changes (Mooney et al, 2018; Strauss et al, 2019). It is also possible that afferent sensory functions are modulated in a different way by means of classical tDCS protocols as compared to efferent motor functions.…”

Contribution of TMS and TMS-EEG to the Understanding of Mechanisms Underlying Physiological Brain Aging

“…IHI is most pronounced at ISIs of 8-10 ms and 40-50 ms, which are referred to as short and long latency IHI, respectively (SIHI and LIHI). Most studies, but not all [119,120], testing IHI at rest, revealed that both SIHI [121,122] and LIHI [123][124][125] did not differ between older and young subjects. By contrast, there is evidence that elderly subjects have less lateralized cortical activation during various motor tasks, including hand grip [126].…”