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Objectives: Navigated transcranial magnetic stimulation (nTMS) provides significant benefits over classic TMS. Yet, the acquisition of individual structural magnetic resonance images (MRIindividual) is a time-consuming, expensive, and not feasible prerequisite in all subjects for spatial tracking and anatomical guidance in nTMS studies. We hypothesize that spatial transformation can be used to adjust MRI templates to individual head shapes (MRIwarped) and that TMS parameters do not differ between nTMS using MRIindividual or MRIwarped.
Materials and Methods: Twenty identical TMS sessions, each including four different navigation conditions, were conducted in 10 healthy subjects (one female, 27.4 ± 3.8 years), i.e., twice per subject by two researchers to additionally assess interrater reliabilities. MRIindividual were acquired for all subjects. MRIwarped were obtained through the spatial transformation of a template MRI following a 5-, 9-and 36-point head surface registration (MRIwarped_5, MRIwarped_9, MRIwarped_36). Stimulation hotspot locations, resting motor threshold (RMT), 500 μV motor threshold (500 μV-MT), and mean absolute motor evoked potential difference (MAD) of primary motor cortex (M1) examinations were compared between nTMS using either MRIwarped variants or MRIindividual and non-navigated TMS.
Results: M1 hotspots were spatially consistent between MRIindividual and MRIwarped_36 (insignificant deviation by 4.79 ± 2.62 mm). MEP thresholds and variance were also equivalent between MRIindividual and MRIwarped_36 with mean differences of RMT by −0.05 ± 2.28% maximum stimulator output (%MSO; t(19) = −0.09, p = 0.923), 500 μV-MT by −0.15 ± 1.63%MSO (t(19) = −0.41, p = 0.686) and MAD by 70.5 ± 214.38 μV (t(19) = 1.47, p = 0.158). Intraclass correlations (ICC) of motor thresholds were between 0.88 and 0.97.
Conclusions: NTMS examinations of M1 yield equivalent topographical and functional results using MRIindividual and MRIwarped if a sufficient number of registration points are used.
Background and Objective: Transcranial random noise stimulation (tRNS) is an emerging non-invasive brain stimulation technique to modulate brain function, with previous studies highlighting its considerable benefits in therapeutic stimulation of the motor system. However, high variability of results and bidirectional task-dependent effects limit more widespread clinical application. Task dependency largely results from a lack of understanding of the interaction between externally applied tRNS and the endogenous state of neural activity during stimulation. Hence, the aim of this study was to investigate the task dependency of tRNS-induced neuromodulation in the motor system using a finger-tapping task (FT) versus a go/no-go task (GNG). We hypothesized that the tasks would modulate tRNS’ effects on corticospinal excitability (CSE) and task performance in opposite directions.
Methods: Thirty healthy subjects received 10 min of tRNS of the dominant primary motor cortex in a double-blind, sham-controlled study design. tRNS was applied during two well-established tasks tied to diverging brain states. Accordingly, participants were randomly assigned to two equally-sized groups: the first group performed a simple motor training task (FT task), known primarily to increase CSE, while the second group performed an inhibitory control task (go/no-go task) associated with inhibition of CSE. To establish task-dependent effects of tRNS, CSE was evaluated prior to- and after stimulation with navigated transcranial magnetic stimulation.
Results: In an ‘activating’ motor task, tRNS during FT significantly facilitated CSE. FT task performance improvements, shown by training-related reductions in intertap intervals and increased number of finger taps, were similar for both tRNS and sham stimulation. In an ‘inhibitory’ motor task, tRNS during GNG left CSE unchanged while inhibitory control was enhanced as shown by slowed reaction times and enhanced task accuracy during and after stimulation.
Conclusion: We provide evidence that tRNS-induced neuromodulatory effects are task-dependent and that resulting enhancements are specific to the underlying task-dependent brain state. While mechanisms underlying this effect require further investigation, these findings highlight the potential of tRNS in enhancing task-dependent brain states to modulate human behavior.