Non-Invasive neuroModulation of Deep Brain Structures for Upper Limb Recovery After Stroke by Transcranial Temporal Interference Electric Stimulation.

• The burden of stroke.** Stroke is a major cause of acquired disabilities, with a growing number of survivors due to an aging society and improved acute management. Still, persistent motor impairment is highly common in stroke patients and is associated with a poor level of reintegration in normal life. Effective novel neurorehabilitation strategies are thus urgently required to enhance recovery rates and decrease the burden of stroke for patients, the healthcare systems and society.
• Neuroplasticity and brain networks.** To improve the efficacy of therapeutic strategies for stroke patients, a clear understanding of the neuronal mechanisms underlying post-stroke recovery is essential. Stroke lesions initiate a cascade of changes in brain metabolism, functional activation, neuronal excitability, and structural integrity. Recent neuroimaging studies, including structural and functional analyses, have highlighted dynamic network changes that are particularly important in the process of successful recovery. For upper extremity (UE) motor function, the recovery network notably includes the primary and secondary motor cortices, as well as core deep brain structures such as the striatum. Neuroplastic changes in these areas are critical elements for successful functional reorganization and recovery after a stroke. An important aspect is the enhanced plasticity observed during the acute and subacute phases after a stroke-specifically, the first 12 weeks, known as the "hyper-plastic phase." This period corresponds to a sensitive window during which the brain responds optimally to external stimuli, activity, rehabilitative treatments, and interventional strategies. Intensive interventional approaches during this time are most effective in promoting brain plasticity and recovery. However, spontaneous brain reorganization after a stroke often fails to achieve substantial and satisfactory functional recovery independently.
• Promising but unsatisfactory results from NIBS.** Among various innovative neurorehabilitation strategies, non-invasive brain stimulation (NIBS) is associated with promising results to neuromodulate brain activity and enhance neuroplasticity and motor recovery by potentiating the response to behavioural training. The safe and easy application of NIBS makes the technique particularly interesting for clinicians and scientists. However, despite this exciting potential, the application is so far limited to cortical areas (e.g., motor cortex) and has revealed heterogeneous results regarding motor recovery. This heterogeneity might be due to the so far 'one suits all non-personalized' application of NIBS. Historically, the motor cortex has been the primary target for interventions, despite the possibility that the motor cortex may not be as crucial as other brain regions in motor skill acquisition and consolidation-key components of neurorehabilitation and recovery. However, core deep brain structures, such as the basal ganglia, particularly the striatum, which are critically involved in (re-)learning, reorganization, and recovery processes after stroke, have remained inaccessible to non-invasive interventions.
• Role of the basal ganglia/striatum under physiological conditions.** Deep subcortical structures such as the basal ganglia are relay nodes representing a critical hub in the motor network. In particular, the striatum plays a substantial role in key aspects of sensorimotor processing and learning, essentially important for the execution and (re-)acquisition of motor skills. Lesion studies in animal models demonstrated the significant role of the striatum in the acquisition and the retention of motor behavior. For example, lesions of the dorsal striatum in primates resulted in significant motor impairment, e.g., slowness of movement and altered acquisition of motor skills while lesions of the dorsolateral striatum in rats led to the disruption of motor habit formation. In humans, the classical studies of patients with neurological disorders affecting the striatum (as Parkinson's Disease) demonstrated striatal involvement not only in movement performance but also in the acquisition of new motor skills. This was further underlined in non-human primate models of Parkinsonism. Furthermore, imaging data were able to delineate the involvement of basal ganglia in the motor learning network in healthy humans.

In sum, the striatum is a key structure of motor control and (re-)learning. The striatum contributes to multiple levels including initiation of movements, facilitation of goal-directed actions, habit formation and especially motor skill acquisition, consolidation and retention. This makes the striatum a very promising target for interventional strategies based on striatal neuromodulation combined with motor (re-)learning after brain lesions. This concept is further supported by current evidence that the striatum is a highly neuroplastic area given the striatum's composition and known associations with critical neurotransmitter systems.

• Striatum in stroke - animal and human models.** Emerging evidence from both animal and human studies underscores the pivotal role of the striatum in post-stroke recovery mechanisms, highlighting the striatum as a promising target for neuromodulation therapies. In animal stroke models, a consistent regenerative response has been observed after focal strokes, resulting in the replacement of lost striatal neurons. Despite disturbances in movement-related neuronal activity following motor cortical strokes, the striatum continues to play a substantial role in controlling residual motor functions. During rehabilitation, reorganization of striatal activity strongly correlates with improvements in animals' motor abilities. Moreover, optogenetic stimulation of striatal activity promotes neurogenesis and long-term functional recovery, demonstrating a causal relationship between striatal activity and motor function restoration. In human studies, neuroimaging has revealed that lesions in the basal ganglia are associated with abnormal interaction patterns with cortical areas after a stroke. Structural connectivity with the basal ganglia two weeks post-stroke significantly contributes to predicting motor recovery at three months. A large-scale study involving over 800 individuals found that reduced volume of the ipsilesional striatum is strongly associated with poorer outcomes in the chronic phase. Collectively, these findings suggest that neuroplastic changes in the striatum are crucial for supporting motor recovery. Interestingly, the striatum is also involved in reward processing and motivation as well as visuospatial attention-aspects that play significant roles in the relearning and reacquisition of motor skills following a stroke. Deficits in reward processing among stroke patients have been linked to reduced activity in the ventral striatum, independent of structural damage. Furthermore, individuals with lesions in the basal ganglia exhibit poorer reward sensitivity, higher levels of apathy, and impaired visuospatial attention. Therefore, the basal ganglia, particularly the striatum, serve as key nodes within brain networks that facilitate motor recovery and influence critical aspects of motor skill reacquisition-such as reward, motivation, and attention-which are fundamental mechanisms underpinning neurorehabilitation.
• Striatum and LTP-like plasticity.** Approximately 95% of striatal cells are GABAergic medium spiny neurons (MSNs), which receive glutamatergic inputs from the cortex and dopaminergic inputs from the substantia nigra. MSNs have the capacity to undergo long-term potentiation (LTP) or long-term depression (LTD)-key neuroplasticity mechanisms-in response to high-frequency stimulation. Theta burst stimulation, initially demonstrated in hippocampal slices, can induce LTP- and LTD-like plasticity in the striatum, particularly when delivered in patterns that mirror natural striatal activation. This underscores the significance of activity-dependent effects linked to physiological striatal firing in enhancing behavioral outcomes. Importantly, LTP at corticostriatal synapses is associated with motor behavior and may constitute a key cellular substrate for motor learning and skill acquisition. These attributes make the striatum an excellent target for neuromodulatory interventions, especially for LTP-inducing theta-burst stimulation. Collectively, this evidence reinforces the view that the striatum is a highly neuroplastic structure playing a crucial role in motor skill acquisition-two essential aspects for functional reorganization and motor recovery after stroke.
• Transcranial temporal interference stimulation (tTIS).** The aforementioned animal and human studies have highlighted the critical role of the striatum in motor learning, recovery processes, and neuroplasticity. These findings strongly support the striatum as a novel, promising target for neuromodulation-based interventions to effectively support recovery, particularly during the subacute, hyperplastic phase following a stroke. However, traditional NIBS techniques, such as transcranial direct current stimulation (tDCS), alternating current stimulation (tACS), and transcranial magnetic stimulation (TMS), are unable to reach deep brain structures like the striatum due to the inherent trade-off between focality and depth. The recent introduction of transcranial temporal interference stimulation (tTIS) presents an exciting, implementable, and promising non-invasive approach. tTIS enables targeted, non-invasive modulation of striatal activity with an excellent balance between depth and focality. Striatal tTIS represents a groundbreaking, novel interventional strategy to enhance motor recovery following brain injury.

In first-in-human proof-of-concept studies, **the investigators demonstrated** that striatal neuromodulation by tTIS is feasible and safe. Applying theta burst patterned tTIS (iTBS-tTIS) to the striatum led to a significant, improvement in motor skill acquisition and increased activity changes in the striatum and the connected network. The behavioral improvement was most pronounced in healthy old adults, typically in comparable age as stroke patients. Additional proof-of-concept studies further support the feasibility and efficacy of tTIS in brain lesioned patient cohorts. For instance, **the investigators conducted** a study involving 15 patients with traumatic brain injury (TBI) and, demonstrating that tTIS significantly improved motor learning and retention, with clinically meaningful effects observed for at least 24 hours after stimulation. Moreover, a study in patients with Parkinson's disease demonstrated that tTIS can improve motor symptoms, particularly bradykinesia and tremor. Collectively, these results highlight that tTIS is a safe and well-tolerated technique, demonstrating strong potential for improving motor outcomes in various neurological conditions. To achieve not only the demonstrated shorter-lasting, but long-lasting effects in the present project, **the investigators will significantly enhance the dosage and apply an accelerated stimulation protocol with three stimulation sessions per day for one week.**

• Accelerated stimulation protocols for neurorehabilitation.** Recent clinical guidelines from leading organizations such as the National Institute for Health and Care Excellence (NICE), the Intercollegiate Stroke Working Party (ISWP), and the European Stroke Organisation (ESO) emphasize the critical importance of increasing the intensity of upper limb motor rehabilitation in stroke recovery. Accelerated protocols-entailing multiple therapy sessions per day-have emerged as an effective strategy to significantly enhance functional outcomes and improve the quality of life for stroke survivors. Studies have highlighted the benefits and safety of accelerated brain stimulation protocols across various conditions, including stroke, depression, Parkinson's disease and Alzheimer's disease. These protocols, which deliver multiple stimulation sessions per day over consecutive days, have demonstrated equal or greater effectiveness compared to traditional, longer treatment paradigms. Moreover, accelerated protocols reduce the overall time commitment for patients, minimizing the impact on **patients' professional responsibilities and transportation demands**. Building on this evidence, **the investigators propose** to implement an accelerated striatal tTIS protocol, aiming to achieve more rapid, intensive, and effective motor rehabilitation.
• Primary research question.** Current evidence highlights that (a) the striatum is a core structure involved in post-stroke motor recovery, and (b) there is an innovative opportunity to neuromodulate the striatum non-invasively, safely, and with precise depth-focality. This breakthrough could pave the way for novel and disruptive treatment strategies to enhance stroke recovery. **The investigators hypothesize** that personalized, accelerated striatal tTIS, combined with upper extremity rehabilitative training, will be feasible, safe, and lead to greater behavioral improvement and recovery than placebo stimulation with rehabilitative training in chronic stroke patients. The primary endpoints are composite upper extremity motor functions, feasibility and safety. To test this hypothesis, **the investigators will conduct** a double-blind, placebo-controlled, multicenter clinical trial. **The investigators will apply** intermittent theta-burst tTIS to the striatum (iTBS-tTISStriatum), with individualized electrode positioning based on each patient's anatomy, alongside intensive rehabilitation therapy. This will be the first clinical trial to non-invasively target deep brain structures (striatum) to enhance stroke recovery in humans.