P254 The interaction of cross-limb transfer gains, intermittent theta-burst stimulation and subsequent learning in young and older adults

Abstract

Introduction Recently we found evidence that following right hand motor training intermittent theta-burst stimulation (iTBS) applied to the ipsilateral primary motor cortex (M1) increased corticospinal excitability (CSE) and facilitated performance in the untrained hand (i.e., cross-limb transfer, CLT). Objectives Here we examine whether the interaction of CLT and iTBS can be exploited to drive improved learning in one hand following learning with the other hand, and to assess any age-related differences in this interaction. Materials and methods Eighteen young (Mage = 25.2 ± 5.7 years, 8 males) and seven healthy older adults (Mage = 66.4 ± 3.1 years, 6 males) practiced a ballistic motor task with their right index finger (150 trials, T1), after which iTBS was applied to the untrained M1. Another 150 training trials (T2) were then conducted. During T2, 13 participants (8 young, 7 older) practiced with their left hand (RL group) while 12 young participants continued practicing with their right hand (RR group). Performance (peak acceleration) and CSE were assessed before T1, pre- and post-iTBS, and following T2. Results Although iTBS facilitated untrained hand MEPs in both age groups (+29%, p = .005), untrained hand performance was facilitated by iTBS in young (+25%; p = .04), but not in older (−2%, p = .92), adults. Following T2, no differences in left hand performance gains (adjusted for post-iTBS performance) between RL and RR (12% vs. 9%, p=.88, Fig. 1A) were observed, although CSE gains in the left hand following T2 were greater following RL than RR (34% vs. −4%, p = .02, Fig. 1A). Conclusion In young but not older adults, iTBS-induced LTP-like plasticity can be utilised to drive performance gains in the untrained limb above and beyond those gains already induced by cross-limb transfer processes. With respect to left hand performance gains, no advantage was observed following left hand T2 training (‘training gains’) compared to right hand T2 training (‘CLT gains’). Thus, homeostatic plasticity effects following iTBS administered to the untrained M1 following right hand motor learning appear insufficient to drive greater subsequent training gains when training is then undertaken with the left hand relative to those gains which would continue to occur (during right hand training) as a result of cross-limb transfer. Download : Download high-res image (447KB) Download : Download full-size image