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BACKGROUND: The dorsal premotor cortex (PMd) is a key region in bimanual coordination. However, causal evidence linking PMd functionality during motor planning and execution to movement quality is lacking. OBJECTIVE: We investigated how left (PMdL) and right PMd (PMdR) are causally involved in planning and executing bimanual movements, using short-train repetitive transcranial magnetic stimulation (rTMS). Additionally, we explored to what extent the observed rTMS-induced modulation of performance could be explained by rTMS-induced modulation of PMd-M1 interhemispheric interactions (IHI). METHODS: Twenty healthy adults (mean age ± SD = 22.85 ± 3.73 years) participated in two sessions, in which either PMdL or PMdR was targeted with rTMS (10 Hz) in a pseudo-randomized design. PMd functionality was transiently modulated during the planning or execution of a complex bimanual task, whereby the participant was asked to track a moving dot by controlling two dials. The effect of rTMS on several performance measures was investigated. Concurrently, rTMS-induced modulation of PMd-M1 IHI was measured using a dual-coil paradigm, and associated with the rTMS-induced performance modulation. RESULTS: rTMS over PMdL during planning increased bilateral hand movement speed (p = 0.03), thereby improving movement accuracy (p = 0.02). In contrast, rTMS over PMdR during both planning and execution induced deterioration of movement stability (p = 0.04). rTMS-induced modulation of PMd-M1 IHI during planning did not predict rTMS-induced performance modulation. CONCLUSION: The current findings support the growing evidence on PMdL dominance during motor planning, as PMdL was crucially involved in planning the speed of each hand, subserving bimanual coordination accuracy. Moreover, the current results suggest that PMdR fulfills a role in continuous adjustment processes of movement. ispartof: BRAIN STIMULATION vol:14 issue:2 pages:423-434 ispartof: location:United States status: published

CITATION Hehl M, Swinnen SP, Van Malderen S and Cuypers K (2022) No evidence for a difference in lateralization and distinctiveness level of transcranial magnetic stimulation-derived cortical motor representations over the adult lifespan. This study aimed to investigate the presence and patterns of age-related differences in TMS-based measures of lateralization and distinctiveness of the cortical motor representations of two different hand muscles. In a sample of seventy-three right-handed healthy participants over the adult lifespan, the first dorsal interosseus (FDI) and abductor digiti minimi (ADM) cortical motor representations of both hemispheres were acquired using transcranial magnetic stimulation (TMS). In addition, dexterity and maximum force levels were measured. Lateralization quotients were calculated for homolog behavioral and TMS measures, whereas the distinctiveness between the FDI and ADM representation within one hemisphere was quantified by the center of gravity (CoG) distance and cosine similarity. The presence and patterns of age-related changes were examined using linear, polynomial, and piecewise linear regression. No age-related differences could be identified for the lateralization quotient of behavior or cortical motor representations of both intrinsic hand muscles. Furthermore, no evidence for a change in the distinctiveness of the FDI and ADM representation with advancing age was found. In conclusion this work showed that lateralization and distinctiveness of cortical motor representations, as determined by means of TMS-based measures, remain stable over the adult lifespan. Funding This work was supported by the Research Fund KULeuven (C16/15/070), the Research Foundation Flanders grant (G039821N), the Excellence of Science grant (EOS 30446199, MEMODYN), and the Infrastructure fund from the Research Foundation Flanders [AUHL/11/01 (R-3987) and I005018N]. MH (11F6921N) and SVM (11L9322N) were funded by a fellowship grant from FWO. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. SVM was supported by the UHasselt Special Research Fund grant (BOF21INCENT15). Acknowledgments We wish to thank H. Thijs and R. Clerckx for their assistance in the statistical analysis and the automatization of data processing, respectively

Dual-site transcranial magnetic stimulation (ds-TMS) is well suited to investigate the causal effect of distant brain regions on the primary motor cortex, both at rest and during motor performance and learning. However, given the broad set of stimulation parameters, clarity about which parameters are most effective for identifying particular interactions is lacking. Here, evidence describing inter-and intra-hemispheric interactions during rest and in the context of motor tasks is reviewed. Our aims are threefold: (1) provide a detailed overview of ds-TMS literature regarding inter-and intra-hemispheric connectivity; (2) describe the applicability and contributions of these interactions to motor control, and; (3) discuss the practical implications and future directions. Of the 3659 studies screened, 109 were included and discussed. Overall, there is remarkable variability in the experimental context for assessing ds-TMS interactions, as well as in the use and reporting of stimulation parameters, hindering a quantitative comparison of results across studies. Further studies examining ds-TMS interactions in a systematic manner, and in which all critical parameters are carefully reported, are needed. This work was supported by the KULeuven Special Research Fund grant (C16/15/070), Research Foundation Flanders grant (G089818N and G039821N), and the Excellence of Science grant (EOS 30446199, MEMODYN). SVM (11L9322N) and MH (11F6921N) are funded by a grant from the Research Foundation Flanders. SVM is supported by the UHasselt Special Research Fund grant (BOF21INCENT15).