Functional substitution and long-term dependency in BCI–FES-based neurorehabilitation

Dear Editor, Wang et al present a comprehensive and timely review of brain–computer interface-driven functional electrical stimulation (BCI–FES) as an innovative approach to motor rehabilitation following central nervous system injury1. By coupling cortical intention with peripheral electrical stimulation, BCI–FES systems represent a significant conceptual advance over conventional rehabilitation strategies and offer new possibilities for patients with severe motor impairment. However, the translational trajectory of BCI–FES raises an important issue that warrants closer attention. The capacity of these systems to bypass damaged corticospinal pathways through an external artificial loop, while enabling immediate motor output, also introduces the possibility that functional substitution may increasingly supersede endogenous recovery as duration of use extends. This concern becomes particularly salient as BCI–FES technologies move beyond short-term laboratory protocols toward prolonged and unsupervised application2,3. As shown in Figure 1, non-invasive EEG-based BCI–FES studies are largely confined to short-duration, laboratory-based interventions, whereas invasive electrocorticography (ECoG)-based systems are characterized by substantially longer periods of use, including home-based deployment lasting months to years. This clear divergence suggests that BCI–FES may not constitute a single rehabilitative modality, but rather encompasses a spectrum ranging from time-limited therapy to long-term assistive or neuroprosthetic solutions4. Figure 1.: The invasiveness, duration, and context of use in BCI–FES studies. From a neurobiological perspective, sustained motor recovery depends not only on task execution but on continuous engagement and reorganization of intrinsic motor networks. If motor output is reliably achieved through an external pathway that assumes the functional role of a damaged circuit, residual corticospinal or subcortical pathways may be subjected to reduced functional demand. Over time, this could limit the reactivation of endogenous motor control, particularly in individuals with partial preservation of descending pathways. Although long-term outcome data remain scarce, the extended durations observed in invasive BCI–FES applications underscore the relevance of this potential trade-off5. These considerations do not diminish the clinical value of BCI–FES, but they do highlight the need for clearer distinction between restorative and assistive objectives. Design principles such as intention-dependent stimulation, adaptive assistance, and systematic reduction of external drive should be integral to clinical implementation if the primary goal is recovery rather than permanent compensation. As BCI–FES systems continue to advance toward wider clinical and home-based use, explicit evaluation of dependence, withdrawal effects, and durability of motor gains will be essential. Addressing the balance between functional substitution and endogenous recovery will ultimately determine the role of BCI–FES within neurorehabilitation, and whether it should be framed primarily as a rehabilitative therapy or as a long-term neural assistive technology. In accordance with the transparency in the reporting of artificial intelligence (TITAN) guideline, generative artificial intelligence was used solely to assist with language refinement, and the authors take full responsibility for the accuracy, originality, and interpretation of all content6.