Serotonergic System Dysregulation in Alzheimer's Disease.

Stroke is increasingly conceptualized not merely as a focal lesion but as a disorder of distributed brain networks, characterized by large-scale disruptions in interhemispheric balance, functional connectivity, and dynamic neural communication (). Although endogenous neuroplastic mechanisms enable partial recovery, many survivors experience persistent motor and cognitive impairments that reflect maladaptive network reorganization rather than isolated cortical damage (, ). These systems-level alterations underscore the necessity of therapeutic strategies capable of modulating neural circuits rather than targeting single cortical sites. Non-invasive brain stimulation (NIBS), including transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES), has emerged as a promising neuromodulatory approach for investigating the involvement of targeted brain regions in functional processes and for modulating dysfunctional network organization after stroke (). However, a fundamental challenge in the clinical application of NIBS is the high inter-individual variability in treatment response. Even among neurologically intact individuals and across standardized plasticity-inducing protocols, motor evoked potentials (MEPs) and cortical excitability responses exhibit significant heterogeneity (, ). This variability highlights a limitation of empirically applied stimulation paradigms and emphasizes the need for state-dependent, biomarker-informed intervention strategies that account for individual functional network states and neurophysiological characteristics. Neuroimaging provides a critical bridge between mechanistic understanding and clinical translation. Electroencephalography (EEG) enables the discovery of oscillatory interaction networks and the characterization of phase-interaction mappings that coordinate distributed neuronal activity (). Complementarily, functional magnetic resonance imaging (fMRI), including resting-state and task-based paradigms, delineates large-scale functional brain organization by characterizing regional activation patterns and interactions among anatomically distributed brain regions, thereby providing insights into functional connectivity and network reorganization after stroke (). These modalities offer complementary temporal and spatial perspectives on stimulation-induced neuroplasticity, enabling multi-scale characterization of post-stroke network reorganization. This review specifically focuses on repetitive transcranial magnetic stimulation (rTMS), transcranial direct current stimulation (tDCS), and emerging transcranial alternating current stimulation (tACS) paradigms integrated with EEG and fMRI biomarkers in post-stroke recovery. Other NIBS modalities, such as transcranial random noise stimulation (tRNS), are discussed where relevant but were not systematically reviewed. The present synthesis primarily focuses on scalp EEG methodologies, which remain the dominant electrophysiological approach in post-stroke neuromodulation studies. Future opportunities lie in invasive electrophysiological recordings, which may enable higher-resolution characterization of stimulation-induced mechanisms and circuit-level neuroplasticity.